Biomedical subjects

Marcel van Herk

Publications and source records attributed to Marcel van Herk.

At least 19 recordsLinked to original sources

PubMed2026

Technical note: reconstructing dose distributions from manually planned electron boosts in breast radiotherapy.

PURPOSE: In breast radiotherapy, delivery of manually-calculated electron boosts limits retrospective dose-response analyses as dose distribution is unavailable. This work evaluates the feasibility of reconstructing dose distributions from manually planned electron boosts in breast-conserving radiotherapy. METHODS: Only 72 out of 198 breast cancer patients had complete stored dose distributions from sequential electron boosts in the REQUITE study. Arbitrary data from 70/72 patients were used to develop and validate dose reconstruction method. Twenty patients were used to determine optimal parameters for Monte-Carlo-based (MC) electron dose reconstruction on RayStation (v.11B-R), considering CT-calibration curve, MC-history number, andcalculation grid resolution. Remaining 50 patients were used to quantify dose reconstruction accuracy. The similarity between reconstructed and stored dose was evaluated using 3D-gamma index and dosimetric parameters extracted from breast and tumour bed contours. Dose difference location was evaluated using dose-location histogram. RESULTS: Calculation grid resolution significantly impacted electron dose distribution (p < 0.01), where the finest grid (0.15 cm) showed highest similarity to stored doses. CT-calibration curve and MC-history number had a negligible influence on dose reconstruction. Dosimetric difference between reconstructed and stored doses was < 1 Gy for breast and tumour bed. Reconstructed dose was achieved > 90% gamma passing rate in the validation set. However, around 2.5 Gy dose differences were observed at the skin and tissue interface regions. CONCLUSIONS: Retrospective electron boost dose reconstruction is feasible with acceptable accuracy, and could increase data completeness in large cohort studies. Caution is advised when assessing dose near tissue interface and further validation is needed outside the REQUITE dataset.

Electrons↗

PubMed2006

Short-term and long-term reproducibility of lung tumor position using active breathing control (ABC).

PURPOSE: To evaluate the short-term and long-term reproducibility of lung tumor position for scans acquired using an active breathing control (ABC) device. METHODS AND MATERIALS: Ten patients with lung cancer were scanned over three sessions during the course of treatment. For each session, two scans were acquired at deep inhale, and one scan each at half of deep inhale and at exhale. Long-term reproducibility was evaluated by comparing the same breathing state scans from two sessions, with setup variation removed by skeletal alignment. Tumor alignment was based on intensity matching of a small volume around the tumor. For short-term reproducibility, the two inhale volumes from the same session were compared. RESULTS: For the short-term reproducibility, the mean and the standard deviation (SD) of the displacement of the center of tumor were 0.0 (1.5) mm in anteroposterior (AP), 0.3 (1.4) mm in superior/inferior (SI), and 0.2 (0.7) mm in right/left (RL) directions. For long-term reproducibility, the mean (SD) were -1.3 (3.1) mm AP, -0.5 (3.8) mm SI, and 0.3 (1.6) mm RL for inhale and -0.2 (2.8) mm AP, 0.2 (2.1) mm SI, and -0.7 (1.1) mm RL for exhale. CONCLUSION: The ABC device demonstrates very good short-term and long-term reproducibility. Increased long-term variability in position, primarily in the SI and AP directions, indicates the role of tumor-directed localization in combination with breath-held immobilization.

Carcinoma, Non-Small-Cell Lung↗

PubMed2006

Mid-ventilation CT scan construction from four-dimensional respiration-correlated CT scans for radiotherapy planning of lung cancer patients.

PURPOSE: Four-dimensional (4D) respiration-correlated imaging techniques can be used to obtain (respiration) artifact-free computed tomography (CT) images of the thorax. Current radiotherapy planning systems, however, do not accommodate 4D-CT data. The purpose of this study was to develop a simple, new concept to incorporate patient-specific motion information, using 4D-CT scans, in the radiotherapy planning process of lung cancer patients to enable smaller error margins. METHODS AND MATERIALS: A single CT scan was selected from the 4D-CT data set. This scan represented the tumor in its time-averaged position over the respiratory cycle (the mid-ventilation CT scan). To select the appropriate CT scan, two methods were used. First, the three-dimensional tumor motion was analyzed semiautomatically to calculate the mean tumor position and the corresponding respiration phase. An alternative automated method was developed to select the correct CT scan using the diaphragm motion. RESULTS: Owing to hysteresis, mid-ventilation selection using the three-dimensional tumor motion had a tumor position accuracy (with respect to the mean tumor position) better than 1.1 +/- 1.1 mm for all three directions (inhalation and exhalation). The accuracy in the diaphragm motion method was better than 1.1 +/- 1.1 mm. Conventional free-breathing CT scanning had an accuracy better than 0 +/- 3.9 mm. The mid-ventilation concept can result in an average irradiated volume reduction of 20% for tumors with a diameter of 40 mm. CONCLUSION: Tumor motion and the diaphragm motion method can be used to select the (artifact-free) mid-ventilation CT scan, enabling a significant reduction of the irradiated volume.

