Biomedical subjects

R D Zwicker

Publications and source records attributed to R D Zwicker.

At least 19 recordsLinked to original sources

PubMed2001

Intensity-modulated stereotactic radiosurgery using dynamic micro-multileaf collimation.

PURPOSE: The implementation of dynamic leaf motion on a micro-multileaf collimator system provides the capability for intensity-modulated stereotactic radiosurgery (IMSRS), and the consequent potential for improved dose distributions for irregularly shaped tumor volumes adjacent to critical organs. This study explores the use of IMSRS to provide improved tumor coverage and normal tissue sparing for small cranial tumors relative to plans based on multiple fixed uniform-intensity beams or traditional circular collimator arc-based stereotactic techniques. METHODS AND MATERIALS: Four patient cases involving small brain lesions are presented and analyzed. The cases were chosen to include a representative selection of target shapes, number of targets, and adjacent critical areas. Patient plans generated for these comparisons include standard arcs with multiple circular collimators, and fixed noncoplanar static fields with uniform-intensity beams and IMSRS. Parameters used for evaluation of the plans include the percentage of irradiated volume to tumor volume (PITV), normal tissue dose-volume histograms, and dose-homogeneity ratios. All IMSRS plans were computed using previously established IMRT techniques adapted for use with the BrainLAB M3 micro-multileaf collimator. The algorithms comprising the IMRT system for optimization of intensity distributions and conversion into leaf trajectories of the BrainLab M3 were developed at our institution. The ADAC Pinnacle(3) radiation treatment-planning system was used for dose calculations and for input of contours for target volumes and normal critical structures. RESULTS: For all cases, the IMSRS plans showed a high degree of conformity of the dose distribution with the target shape. The IMSRS plans provided either (1) a smaller volume of normal tissue irradiated to significant dose levels, generally taken as doses greater than 50% of the prescription, or (2) a lower dose to an important adjacent critical organ. The reduction in volume of normal tissue irradiated in the IMSRS plans ranged from 10% to 50% relative to the other arc and uniform fixed-field plans. CONCLUSION: The case studies presented for IMSRS demonstrate significant dosimetric improvements for small, irregularly shaped lesions of the brain when compared to treatments using multiple static fields or standard SRS arc techniques with circular collimators. For all cases, the IMSRS plan yielded a smaller volume of normal tissue irradiated, and/or a reduction in the volume of an adjacent critical organ (i.e., brainstem) irradiated to significant dose levels.

Brain Neoplasms↗

PubMed2001

Biologic treatment planning for high-dose-rate brachytherapy.

PURPOSE: Interstitial brachytherapy treatment plans are conventionally optimized with respect to total target dose and dose homogeneity, which does not account for the biologic effects of dose rate. In an HDR implant, with a stepping source, the dose rate dramatically changes during the course of treatment, depending on location, as the source moves from dwell position to dwell position. These widely varying dose rates, together with the related sequencing of the dwell positions, may impart different biologic effects at points receiving the same total dose. This study applies radiobiologic principles to account for the potential biologic impact of dose delivery at varying dose rates within an HDR implant. METHODS AND MATERIALS: The model under study uses a generalized version of the linear-quadratic (LQ) cell kill formula to calculate the surviving fraction of cells subjected to HDR irradiation. Using a planar interstitial HDR implant with the dwell times optimized to produce a homogeneous dose distribution along a reference plane parallel to the implant plane, surviving fractions were compared at selected reference points subjected to the same total dose. Biologic effect homogeneity was compared to dose homogeneity by plotting the effects at the reference points. The effects were examined with LQ parameters alpha, beta, and sublethal repair time T(1) varied over a range typical of human cells. RESULTS: In a region in which dose is relatively uniform, surviving fraction for some values of the model parameters are found to vary by as much as an order of magnitude due to differences in the HDR irradiation profiles at different dose points. This effect is more pronounced for shorter repair times and smaller alpha/beta ratios, and increases with increasing total irradiation time. CONCLUSION: Conventional HDR treatment planning currently considers dose distribution as the primary indicator of clinical effect. Our results demonstrate that plans optimized to maximize homogeneity within a target volume may not reflect the effect of the sequential nature of HDR dose delivery on cell kill. Biologic effect modeling may improve our understanding and ability to predict the adverse effects of our treatment, such as fat necrosis and fibrosis. Accounting for irradiation history and repair kinetics in the evaluation of HDR brachytherapy plans may add an important new dimension to our planning capabilities.

