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I J Das

Publications and source records attributed to I J Das.

At least 37 records · Page 2Linked to original sources
PubMed1997

Efficacy of a belly board device with CT-simulation in reducing small bowel volume within pelvic irradiation fields.

PURPOSE/OBJECTIVE: Acute and chronic small bowel toxicity associated with pelvic irradiation limits dose escalation for both chemotherapy and radiotherapy for rectal cancer. Various surgical and technical maneuvers including compression and belly board devices (BBD) have been used to reduce small bowel volume in treatment fields. However, quantitative dose volume advantages of such methods have not been reported. In this study, the efficacy of BBD with CT-simulation is presented with dose-volume histogram (DVH) analyses for rectal cancer. METHODS AND MATERIALS: Twelve consecutive patients referred to our department with rectal cancer were included in this study. Patients were given oral contrast 1.5 h prior to scanning and instructed not to empty their bladder during the procedure. The initial CT scan without BBD was taken in the prone position with an immobilization cast. A second CT study was performed with a commercially available BBD consisting of an 18-cm thick hard sponge with an adjustable opening (maximum 42 x 42 cm2). All patients were positioned prone over the BBD so that the opening was above the treatment volume and usually extended from the diaphragm to the bottom of the fourth lumbar spine. Image fusion between both sets of CT scans (with and without BBD) was performed using common bony landmarks to maintain the same target volume. The critical structures including small bowel and bladder were delineated on each slice for DVH analysis. On each study, a three-field optimized plan with conformal blocks in beams-eye-view was generated for volumetric analysis. The DVHs with and without BBD were evaluated for each patient. RESULTS: The median age and body weight of 12 patients (4 females and 8 males) were 57.5 years and 82.7 kg, respectively. The changes in posterior-anterior (PA) and lateral separation with and without BBD at central axis slices were analyzed. The changes in lateral separation were minimal (<0.8 cm); however, the PA separation was reduced by 11.3 +/- 3.3% when BBD was used. The reduction in PA separation was directly related to the reduction in small bowel volume. The small bowel volume was significantly reduced with a median reduction of 70% (range 10-100%) compared to the small bowel volume without BBD. The small bowel volume reduction did not correlate either with body weight, age, gender, or sequence of radiation treatment with surgery (pre-op vs. post-op). The DVH analysis of small bowel with BBD showed significant volume reduction at each dose level. For 50% patients, the DVH analysis demonstrated an increase in bladder volume with BBD. All patients treated with the BBD completed their treatment without any break and without significant acute gastrointestinal or genitourinary toxicity. CONCLUSIONS: For rectal cancers, small bowel is the dose-limiting structure for acute and chronic toxicity. The use of the BBD should improve the tolerance of aggressive combined modality treatment by reducing the small bowel volume within the pelvis compared to the prone position alone. The BBD provides an easy, economical, comfortable, and noninvasive technique to displace small bowel from pelvic treatment fields. The small bowel volume is dramatically reduced at each dose level. The volume reduction does not correlate with gender, age, weight, pelvic separation, and sequence of radiation treatment vs. surgery.

Aged↗
PubMed1997

Dosimetric comparison of treatment planning systems in irradiation of breast with tangential fields.