Algorithms↗

PubMed2006

Anatomy changes in radiotherapy detected using portal imaging.

BACKGROUND AND PURPOSE: Localisation images normally acquired to verify patient positioning also contain information about the patient's internal anatomy. The aim of this study was to investigate the anatomical changes observed in localisation images and examples of dosimetric consequences. PATIENTS AND METHODS: Localisation images were obtained weekly prior to radiotherapy with an electronic portal imaging device (EPID). A series of 'difference images' was created by subtracting the first localisation image from that of subsequent fractions. Images from 81 lung, 40 head and neck and 34 prostate cancer patients were classified according to the changes observed. Changes were considered relevant if the average pixel value over an area of at least 1cm(2) differed by more than 5%, to allow for variations in linac output and EPID signal. Two patients were selected to illustrate the dosimetric effects of relevant changes. Their plans were re-calculated with repeat CT scans acquired after 4 weeks of treatment and compared with the difference images of the corresponding days. RESULTS: Progressive changes were detected for 57% of lung and 37% of head and neck cancer patients studied. Random changes were observed in 37% of lung, 28% of head and neck and 82% of prostate cancer patients. For a lung case, an increase of 10.0% in EPID dose due to tumour shrinkage corresponded to an increase of 9.8% in mean lung dose. Gas pockets in the rectum region of the prostate case increased the EPID dose by 6.3%, and resulted in a decrease of the minimum dose to the planning target volume of 26.4%. CONCLUSIONS: Difference images are an efficient means of qualitatively detecting anatomical changes for various treatment sites in clinical practice. They can be used to identify changes for a particular patient, to indicate if the dose delivered to the patient would differ from planning and to detect if there is a need for re-planning.

Diagnostic Imaging↗

PubMed2006

Mapping of treatment-induced apoptosis in normal structures: 99mTc-Hynic-rh-annexin V SPECT and CT image fusion.

PURPOSE: The purpose of this study was to map treatment-induced (99m)Tc-Hynic-rh-annexin V uptake in normal tissues using co-registration of SPECT and CT. METHODS: Nineteen patients (11 male, 8 female, mean age 57 years) with various malignant tumours (12 lymphomas, four non-small cell lung cancers and three head and neck squamous cell carcinomas) underwent (99m)Tc-Hynic-rh-annexin V scintigraphy and CT before and within 48 h after the start of anticancer therapy. SPECT and CT were performed separately, with the patient in a reproducible position. Volume-based automated and manual methods were used to match functional and anatomical data. SPECT/CT co-registration was used to evaluate treatment-induced changes in the normal structures. RESULTS: A significant radiation field-related increase in early post-treatment (99m)Tc-Hynic-rh-annexin V uptake in salivary glands and bone marrow was detected in eight of nine patients. Radiation field-related increase in bone marrow activity above the baseline value was detected in all 13 irradiated patients. A minimal, symmetrical increase in activity in the salivary glands was detected after the initial course of platinum-based chemotherapy, and a diffuse prominent increase in (99m)Tc-Hynic-rh-annexin V in the bone marrow was detected in all cases. Precise delineation between the tumour and normal tissue tracer accumulation was accomplished in all cases using SPECT/CT co-registered volumes, enhanced by the "colourwash" technique. CONCLUSION: Mapping of early treatment-related changes in annexin V uptake by SPECT/CT co-registration permits accurate evaluation of tracer distribution in normal structures and precise delineation from tumour uptake. The associations between tracer distribution in the normal tissues and treatment regimen found in this study may contribute to the evaluation of dose-effect relations in various treatment schedules.

Adult↗

PubMed2006

Tumor motion and deformation during external radiotherapy of bladder cancer.