Algorithms↗

PubMed2001

Brachytherapy in the treatment of breast cancer.

Whole-breast external-beam radiation therapy (EBRT) involves a 6-week course of fractionated treatments. In contrast, brachytherapy can be completed in a 4- to 5-day treatment course. Because of this shortened time frame, there has been interest in breast brachytherapy as a sole modality after lumpectomy. The American Brachytherapy Society (ABS) has issued guidelines specifically for the use of brachytherapy in breast carcinoma. In these guidelines, the ABS addresses important areas of controversy related to the indications, execution, and evaluation of breast implants when utilized in the following settings: as the sole treatment modality following lumpectomy, as an alternative to a 6-week course of EBRT following lumpectomy, as a boost following whole-breast irradiation, and for the treatment of local recurrences following breast-conservation treatment. The ABS recommends a precise definition and meticulous delineation of the clinical target volume. In addition, the Society recommends the routine use of dose-volume histograms and a dose-homogeneity index as tools to ensure reproducible brachytherapy and to allow interinstitutional comparisons. Brachytherapy as a sole modality is currently considered investigational and should be performed in the context of a controlled clinical trial. Practitioners and cooperative groups are encouraged to use these guidelines to formulate treatment and dose-reporting policies, but responsibility for medical decisions ultimately rests with the treating radiation oncologist.

Brachytherapy↗

PubMed2000

Interstitial high-dose-rate brachytherapy boost: the feasibility and cosmetic outcome of a fractionated outpatient delivery scheme.

PURPOSE: To evaluate the feasibility, potential toxicity, and cosmetic outcome of fractionated interstitial high dose rate (HDR) brachytherapy boost for the management of patients with breast cancer at increased risk for local recurrence. METHODS AND MATERIALS: From 1994 to 1996, 18 women with early stage breast cancer underwent conventionally fractionated whole breast radiotherapy (50-50.4 Gy) followed by interstitial HDR brachytherapy boost. All were considered to be at high risk for local failure. Seventeen had pathologically confirmed final surgical margins of less than 2 mm or focally positive. Brachytherapy catheter placement and treatment delivery were conducted on an outpatient basis. Preplanning was used to determine optimal catheter positions to enhance dose homogeneity of dose delivery. The total HDR boost dose was 15 Gy delivered in 6 fractions of 2.5 Gy over 3 days. Local control, survival, late toxicities (LENT-SOMA), and cosmetic outcome were recorded in follow-up. In addition, factors potentially influencing cosmesis were analyzed by logistic regression analysis. RESULTS: The minimum follow-up is 40 months with a median 50 months. Sixteen patients were alive without disease at last follow-up. There have been no in-breast failures observed. One patient died with brain metastases, and another died of unrelated causes without evidence of disease. Grade 1-2 late toxicities included 39% with hyperpigmentation, 56% with detectable fibrosis, 28% with occasional discomfort, and 11% with visible telangiectasias. Grade 3 toxicity was reported in one patient as persistent discomfort. Sixty-seven percent of patients were considered to have experienced good/excellent cosmetic outcomes. Factors with a direct relationship to adverse cosmetic outcome were extent of surgical defect (p = 0.00001), primary excision volume (p = 0.017), and total excision volume (p = 0.015). CONCLUSIONS: For high risk patients who may benefit from increased doses, interstitial HDR brachytherapy provides a convenient outpatient method for boosting the lumpectomy cavity following conventional whole breast irradiation without overdosing normal tissues. The fractionation scheme of 15 Gy in 6 fractions over 3 days is well tolerated. The volume of tissue removed from the breast at lumpectomy appears to dominate cosmetic outcome in this group of patients.

Adult↗

PubMed2000

Internal mammary node coverage: an investigation of presently accepted techniques.