PURPOSE: The objectives of this study are: (1) to investigate the dosimetric differences of the different treatment planning systems (TPS) in breast irradiation with tangential fields, and (2) to study the effect of beam characteristics on dose distributions in tangential breast irradiation with 6 MV linear accelerators from different manufacturers. METHODS AND MATERIALS: Nine commercial and two university-based TPS are evaluated in this study. The computed tomographic scan of three representative patients, labeled as "small", "medium" and "large" based on their respective chest wall separations in the central axis plane (CAX) were used. For each patient, the tangential fields were set up in each TPS. The CAX distribution was optimized separately with lung correction, for each TPS based on the same set of optimization conditions. The isodose distributions in two other off-axis planes, one 6 cm cephalic and the other 6 cm caudal to the CAX plane were also computed. To investigate the effect of beam characteristics on dose distributions, a three-dimensional TPS was used to calculate the isodose distributions for three different linear accelerators, the Varian Clinac 6/100, the Siemens MD2 and the Philips SL/7 for the three patients. In addition, dose distributions obtained with 6 MV X-rays from two different accelerators, the Varian Clinac 6/100 and the Varian 2100C, were compared. RESULTS: For all TPS, the dose distributions in all three planes agreed qualitatively to within +/- 5% for the "small" and the "medium" patients. For the "large" patient, all TPS agreed to within +/- 4% on the CAX plane. The isodose distributions in the caudal plane differed by +/- 5% among all TPS. In the cephalic plane in which the patient separation is much larger than that in the CAX plane, six TPS correctly calculated the dose distribution showing a cold spot in the center of the breast contour. The other five TPS showed that the center of the breast received adequate dose. Isodose distributions for 6 MV X-rays from three different accelerators differed by about +/- 3% for the "small" patient and more than +/- 5% for the "large" patient. For two different 6 MV machines of the same manufacturer, the isodose distribution agreed to within +/- 2% for all three planes for the "large" patient. CONCLUSION: The differences observed among the various TPS in this study were within +/- 5% for both the "small" and the "medium" patients while doses at the hot spot exhibit a larger variation. The large discrepancy observed in the off-axis plane for the "large" patient is largely due to the inability of most TPS to incorporate the collimator angles in the dose calculation. Only six systems involved agreed to within +/- 5% for all three patients in all calculation planes. The difference in dose distributions obtained with three accelerators from different manufacturers is probably due to the difference in beam profiles. On the other hand, the 6 MV X-rays from two different models of linear accelerators from the same manufacturer have similar beam characteristics and the dose distributions are within +/- 2% of each other throughout the breast volume. In general, multi-institutional breast treatment data can be compared within a +/- 5% accuracy.

Body Constitution↗
PubMed1997

Dose estimation to critical organs from vertex field treatment of brain tumors.

PURPOSE: Radiation management of intracranial tumors may require a noncoplanar vertex field that often irradiates the entire length of the body. In view of radiation related risks to the normal tissues dose estimation to the extracranial organs such as the thyroid gland, spinal cord, heart, and genitalia is performed for a vertex field. METHODS AND MATERIALS: A vertex field used clinically was reproduced on an anthropomorphic Rando phantom to measure radiation dose to various organs in the primary beam. Three photon beams (4, 6, and 10 MV), and two high energy electron beams (16 and 20 MeV) were used. Dosimetry was performed with an ion chamber sandwiched between phantom slices at the appropriate positions. All doses were normalized to the target dose at a depth of 5 cm. The effect of the head position was studied by rotating the gantry angle up to +/-20 degrees to mimic the extension and flexion of the head. Theoretical calculation was performed using an exponential best fit to the depth dose table to estimate the dose to various points and compare with the measured dose. RESULTS: The measured normalized dose to the cervical cord, thyroid, heart, and female and male gonads are 60, 36, 16, 2.5, and 1.6%, respectively, for a 6 MV photon beam. The dose from 4 MV and 10 MV are slightly lower and higher, respectively. Doses from electron beams are about a factor of 4-10 lower than those of the photon beams. The measured gonadal dose from the primary beam is <5% of the target dose for all energies used in the study. The actual value, however, is dependent on the body structure, length, and the posture of the patient. A +5 degree head flexion had little effect on the dose to the various parts of the body. The head rotations greater than +/-10 degrees produced relatively lower doses by a factor of 10(-2) to the organs at distances greater than 40 cm from the prescription point. The radiation doses to the different critical organs estimated from the fitted curves are lower than the measured doses up to 35%. CONCLUSIONS: When a vertex field is used for the treatment of the brain tumors, the entire axial length of the body is irradiated which adds to the integral dose. Unlike the scattered and leakage radiation, the primary dose to extracranial critical organs is greater for higher energies. For a 10 MV beam the ovary and testis at a distance of 80 cm and 90 cm may receive a dose of 4.2 and 3%, respectively, of the target dose. The gonadal dose could be quite significant if the entire treatment is delivered using a vertex field. For pediatric and smaller patients, dose to the critical organs at known distances could be estimated from the empirical equation obtained from the measured data. While the risk-benefit ratio is often evaluated and acceptable for treating malignant tumors, the long-term complications need thorough assessment in younger and curable patients. In view of radiation carcinogenesis and genetic burden, dose reduction to critical organs should be considered using a 3D planning system to arrange beams in other nonaxial planes and by considering electron beams for the vertex field.