PURPOSE: First, to quantify bladder-tumor motion in 3 dimensions during a 4-week to 5-week course of external radiotherapy. Second, to relate the motion to the tumor location on the bladder wall. Third, to extensively evaluate gross tumor volume (GTV) shape and volume changes during the course of the treatment. METHODS AND MATERIALS: Multiple repeat computed tomography (CT) images were obtained for 21 bladder cancer patients. These scans were matched to the rigid bony anatomy. For each patient, the main direction and magnitude of the tumor movement was determined by use of principle-component analysis. To study GTV shape changes, all GTVs were registered to the GTV in the planning CT scan, and the residual shape errors were determined by measurement of edge variations perpendicular to the median surface. RESULTS: Gross tumor volume translations were largest in cranial-caudal and anterior-posterior direction (SD, 0.1 to approximately 0.9 cm). The translations were strongly correlated with the tumor location on the bladder wall. The average value of the local standard deviations of the GTV shape ranged from 0.1 to approximately 0.35 cm. CONCLUSIONS: Despite large differences in bladder filling, variations in GTV shape were small compared with variations in GTV position. Geometric uncertainties in the GTV position depended strongly on the tumor location on the bladder wall.

Female↗

PubMed2006

Accurate two-dimensional IMRT verification using a back-projection EPID dosimetry method.

The use of electronic portal imaging devices (EPIDs) is a promising method for the dosimetric verification of external beam, megavoltage radiation therapy-both pretreatment and in vivo. In this study, a previously developed EPID back-projection algorithm was modified for IMRT techniques and applied to an amorphous silicon EPID. By using this back-projection algorithm, two-dimensional dose distributions inside a phantom or patient are reconstructed from portal images. The model requires the primary dose component at the position of the EPID. A parametrized description of the lateral scatter within the imager was obtained from measurements with an ionization chamber in a miniphantom. In addition to point dose measurements on the central axis of square fields of different size, we also used dose profiles of those fields as reference input data for our model. This yielded a better description of the lateral scatter within the EPID, which resulted in a higher accuracy in the back-projected, two-dimensional dose distributions. The accuracy of our approach was tested for pretreatment verification of a five-field IMRT plan for the treatment of prostate cancer. Each field had between six and eight segments and was evaluated by comparing the back-projected, two-dimensional EPID dose distribution with a film measurement inside a homogeneous slab phantom. For this purpose, the y-evaluation method was used with a dose-difference criterion of 2% of dose maximum and a distance-to-agreement criterion of 2 mm. Excellent agreement was found between EPID and film measurements for each field, both in the central part of the beam and in the penumbra and low-dose regions. It can be concluded that our modified algorithm is able to accurately predict the dose in the midplane of a homogeneous slab phantom. For pretreatment IMRT plan verification, EPID dosimetry is a reliable and potentially fast tool to check the absolute dose in two dimensions inside a phantom for individual IMRT fields. Film measurements inside a phantom can therefore be replaced by EPID measurements.

Algorithms↗

PubMed2005

Automatic prostate localization on cone-beam CT scans for high precision image-guided radiotherapy.

PURPOSE: Previously, we developed an automatic three-dimensional gray-value registration (GR) method for fast prostate localization that could be used during online or offline image-guided radiotherapy. The method was tested on conventional computed tomography (CT) scans. In this study, the performance of the algorithm to localize the prostate on cone-beam CT (CBCT) scans acquired on the treatment machine was evaluated. METHODS AND MATERIALS: Five to 17 CBCT scans of 32 prostate cancer patients (332 scans in total) were used. For 18 patients (190 CBCT scans), the CBCT scans were acquired with a collimated field of view (FOV) (craniocaudal). This procedure improved the image quality considerably. The prostate (i.e., prostate plus seminal vesicles) in each CBCT scan was registered to the prostate in the planning CT scan by automatic 3D gray-value registration (normal GR) starting from a registration on the bony anatomy. When these failed, registrations were repeated with a fixed rotation point locked at the prostate apex (fixed apex GR). Registrations were visually assessed in 3D by one observer with the help of an expansion (by 3.6 mm) of the delineated prostate contours of the planning CT scan. The percentage of successfully registered cases was determined from the combined normal and fixed apex GR assessment results. The error in gray-value registration for both registration methods was determined from the position of one clearly defined calcification in the prostate gland (9 patients, 71 successful registrations). RESULTS: The percentage of successfully registered CBCT scans that were acquired with a collimated FOV was about 10% higher than for CBCT scans that were acquired with an uncollimated FOV. For CBCT scans that were acquired with a collimated FOV, the percentage of successfully registered cases improved from 65%, when only normal GR was applied, to 83% when the results of normal and fixed apex GR were combined. Gray-value registration mainly failed (or registrations were difficult to assess) because of streaks in the CBCT scans caused by moving gas pockets in the rectum during CBCT image acquisition (i.e., intrafraction motion). The error in gray-value registration along the left-right, craniocaudal, and anteroposterior axes was 1.0, 2.4, and 2.3 mm (1 SD) for normal GR, and 1.0, 2.0, and 1.7 mm (1 SD) for fixed apex GR. The systematic and random components of these SDs contributed approximately equally to these SDs, for both registration methods. CONCLUSIONS: The feasibility of automatic prostate localization on CBCT scans acquired on the treatment machine using an adaptation of the previously developed three-dimensional gray-value registration algorithm, has been validated in this study. Collimating the FOV during CBCT image acquisition improved the CBCT image quality considerably. Artifacts in the CBCT images caused by large moving gas pockets during CBCT image acquisition were the main cause for unsuccessful registration. From this study, we can conclude that CBCT scans are suitable for online and offline position verification of the prostate, as long as the amount of nonstationary gas is limited.