PURPOSE: Recent publications have generated a renewed interest in regional nodal treatment to include the ipsilateral supraclavicular and internal mammary nodes (IMN). The purpose of this study is to evaluate three presently accepted treatment techniques for coverage of the intact breast and ipsilateral lymph node regions and to construct recommendations regarding the utilization of these techniques. METHODS AND MATERIALS: Anatomic data were obtained from five randomly selected patients with computerized tomography (CT) in treatment position. Three patients presented with cancer of the left breast and two with cancer of the right. Using the Pinnacle 3-D planning system, normal tissue volumes of breast, ipsilateral lung, heart, sternum, and the IMN target were delineated for each patient. Three accepted techniques used to treat ipsilateral breast, internal mammary and supraclavicular nodes (extended tangents, 5-field, partly wide tangents) were configured and compared to a supraclavicular field matched to standard tangential fields. A dosage of 50 Gy in 25 fractions was prescribed to the target volume. Dose-volume histograms (DVH) were generated and analyzed with regard to target volume coverage and lung/heart volumes treated. RESULTS: All of the treatment techniques covering IMN include at least 10% more lung and heart volume than that covered by standard tangential fields. The relative lung and heart volumes treated with each technique were consistent from patient to patient. The 5-field technique clearly treats the largest volume of normal tissue; however, most of this volume receives less than 50% of the dose prescribed. The percent of heart and ipsilateral lung treated to 20 Gy, 30 Gy, and 40 Gy have been calculated and compared. Due to the increase in chest wall thickness and depth of IMN superiorly, complete coverage was not achieved with any technique if the IMN target extended superiorly into the medial supraclavicular field where dose fall-off resulted in a significant underdosing at depth. For the same anatomic reasons, the 5-field technique underdosed 10-15% of the IMN target volume in 4 of the 5 cases. This technique also yielded a greater dose heterogeneity, which was not seen with the other techniques evaluated and correlated with the change of anterior chest wall thickness. CONCLUSIONS: Anatomic variation in chest wall thickness and IMN depth strongly suggests the routine use of multislice CT planning to ensure complete coverage of the target volume and optimal sparing of normal tissue. All of the techniques can be constructed to look acceptable at central axis. To cover the superior most aspect of the IMN chain either high tangential fields, a supraclavicular field photon beam of energy >6 MV, or an AP/PA supraclavicular setup should be considered. The 5-field technique has the most difficulty in compensating for the increased depth of the IMN in the superior aspect of the tangent fields with up to +/-40% variation of the dose noted in isolated areas within the target volume. Based on our evaluation, the partly wide tangent technique offers many advantages. It provides optimal coverage of the target volume, reduces coverage of normal tissue volumes to an acceptable level, and is easily reproducible with a high degree of dose homogeneity throughout the target.

Breast Neoplasms↗

PubMed2000

Hypofractionated stereotactic radiotherapy as an alternative to radiosurgery for the treatment of patients with brain metastases.

PURPOSE: Modeling studies have demonstrated a potential biologic advantage of fractionated stereotactic radiotherapy for malignant brain tumors as compared to radiosurgery (SRS), even when only a few fractions are utilized. We prospectively evaluated the feasibility, toxicity, efficacy and cost of hypofractionated stereotactic radiotherapy (HSRT) in the treatment of selected radiosurgery-eligible patients with brain metastases. METHODS AND MATERIALS: Patients with a limited number of brain metastases not involving the brainstem or optic chiasm underwent linac-based HSRT delivered in 3 fractions using a relocatable stereotactic frame. Depth-helmet and reference point measurements were recorded to address treatment accuracy. All patients underwent whole brain radiotherapy to a dose of 30 Gy. Toxicity, response, and survival duration were recorded for each patient. Prognostic factors were assessed by Cox regression analysis. Cost comparisons with a cohort of SRS treated patients were performed. RESULTS: Thirty-two patients with 57 brain metastases were treated with HSRT. Twenty-three and 9 patients underwent HSRT for upfront and salvage treatment, respectively. The median dose delivered was 27 Gy, given in 3 fractions of 9 Gy. From 3328 depth-helmet measurements, the absolute median setup deviation in AP, lateral, and vertical orientations was approximately 1.0 mm. No significant acute toxicity was seen. Late toxicities included seizures in four patients, and radionecrosis in two patients. The median survival duration from treatment was 12 months. KPS (p = 0.039) and RTOG-RPA class (p = 0.039) were identified as significant prognostic factors for survival. HSRT was $4119 less costly than SRS. CONCLUSION: HSRT, as delivered in this study, is more comfortable for patients and less costly than SRS in the treatment of selected patients with brain metastases. Proper dose selection and radiobiologic/toxicity trade-offs with SRS await further study.

Adolescent↗

PubMed2000

Gynecologic brachytherapy: digital fluoroscopy for placement verification and treatment planning.