Brain Neoplasms↗
PubMed1997

Patterns of dose variability in radiation prescription of breast cancer.

PURPOSE: Comparison of radiation outcome of various treatment protocols is difficult due to the variability of dose prescription. A retrospective analysis of the pattern and intercomparison of dose prescriptions is presented for the treatment of breast cancer. MATERIALS AND METHODS: To represent the clinical practice for breast irradiation with tangential fields, commonly used prescription points were chosen that lie on the perpendicular bisector of the chest wall separation (s) that represents the breast apex height (h). These points are located at 1.5 cm from the posterior beam edge, at the chest wall-lung interface (2-3 cm), at distances of h/3 and h/2, and at the isocenter. One hundred consecutive patients treated with intact breast following excisional biopsy were used in this study. For analysis, treatment planning was carried out without lung correction with a 6 MV beam for all patients, even though some of the patients were treated with high energy beams for dose uniformity. Dose distributions were optimized with wedges and beam weights to provide a symmetrical dose distribution on the central axis plane. The statistical analyses of the different parameters, s, h, maximum dose, and doses at various prescription points were carried out. RESULTS: The maximum dose (hot spot) in breast varied from +5% to +27% above the prescribed dose among the patient population. The hot spot was directly related to s, and appeared to be independent of h and the ratio h/s. Among 55%, 40%, and 5% of the patients, the magnitude of the hot spot was 5-10%, 10-15%, and >15%, respectively. Except for the magnitude of the hot spot, the doses at various prescription points were independent of the breast size. For a prescription point at h/3 or at the lung-chest wall interface, the dose variation within +/- 1% is observed for 90% of the patient population. On the other hand, the average dose variation is about +/- 3% among other protocols with dose prescription point varying up to the h/2 point. With the prescription point at the isocenter, an average and maximum variation of 4-5% and 11% were observed, respectively. The maximum dose inhomogeneity for some patients was significantly higher, i.e. up to +27% even without the lung correction. CONCLUSIONS: A wide variation in prescription dose is observed among the different treatment protocols commonly used in breast treatment. For a total dose of 46-50 Gy delivered at 2 Gy/fraction to the breast, the prescribed dose may vary between 50 and 55 Gy and the hot spot dose per fraction may range between 2.3 and 2.5 Gy depending on the protocol and breast size. Thus dose normalization at hot spot and the isocenter should be discouraged unless the total dose to the breast is modified. A uniform definition of dose prescription for breast treatment is greatly required for intercomparison of clinical data.

Breast Neoplasms↗
PubMed1997

Forward dose perturbation at high atomic number interfaces in kilovoltage x-ray beams.

High atomic number (Z) materials such as lead, used for field shaping and shielding normal tissues in kilovoltage beams could produce significant dose enhancement in the forward direction contrary to our normal belief with respect to the attenuation of photon beams. Such a dose enhancement has not been studied in kilovoltage beams, which is investigated in this study. Using a Siemens ortho-voltage unit (60-240 kVp) and a thin window (5 microns) parallel plate ion chamber, forward dose perturbation factor (FDPF) was measured at interfaces created by high- and low-Z materials. The FDPF is defined as the ratio of doses with and without an interface (FDPF = Di/Dh; where Di is the dose at an interface and Dh is the dose in a homogeneous medium). Results indicate that dose enhancement (FDPF > 1) as high as 20-fold can be observed for a thin (> or = 0.02 mm) Pb sheet in contact with soft tissue. The magnitude of FDPF is relatively independent of field size and falls off exponentially with Pb thickness. The typical photon beam attenuation takes at a thickness > 1 mm. This intense dose enhancement is localized within 250 microns of the interface. The FDPF is energy dependent but saturates above 140 kVp, unlike the backscatter dose perturbation that peaks around 200 kVp. The FDPF varies inversely with the thickness of high Z and distance between the surface and high-Z medium. The FDPF falls off rapidly to a level of photon transmission usually predicted by exponential attenuation when distance is increased. In conclusion, with kilovoltage beam, a high-Z medium placed in contact with soft tissue may not attenuate radiation dose unless adequate thickness and proper distance between the surface and high-Z medium is used. The localized intense dose enhancement (approximately 20-fold) created by the high-Z interface could be exploited for clinical use.