Algorithms↗

PubMed2005

Observer variation in target volume delineation of lung cancer related to radiation oncologist-computer interaction: a 'Big Brother' evaluation.

BACKGROUND AND PURPOSE: To evaluate the process of target volume delineation in lung cancer for optimization of imaging, delineation protocol and delineation software. PATIENTS AND METHODS: Eleven radiation oncologists (observers) from five different institutions delineated the Gross Tumor Volume (GTV) including positive lymph nodes of 22 lung cancer patients (stages I-IIIB) on CT only. All radiation oncologist-computer interactions were recorded with a tool called 'Big Brother'. For each radiation oncologist and patient the following issues were analyzed: delineation time, number of delineated points and corrections, zoom levels, level and window (L/W) settings, CT slice changes, use of side windows (coronal and sagittal) and software button use. RESULTS: The mean delineation time per GTV was 16 min (SD 10 min). The mean delineation time for lymph node positive patients was on average 3 min larger (P = 0.02) than for lymph node negative patients. Many corrections (55%) were due to L/W change (e.g. delineating in mediastinum L/W and then correcting in lung L/W). For the lymph node region, a relatively large number of corrections was found (3.7 corr/cm2), indicating that it was difficult to delineate lymph nodes. For the tumor-atelectasis region, a relative small number of corrections was found (1.0 corr/cm2), indicating that including or excluding atelectasis into the GTV was a clinical decision. Inappropriate use of L/W settings was frequently found (e.g. 46% of all delineated points in the tumor-lung region were delineated in mediastinum L/W settings). Despite a large observer variation in cranial and caudal direction of 0.72 cm (1 SD), the coronal and sagittal side windows were not used in 45 and 60% of the cases, respectively. For the more difficult cases, observer variation was smaller when the coronal and sagittal side windows were used. CONCLUSIONS: With the 'Big Brother' tool a method was developed to trace the delineation process. The differences between observers concerning the delineation style were large. This study led to recommendations on how to improve delineation accuracy by adapting the delineation protocol (guidelines for L/W use) and delineation software (double window with lung and mediastinum L/W settings at the same time, enforced use of coronal and sagittal views) and including FDG-PET information (lymph nodes and atelectasis).

Aged↗

PubMed2005

Reduction of observer variation using matched CT-PET for lung cancer delineation: a three-dimensional analysis.