The authors evaluated the feasibility of using digital fluoroscopic images for device placement verification and dosimetric planning for gynecologic brachytherapy. Adequate images were obtained rapidly, and the limited pincushion distortion on digital fluoroscopic images produced negligible variations in brachytherapy dose calculations compared with those calculated with standard radiographs. Intraoperative digital fluoroscopy can facilitate both placement verification and dosimetric planning for gynecologic brachytherapy.

Brachytherapy↗

PubMed1999

Optimization of planar high-dose-rate implants.

PURPOSE: Brachytherapy has long been used to deliver localized radiation to the breast and other cancer sites. For interstitial implants, proper source positioning is critical in obtaining satisfactory dose distributions. The present work examines techniques for optimizing source guide placement in high-dose-rate (HDR) biplanar implants, and examines the effects of suboptimal catheter placement. METHODS AND MATERIALS: Control of individual dwell times in HDR implants allows a high degree of dose uniformity in planes parallel to the implant planes. Biplanar HDR implants can be considered optimized when the dose at the implant center is equal to the dose at the symmetric target boundaries. It is shown that this optimal dose uniformity is achieved when the interplanar separation is related to the target thickness T through the direct proportionality, s = T/square root2. To quantify the significance of source positioning, the average dose and a related quantity, equivalent uniform dose (EUD), were calculated inside the treatment volume for two conditions of suboptimal catheter geometry. In one case, the interplanar spacing was varied from 1 cm up to the target thickness T, while a second study examined the effects of off-center placement of the implant planes. RESULTS: Both the average dose and EUD were minimized when the interplanar spacing satisfied the relationship s = T/square root2. EUD, however, was significantly smaller than the average dose, indicating a reduced relative cell killing in the high dose regions near the dwell points. It was also noted that in contrast to the average dose, the EUD is a relatively weak function of catheter misplacement, suggesting that the biological consequences of suboptimal implant geometry may be less significant than is indicated by the increase in average dose. CONCLUSION: A concise formula can be used to determine the interplanar separation needed for optimal dose uniformity in Manchester-type implants. Deviations from optimal source geometry result in an increase in the average dose inside the treatment volume, but the weaker dependence of the EUD suggests that the surviving fraction of cells may not be not strongly affected by suboptimal source geometry.

Algorithms↗

PubMed1999

Cosmetic outcome in patients receiving an interstitial implant as part of breast-conservation therapy.

PURPOSE: To study factors related to breast cosmetic outcome in patients treated with an interstitial implant as part of breast-conservation therapy. MATERIALS AND METHODS: One hundred fifty-six patients with stage I or II breast carcinoma who received 50 Gy of external-beam irradiation followed by a 20-Gy interstitial boost were examined. The dose homogeneity index (DHI) was calculated for each evaluable implant and was examined in light of other patient-, treatment-, and tumor-related variables previously demonstrated to affect cosmesis. RESULTS: Of the variables examined, both the DHI (P = .021) and the total excision volume (P = .019) were significantly related to cosmetic outcome (excellent vs less than excellent) in a univariate model. In the multivariate analysis, only the total excision volume remained significant (P = .032). The mean total excision volume +/- SD in patients with excellent cosmetic outcome (81.8 cm3 +/- 84.0) was significantly less than that in patients with less than excellent cosmetic outcome (120 cm3 +/- 84). The probability of excellent cosmetic outcome linearly increased with an increase in DHI. The mean DHI was 0.74 +/- 0.12 for the cases with excellent cosmetic outcome and 0.68 +/- 0.10 for those with less than excellent cosmetic outcome. CONCLUSION: To achieve optimal cosmesis, DHI should be maximized. The volume of tissue removed, however, remains the most significant determinant.

Adult↗

PubMed1998

A comparison of three stereotactic radiotherapy techniques; ARCS vs. noncoplanar fixed fields vs. intensity modulation.