PURPOSE: Target delineation using only CT information introduces large geometric uncertainties in radiotherapy for lung cancer. Therefore, a reduction of the delineation variability is needed. The impact of including a matched CT scan with 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) and adaptation of the delineation protocol and software on target delineation in lung cancer was evaluated in an extensive multi-institutional setting and compared with the delineations using CT only. METHODS AND MATERIALS: The study was separated into two phases. For the first phase, 11 radiation oncologists (observers) delineated the gross tumor volume (GTV), including the pathologic lymph nodes of 22 lung cancer patients (Stages I-IIIB) on CT only. For the second phase (1 year later), the same radiation oncologists delineated the GTV of the same 22 patients on a matched CT-FDG-PET scan using an adapted delineation protocol and software (according to the results of the first phase). All delineated volumes were analyzed in detail. The observer variation was computed in three dimensions by measuring the distance between the median GTV surface and each individual GTV. The variation in distance of all radiation oncologists was expressed as a standard deviation. The observer variation was evaluated for anatomic regions (lung, mediastinum, chest wall, atelectasis, and lymph nodes) and interpretation regions (agreement and disagreement; i.e., >80% vs. <80% of the radiation oncologists delineated the same structure, respectively). All radiation oncologist-computer interactions were recorded and analyzed with a tool called "Big Brother." RESULTS: The overall three-dimensional observer variation was reduced from 1.0 cm (SD) for the first phase (CT only) to 0.4 cm (SD) for the second phase (matched CT-FDG-PET). The largest reduction in the observer variation was seen in the atelectasis region (SD 1.9 cm reduced to 0.5 cm). The mean ratio between the common and encompassing volume was 0.17 and 0.29 for the first and second phases, respectively. For the first phase, the common volume was 0 in 4 patients (i.e., no common point for all GTVs). In the second phase, the common volume was always >0. For all anatomic regions, the interpretation differences among the radiation oncologists were reduced. The amount of disagreement was 45% and 18% for the first and second phase, respectively. Furthermore, the mean delineation time (12 vs. 16 min, p<0.001) and mean number of corrections (25 vs. 39, p<0.001) were reduced in the second phase compared with the first phase. CONCLUSION: For high-precision radiotherapy, the delineation of lung target volumes using only CT introduces too great a variability among radiation oncologists. Implementing matched CT-FDG-PET and adapted delineation protocol and software reduced observer variation in lung cancer delineation significantly with respect to CT only. However, the remaining observer variation was still large compared with other geometric uncertainties (setup variation and organ motion).

Aged↗

PubMed2005

Quantification of shape variation of prostate and seminal vesicles during external beam radiotherapy.

PURPOSE: The prostate is known to translate and rotate under influence of rectal filling changes and many studies have addressed the magnitude of these motions. However, prostate shape variations also have been reported. For image-guided radiotherapy, it is essential to know the relative magnitude of translations, rotations, and shape variation so that the most appropriate correction strategy can be chosen. However, no quantitative analysis of shape variation has been performed. It is, therefore, the purpose of this article to develop a method to determine shape variation of complex organs and apply it to determine shape variation during external beam radiotherapy of a GTV (gross tumor volume) consisting of prostate and seminal vesicles. METHODS AND MATERIALS: For this study, the data of 19 patients with prostate cancer were used. Each patient received a planning computed tomography (CT) scan and 8-12 (11 on average) repeat CT scans that were made during the course of conformal radiotherapy. One observer delineated the GTV in all scans, and volume variations were measured. After matching the GTVs for each patient for translation and rotation, a coverage probability matrix was constructed and the 50% isosurface was taken to determine the average GTV surface. Perpendicular distances between the average GTV and the individual GTVs were calculated for each point of the average GTV, and their variation was expressed in terms of local standard deviation (SD). The local SDs of the shape variation of all 19 patients were mapped onto a reference case by matching and morphing of the individual average GTVs. Repeated delineation of the GTV was done for 6 patients to determine intraobserver variation. Finally, the measured shape variation was corrected for intraobserver variation to estimate the "real" shape variation. RESULTS: No significant variations in GTV volume were observed. The measured shape variation (including delineation variation) was largest at the tip of the vesicles (SD = 2.0 mm), smallest at the left and right side of the prostate (SD = 1.0 mm), and average elsewhere (SD = 1.5 mm). At the left, right, and cranial sides of the prostate, the intraobserver variation was of the same order of magnitude as the measured shape variation; elsewhere it was smaller. However, the accuracy of the estimated SD for intraobserver variation was about half of the accuracy of the estimated SD for the measured shape variation, giving an overall uncertainty of maximum 0.6 mm SD in the estimate of the "real" shape variation. The "real" shape variation was small at the left, right, and cranial side of the prostate (SD <0.5 mm) and between 0.5 mm and 1.6 mm elsewhere. CONCLUSIONS: We developed a method to quantify shape variation of organs with a complex shape and applied it to a GTV consisting of prostate and seminal vesicles. Deformation of prostate and seminal vesicles during the course of radiotherapy is small (relative to organ motion). Therefore, it is a valid approximation in image-guided radiotherapy of prostate cancer, in first order, to correct only for setup errors and organ motion. Prostate and seminal vesicles deformation can be considered as a second-order effect.

Humans↗

PubMed2005

Strategies to reduce the systematic error due to tumor and rectum motion in radiotherapy of prostate cancer.

BACKGROUND AND PURPOSE: The goal of this work is to develop and evaluate strategies to reduce the uncertainty in the prostate position and rectum shape that arises in the preparation stage of the radiation treatment of prostate cancer. PATIENTS AND METHODS: Nineteen prostate cancer patients, who were treated with 3-dimensional conformal radiotherapy, received each a planning CT scan and 8-13 repeat CT scans during the treatment period. We quantified prostate motion relative to the pelvic bone by first matching the repeat CT scans on the planning CT scan using the bony anatomy. Subsequently, each contoured prostate, including seminal vesicles, was matched on the prostate in the planning CT scan to obtain the translations and rotations. The variation in prostate position was determined in terms of the systematic, random and group mean error. We tested the performance of two correction strategies to reduce the systematic error due to prostate motion. The first strategy, the pre-treatment strategy, used only the initial rectum volume in the planning CT scan to adjust the angle of the prostate with respect to the left-right (LR) axis and the shape and position of the rectum. The second strategy, the adaptive strategy, used the data of repeat CT scans to improve the estimate of the prostate position and rectum shape during the treatment. RESULTS: The largest component of prostate motion was a rotation around the LR axis. The systematic error (1 SD) was 5.1 degrees and the random error was 3.6 degrees (1 SD). The average LR-axis rotation between the planning and the repeat CT scans correlated significantly with the rectum volume in the planning CT scan (r=0.86, P<0.0001). Correction of the rotational position on the basis of the planning rectum volume alone reduced the systematic error by 28%. A correction, based on the data of the planning CT scan and 4 repeat CT scans reduced the systematic error over the complete treatment period by a factor of 2. When the correction was carried out later in the treatment (based on the data of more scans) the overall reduction was less. For the rectum, the first strategy performed best at the upper anterior side, where a reduction of the anterior-posterior displacement of 30% could be achieved. The systematic error could be reduced by 43% for the whole rectum by using the data of 4 repeat CT scans and the planning CT scan. CONCLUSIONS: Both the pre-treatment as well as the adaptive correction strategy reduced the systematic error in the prostate position and rectum position and shape. A smaller systematic error makes it possible to safely reduce the margin around the clinical tumor volume, so that normal tissues can be spared or the prescription dose can be escalated.

Humans↗

PubMed2005

The sensitivity of dose distributions for organ motion and set-up uncertainties in prostate IMRT.

BACKGROUND AND PURPOSE: To determine the effect of organ motion and set-up uncertainties on IMRT dose distributions for prostate. METHODS: For five patients, IMRT techniques were designed to irradiate the CTV (prostate plus seminal vesicles). Technique I delivered 78 Gy to PTV1 (CTV+10 mm margin). Technique II delivered 68 Gy to PTV1, and a 10 Gy boost to PTV2 (CTV+an anisotropic margin of 0 to 5 mm). Technique III delivered 68 Gy to PTV1 and simultaneously 78 Gy to PTV2. Uncertainties were simulated using population statistics of organ motion and set-up accuracy. The average TCP (TCPpop) of the CTV and average NTCP (NTCPpop) of the rectal wall were calculated. RESULTS: The planning TCP was a good predictor for TCPpop for Techniques I and II. Technique III was sensitive for geometrical uncertainties, reducing TCPpop by 0.8 to 2.4% compared to planning. NTCPpop was reduced for Technique III by a factor 2.6 compared to Technique I. For all plans, the planning NTCP was strongly correlated with NTCPpop. CONCLUSIONS: Dose distributions created with Techniques I and II are insensitive for geometrical uncertainties, while Technique III resulted in a reduction of TCPpop. This reduction can be compensated by a small dose escalation, while still resulting in an NTCPpop of the rectal wall that is lower or comparable to Technique I.

Humans↗

PubMed2005

Target definition in prostate, head, and neck.

Target definition is a major source of errors in both prostate and head and neck external-beam radiation treatment. Delineation errors remain constant during the course of radiation and therefore have a large impact on the dose to the tumor. Major sources of delineation variation are visibility of the target including its extensions, disagreement on the target extension, and interpretation or lack of delineation protocols. The visibility of the target can be greatly improved with the use of multimodality imaging. Both in the head and neck and the prostate, computed tomography (CT)-magnetic resonance imaging coregistration decreases the target volume and its variability. CT-positron emission tomography delineation is promising for delineation in head and neck cancer. Despite the better visibility, a different interpretation of the target extension remains a major source of error. The use of coregistration of CT with a second modality, together with improved guidelines for delineation and an online anatomical atlas, increases agreement between observers in prostate, lung, and nasopharynx tumors. Delineation errors should not be treated differently from other geometrical errors. Similar margin recipes for the correction of setup errors and organ motion should be adapted to incorporate the effect of delineation errors. A calculation of a 3-dimensional clinical target volume-planning target volume margin incorporating delineation errors for the head and neck is around 6.1 to 9.7 mm. Given the good local control of IMRT with smaller margins and smaller pathological specimens, it is likely that the delineated CTV frequently overestimates the actual volume.