PURPOSE: Linac arc based stereotactic radiotherapy is being used with increasing frequency to treat brain tumors. This approach can be used for single or fractionated treatments, and is typically carried out with circular collimators which are optimal for small, spherical targets. Treatment planning using fixed noncoplanar beams or intensity-modulated beams may enhance the ability to conform to irregularly shaped and/or large tumors, especially when combined with stereotactic localization. We compare the dose conformity and normal brain dose characteristics of three stereotactic techniques for various nonspherical target shapes. METHODS AND MATERIALS: Three intracranial test targets were constructed using a 3D treatment planning system after a patient underwent CT simulation. The targets included an ellipsoid with major axis dimensions of 4.0, 2.0, and 2.0 cm, a hemisphere with a diameter of 4.0 cm, and an irregularly shaped patient tumor with a maximum dimension of 5.3 cm. The following stereotactic techniques were compared for each target: a) 5 arcs as used in traditional linac radiosurgery/radiotherapy (noncoplanar arcs [ARCS]), b) 6 fixed noncoplanar custom blocked fields (3D), c) intensity modulation using 6 noncoplanar beams and a mini-multileaf collimator (intensity-modulated radiation therapy [IMRTI). Dose volume histograms were performed for each target/technique combination. RESULTS: For the ellipsoid, dose conformity is similar for all three techniques and normal brain isodose distributions are more favorable with the ARCS plan. For the hemisphere and irregular tumor targets, dose conformity and high/low isodose normal brain volumes are more favorable with the IMRT technique. CONCLUSIONS: For the targets described above, the intensity-modulated technique results in improved dose conformity and decreased dose to nontarget brain in high and low isodose regions as compared to the standard noncoplanar arc technique or noncoplanar fixed fields for the hemisphere and tumor targets. Intensity-modulated treatment delivery may allow for an increase in the therapeutic ratio for treating stereotactically defined large and/or irregularly shaped intracranial targets.

Brain Neoplasms↗

PubMed1998

Accelerated radiotherapy regimen for malignant gliomas using stereotactic concomitant boosts for dose escalation.

The purpose of this pilot study was to determine the feasibility and toxicities of an accelerated treatment program by using a concomitant stereotactic radiotherapy boost given weekly during a course of standard external-beam irradiation (EBXRT) in patients with malignant gliomas. Twelve patients underwent biopsy or subtotal resection of a malignant glioma and were enrolled on the protocol, which delivered 44 Gy-EBXRT and a 12-Gy stereotactic radiotherapy boost given on 3 consecutive weeks of treatment for a total dose of 80 Gy over 33 days. Three patients with anaplastic astrocytoma and nine patients with glioblastoma multiforme had median survival times of 33 months and 16 months, respectively. All of the tumor recurrences were within or were closely adjacent to the region of high-dose irradiation. None of the patients required a treatment break, and there were no acute complications. Two patients developed seizures in the follow-up period, and four patients were diagnosed with radionecrosis at the time of the second operation. The treatment program was found to be feasible and was well tolerated, and it resulted in a rate of late complications similar to those of radiosurgery or interstitial brachytherapy.

Adult↗

PubMed1997

The biological effectiveness of intermittent irradiation as a function of overall treatment time: development of correction factors for linac-based stereotactic radiotherapy.

PURPOSE: Continuous irradiation of relatively short duration as administered in gamma-ray stereotactic radiosurgery (SRS) is biologically not equivalent to the more protracted intermittent exposures during accelerator-based radiosurgery with multiple arcs. Accelerator-based SRS and fractionated stereotactic radiotherapy (SRT) is currently performed with a high degree of variability in equipment and techniques resulting in highly variable treatment delivery times. The present work is designed to quantify the effects of radiation delivery times on biological effectiveness. For this, the intermittent radiation delivery schemes, typical for linac-based SRS/SRT, have been simulated in vitro to derive biological correction factors. METHODS AND MATERIALS: The experiments were carried out using U-87MG human glioma cells in suspension at 37 degrees C irradiated with 6 MV X-rays to clinically relevant doses ranging from 6 to 18 Gy, delivered over total irradiation times from 16 min to 3 h. The resulting cell survival data was used to calculate dose correction factors to compensate for wide variations in dose delivery times. RESULTS: At each total dose level, cell survival increased with increasing total irradiation time. The increase in survival was more pronounced at higher dose levels. At a total dose of 12 Gy, cell survival increased by a factor of 4.7 when irradiation time was increased from 16 to 112 min. Dose correction factors were calculated to allow biologically equivalent irradiations over the range of exposure times. Cells irradiated with corrected total doses of 11.5 Gy delivered incrementally in 16 min up to 13.3 Gy in 112 min were found to exhibit the same survival within the experimental limits of accuracy. CONCLUSIONS: For a given total dose, variations in dose delivery time typical of SRS/SRT techniques will result in significant changes in cell survival. In the dose range studied, an isoeffect dose correction factor of 2 to 3 cGy/min was shown to compensate for the change in delivery time for U-87 MG human gloma cells in vitro.