Head and Neck Neoplasms↗

PubMed2005

Respiratory correlated cone beam CT.

A cone beam computed tomography (CBCT) scanner integrated with a linear accelerator is a powerful tool for image guided radiotherapy. Respiratory motion, however, induces artifacts in CBCT, while the respiratory correlated procedures, developed to reduce motion artifacts in axial and helical CT are not suitable for such CBCT scanners. We have developed an alternative respiratory correlated procedure for CBCT and evaluated its performance. This respiratory correlated CBCT procedure consists of retrospective sorting in projection space, yielding subsets of projections that each corresponds to a certain breathing phase. Subsequently, these subsets are reconstructed into a four-dimensional (4D) CBCT dataset. The breathing signal, required for respiratory correlation, was directly extracted from the 2D projection data, removing the need for an additional respiratory monitor system. Due to the reduced number of projections per phase, the contrast-to-noise ratio in a 4D scan reduced by a factor 2.6-3.7 compared to a 3D scan based on all projections. Projection data of a spherical phantom moving with a 3 and 5 s period with and without simulated breathing irregularities were acquired and reconstructed into 3D and 4D CBCT datasets. The positional deviations of the phantoms center of gravity between 4D CBCT and fluoroscopy were small: 0.13 +/- 0.09 mm for the regular motion and 0.39 +/- 0.24 mm for the irregular motion. Motion artifacts, clearly present in the 3D CBCT datasets, were substantially reduced in the 4D datasets, even in the presence of breathing irregularities, such that the shape of the moving structures could be identified more accurately. Moreover, the 4D CBCT dataset provided information on the 3D trajectory of the moving structures, absent in the 3D data. Considerable breathing irregularities, however, substantially reduces the image quality. Data presented for three different lung cancer patients were in line with the results obtained from the phantom study. In conclusion, we have successfully implemented a respiratory correlated CBCT procedure yielding a 4D dataset. With respiratory correlated CBCT on a linear accelerator, the mean position, trajectory, and shape of a moving tumor can be verified just prior to treatment. Such verification reduces respiration induced geometrical uncertainties, enabling safe delivery of 4D radiotherapy such as gated radiotherapy with small margins.

Algorithms↗

PubMed2005

The applicability of simultaneous TRUS-CT imaging for the evaluation of prostate seed implants.

To study dose-effect relations of prostate implants with I-125 seeds, accurate knowledge of the dose distribution in the prostate is essential. Commonly, a post-implant computed tomography (CT) scan is used to determine the geometry of the implant and to delineate the contours of the prostate. However, the delineation of the prostate on CT slices is very cumbersome due to poor contrast between the prostate capsule and surrounding tissues. Transrectal Ultrasound (TRUS) on the other hand offers good visualization of the prostate but poor visualization of the implanted seeds. The purpose of this study was to investigate the applicability of combining CT with 3D TRUS by means of image fusion. The advantage of fused TRUS-CT imaging is that both prostate contours and implanted seeds will be well visible. In our clinic, post-implant imaging was realized by simultaneously acquiring a TRUS scan and a CT scan. The TRUS transducer was inserted while the patient was on the CT couch and the CT scan was made directly after the TRUS scan, with the probe still in situ. With the TRUS transducer being visible on both TRUS and CT images, the geometrical relationship between both image sets could be defined by registration on the transducer. Having proven the applicability of simultaneous imaging, the accuracy of this registration method was investigated by additional registration on visible seeds, after preregistration on the transducer. In 4 out of 23 investigated cases an automatic grey value registration on seeds failed for each of the investigated cost functions, and in 2 cases for both cost functions, due to poor visibility of the seeds on the TRUS scan. The average deviations of the seed registration with respect to the transducer registration were negligible. However, in a few individual cases the deviations were significant and probably due to movement of the patient between TRUS and CT scan. In case of a registration on the transducer it is important to avoid patient movement in-between the TRUS and CT scan and to keep the time in-between the scans as short as possible. It can be concluded that fusion of a CT scan and a simultaneously made TRUS scan by means of a three-dimensional (3D) transducer is feasible and accurate when performing a registration on the transducer, if necessary, fine-tuned by a registration on seeds. These fused images are likely to be of great value for post-implant dose distribution evaluations.

Algorithms↗

PubMed2005

Reproducibility of the bladder shape and bladder shape changes during filling.

The feasibility of high precision radiotherapy to the bladder region is limited by bladder motion and volume changes. In the near future, we plan to begin treatment delivery of bladder cancer patients with the acquisition of a cone beam CT image on which the complete bladder will be semi-automatically localized. Subsequently, a bladder shape model that was developed in a previous study will be used for bladder localization and for the prediction of shape changes in the time interval between acquisition and beam delivery. For such predictions, knowledge about urinary inflow rate is required. Therefore, a series of MR images was acquired over 1 h with time intervals of 10 min for 18 healthy volunteers. To gain insight in the reproducibility of the bladder shape over longer periods of time, two additional MRI series were recorded for 10 of the volunteers. To a good approximation, the bladder volume increased linearly in time for all individuals. Despite receiving drinking instructions, we found a large variation in the inflow rate between individuals, ranging from 2.1 to 15 cc/min (mean value: 9 +/- 3 cc/min). In contrast, the intravolunteer variation was much smaller, with a mean standard deviation (SD) of 0.4 cc/min. The inflow rate was linearly correlated with age (negative slope). To study the reproducibility of the bladder shape, we compared bladder shapes of equal volume. For all individuals, the caudal part of the bladder was the most reproducible (variations<0.3 cm in all cases). The cranial and posterior parts of the bladder was much less reproducible, with local SD values up to approximately 1.2 cm for bladders with a volume of 200 cc. These large long-term variations were primarily caused by changes in position and filling of the small bowel and rectum. However, for short time intervals, the rectal filling was (nearly) constant. Therefore, the reproducibility of urinary inflow, combined with the previously developed shape model gives us an excellent tool to predict short-term shape changes. We intend to use this tool for further improvement of image-guided radiotherapy for bladder cancer patients.

Adult↗

PubMed2004

Impact of knee support and shape of tabletop on rectum and prostate position.

PURPOSE: To evaluate the impact of different tabletops with or without a knee support on the position of the rectum, prostate, and bulb of the penis; and to evaluate the effect of these patient-positioning devices on treatment planning. METHODS AND MATERIALS: For 10 male volunteers, five MRI scans were made in four different positions: on a flat tabletop with knee support, on a flat tabletop without knee support, on a rounded tabletop with knee support, and on a rounded tabletop without knee support. The fifth scan was in the same position as the first. With image registration, the position differences of the rectum, prostate, and bulb of the penis were measured at several points in a sagittal plane through the central axis of the prostate. A planning target volume was generated from the delineated prostates with a margin of 10 mm in three dimensions. A three-field treatment plan with a prescribed dose of 78 Gy to the International Commission on Radiation Units and Measurements point was automatically generated from each planning target volume. Dose-volume histograms were calculated for all rectal walls. RESULTS: The shape of the tabletop did not affect the rectum and prostate position. Addition of a knee support shifted the anterior and posterior rectal walls dorsally. For the anterior rectal wall, the maximum dorsal shift was 9.9 mm (standard error of the mean [SEM] 1.7 mm) at the top of the prostate. For the posterior rectal wall, the maximum dorsal shift was 10.2 mm (SEM 1.5 mm) at the middle of the prostate. Therefore, the rectal filling was pushed caudally when a knee support was added. The knee support caused a rotation of the prostate around the left-right axis at the apex (i.e., a dorsal rotation) by 5.6 degrees (SEM 0.8 degrees ) and shifts in the caudal and dorsal directions of 2.6 mm (SEM 0.4 cm) and 1.4 mm (SEM 0.6 mm), respectively. The position of the bulb of the penis was not influenced by the use of a knee support or rounded tabletop. The volume of the rectal wall receiving the same dose range (e.g., 40-75 Gy) was reduced by 3.5% (SEM 0.9%) when a knee support was added. No significant differences were observed between the first and fifth scan (flat tabletop with knee support) for all measured points, thereby excluding time trends. CONCLUSIONS: The rectum and prostate were significantly shifted dorsally by the use of a knee support. The rectum shifted more than the prostate, resulting in a dose benefit compared with irradiation without knee support. The shape of the tabletop did not influence the rectum or prostate position.

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