Brain Neoplasms↗

PubMed1997

Electron/photon matched field technique for treatment of orbital disease.

PURPOSE: A number of approaches have been described in the literature for irradiation of malignant and benign diseases of the orbit. Techniques described to date do not deliver a homogeneous dose to the orbital contents while sparing the cornea and lens of excessive dose. This is a result of the geometry encountered in this region and the fact that the target volume, which includes the periorbital and retroorbital tissues but excludes the cornea, anterior chamber, and lens, cannot be readily accommodated by photon beams alone. To improve the dose distribution for these treatments, we have developed a technique that combines a low-energy electron field carefully matched with modified photon fields to achieve acceptable dose coverage and uniformity. METHODS AND MATERIALS: An anterior electron field and a lateral photon field setup is used to encompass the target volume. Modification of these fields permits accurate matching as well as conformation of the dose distribution to the orbit. A flat-surfaced wax compensator assures uniform electron penetration across the field, and a sunken lead alloy eye block prevents excessive dose to the central structures of the anterior segment. The anterior edge of the photon field is modified by broadening the penumbra using a form of pseudodynamic collimation. Direct measurements using film and ion chamber dosimetry were used to study the characteristics of the fall-off region of the electron field and the penumbra of the photon fields. From the data collected, the technique for accurate field matching and dose uniformity was generated. RESULTS: The isodose curves produced with this treatment technique demonstrate homogeneous dose coverage of the orbit, including the paralenticular region, and sufficient dose sparing of the anterior segment. The posterior lens accumulates less than 40% of the prescribed dose, and the lateral aspect of the lens receives less than 30%. A dose variation in the match region of +/-12% is confronted when an unmodified photon field edge is matched with the fall-off of the electron field at the 50% isodose lines. By modifying the penumbra, the dose variation is reduced to +/-2%. Treatment setup accuracy is essential. CONCLUSIONS: The electron/photon matched field technique offers a uniform isodose distribution for treatment of the orbit that has not been previously achieved. With this technique a homogeneous dose can be delivered to the entire orbit while avoiding a significant dose to the anterior segment and minimizing the risk of morbidity.

Electrons↗

PubMed1997

Transverse tomosynthesis on a digital simulator.

The availability of digital radiographic imaging on simulators has led to the investigation of a number of new imaging possibilities, including digital linear tomography (tomosynthesis). It has been shown that a single set of projections in this case is sufficient to reconstruct images from multiple planes, including planes tilted with respect to the tube motion. The present work examines the feasibility of tomosynthetic image reconstruction in transverse planes using a CCD-based digital radiotherapy simulator with conventional isocentric rotational geometry. General transformation equations were derived to permit image reconstruction in arbitrary transverse planes. Transverse images of the skull section of the humanoid phantom have been generated using a 360 degrees gantry sweep. Bone, air, and radiographic markers are well resolved, but the image quality is poor due to the suboptimal scanning geometry available on the simulator.

Biophysical Phenomena↗

PubMed1996

Comparison of dose homogeneity effects due to electron equilibrium loss in lung for 6 MV and 18 MV photons.

PURPOSE: Loss of electronic equilibrium within and adjacent to low density materials can result in a dose reduction along the central axis and near the beam edge for megavoltage photon beams. In this context, Radiation Therapy Oncology Group (RTOG) protocol #91-05 recommends the use of photon beams of energy 12 MV or less for nonsmall cell lung cancer therapy. This work presents data to support the use of higher energy photons for some clinical lung field setups. METHODS AND MATERIALS: Beam profiles were obtained from films inserted into homogeneous (polystyrene) and heterogeneous (polystyrene and lung-equivalent material) phantoms and irradiated in both single-field and parallel-opposed setups with 6 and 18 MV photon beams. Depth-dose curves were obtained with a parallel-plate ion chamber in the heterogeneous phantom to determine the range of field sizes over which the dose reduction at the lung/polystyrene interface becomes clinically significant. RESULTS: Opposed field profiles show less degradation in the penumbra (50-90% width) at the lung/polystyrene interface than single-field profiles. The difference between 6 and 18 MV penumbra widths at the interface also reduced when an opposed field is added. The central axis dose reduction at the interface was negligible for single fields of a width of 8 cm or more. CONCLUSION: Our results show that for opposed fields, the difference in penumbra degradation of the 6 and 18 MV photon beams is clinically insignificant compared to daily setup errors and patient motion. The central axis dose reduction is also shown to be small. Our data support the use of higher energy beams to obtain lower peripheral dose maxima in small clinical geometries.

Carcinoma, Non-Small-Cell Lung↗

PubMed1996

Short communication: Total craniofacial photon shell technique for radiotherapy of extensive angiosarcomas of the head.

Effective radiotherapy for extensive angiosarcomas of the face and scalp is technically difficult due to the complex shape of the volume at risk, which can consist of the superficial tissues of the entire head. This work reports the details of a rotational X-ray technique used to deliver a large part of the tumour dose. The technique consists of four consecutive 90 degree arcs with changing centre blocks to protect critical midline structures. Multilevel CT based treatment planning is carried out to determine the centre block dimensions and beam weights. As a result the radiation dose is delivered with acceptable uniformity over the entire shell of superficial tissues of the head. The overall treatment combines the rotational fields with large lateral field irradiation and/or local boosts with photons or electrons. Two of three patients treated with this technique had local control of the disease until their deaths at 13 and 18 months. A third patient responded well, with only a small region of stable disease at 9 months.

Aged↗

PubMed1995

Clinical use of a digital simulator for rapid setup verification in high dose rate brachytherapy.

PURPOSE: Fractionated high dose rate (HDR) brachytherapy provides a number of technical advantages over conventional implant therapy in that (a) it can be carried out on an outpatient basis, (b) personnel exposure is reduced to insignificant levels, and (c) patient motion during irradiation is minimized, resulting in a more accurate delivery of the planned radiation dose distribution to the target and critical structures. The patient discomfort associated with the repeated applicator insertions and/or treatment setups can be alleviated to the extent that the setup time is held to a minimum. This work describes the use of a prototype digital simulator to obtain fast, high-quality digital images for rapid setup verification. METHODS AND MATERIALS: The digital imaging system of the prototype simulator consists of a charge-coupled device (CCD) camera, which views the x-ray image optically transmitted from a conventional phosphor screen. Treatment is carried out with a remote afterloading HDR unit immediately after setup verification with the patient on the simulator stretcher. The high-resolution digital images are processed and displayed in about 5 s, as opposed to a minimum of approximately 2 min for film. RESULTS: The imaging system has been evaluated for a variety of implant types, both intracavitary and interstitial. The digital radiographs provided permanent high-resolution images as required in most cases for precise applicator positioning. The gray scale manipulation capabilities were found to be useful for imaging in regions of different density, such as lung and soft tissue, in the same radiograph. The advantages of short image acquisition and display times were observed in all cases, but were most evident in the intraluminal procedures, which sometimes involved several pretreatment applicator adjustments at a time of considerable patient discomfort. CONCLUSION: Pretreatment imaging is necessary to fully exploit the technical advantages of HDR brachytherapy. High-quality digital radiography offers unique advantages in HDR setup and verification by providing fast high-resolution, undistorted images with software manipulation capabilities and permanent storage of images.

Brachytherapy↗

PubMed1995

Tissue compensation using dynamic collimation on a linear accelerator.

PURPOSE: The availability of computer-controlled collimators on some accelerators has led to techniques for dynamic beam modification, mainly to simulate beam wedge filters. This work addresses the practical aspects of dynamic tissue compensation in one dimension using available treatment-planning software. METHODS AND MATERIALS: Data derived from the treatment-planning program is used with an iterative calculational routine to determine the monitor unit settings needed for the collimator-controlling computer. The method was first tested by simulating a 60 degrees physical wedge. Further studies were carried out on a specially fabricated plastic phantom that modeled the sagittal contour of the upper torso, neck, and lower head regions. RESULTS: Dynamic wedge point doses generated by the planning program agreed within 1% with the values directly measured in a polystyrene phantom. In the patient phantom, dynamic collimation achieved calculated dose uniformity within 0.5% in a reference plane near the phantom midline. A comparison of computer-generated and measured point doses in this case showed agreement within 3%. CONCLUSIONS: Dynamic collimation can provide effective compensation for contours that vary primarily along one direction. A conventional treatment-planning program can be used to plan dynamic collimation and deliver a prescribed dose with reliable accuracy.

Models, Anatomic↗