Biophysical Phenomena↗
PubMed1996

Characteristics of a dedicated linear accelerator-based stereotactic radiosurgery-radiotherapy unit.

A stereotactic radiosurgery and radiotherapy (SRS/SRT) system on a dedicated Varian Clinac-600SR linear accelerator with Brown-Roberts-Wells and Gill-Thomas-Cosman relocatable frames along with the Radionics (RSA) planning system is evaluated. The Clinac-600SR has a single 6-MV beam with the same beam characteristics as that of the mother unit, the Clinac-600C. The primary collimator is a fixed cone projecting to a 10-cm diameter at isocenter. The secondary collimator is a heavily shielded cylindrical collimator attached to the face plate of the primary collimator. The tertiary collimation consists of the actual treatment cones. The cone sizes vary from 12.5 to 40.0 mm diameter. The mechanical stability of the entire system was verified. The variations in isocenter position with table, gantry, and collimator rotation were found to be < 0.5 mm with a compounded accuracy of < or = 1.0 mm. The radiation leakage under the cones was < 1% measured at a depth of 5 cm in a phantom. The beam profiles of all cones in the x and y directions were within +/- 0.5 mm and match with the physical size of the cone. The dosimetric data such as tissue maximum ratio, off-axis ratio, and cone factor were taken using film, diamond detector, and ion chambers. The mechanical and dosimetric characteristics including dose linearity of this unit are presented and found to be suitable for SRS/SRT. The difficulty in absolute dose measurement for small cone is discussed.

Equipment Design↗
PubMed1996

Characteristics of a scintillator-based daily quality assurance device for radiation oncology beams.

A plastic scintillator based device marketed as a "DCT 444" is investigated for daily quality assurance (QA) of radiation beams. Variations in DCT responses were studied for three identical devices with beam, energy, distance, dose, dose rate, and orientations. The effects of gantry, collimator, and patient support assembly (PSA) rotations were investigated at photon energies of 6, 10, and 18 MV and electron energies in the range of 5-20 MeV from three different linear accelerators. The short and long term linearity, reproducibility, and radiation damage were also investigated in kilovoltage and megavoltage beams. Results indicate that the DCT response is linear with dose and dose rate, but shows directional dependence for all beams. The response is dependent on its orientation and the angle of rotations of collimator, gantry, and PSA of the linear accelerator. When the collimator is rotated through 0-36 degrees, the DCT response varied within +/- 4.5% and +/- 7.2% for the 6 MV and 18 MV beams, respectively. With PSA rotation, deviations up to +/- 11% were noted. The DCT response is symmetric across 0 degrees and peaks at +/- 60% PSA angle for all accelerators and beam energies. The effect of backscattering medium on DCT response is noted for all beams but significant for kilovoltage beams. A deviation as high as 30% was observed when DCT was placed on a thick polystyrene phantom. The long term stability studied over 25 months showed varied signal losses possibly due to radiation damage. However, there is no noticeable signal loss over a short period of time (2-4 weeks). The lightweight DCT device is linear with dose and dose rate, stable, and easy to operate. It stores ten readings that can be downloaded to a personal computer. This study indicates that DCT-444 is a suitable QA tool for central axis dose measurement. However, proper precaution is needed as this device exhibits directional and positional dependence. For a meaningful QA result of a radiation unit, the daily QA setup geometry should be identical to that used for calibration of the DCT device.

Biophysical Phenomena↗
PubMed1996

Dosimetry of high energy photon and electron beams with CEA films.

With all the advantages of film dosimetry in the megavoltage energy range, the use of film as a dosimeter is still limited due to the various difficulties associated with films such as energy dependence, film orientation, and sensitometric nonlinearity. Recently, therapy verification and localization films (CEA TVS and TLF films) from a Swedish manufacturer have become available in vacuum-sealed water-proof packaging in the US. The packaging renders the CEA films useful in a water phantom and ideal for photon and electron dosimetry. A systematic study has been carried out to investigate the potential of dosimetric application of the new films for high energy photon and electron beams. For the TVS films, the characteristic curve is generally energy independent but appears to be dependent on the source of the radiation, i.e., whether it is gamma rays or bremsstrahlung x rays. Compared to Kodak Readypack XV films, the CEA TVS film is linear in optical density over a much larger range of radiation dose. The inter- and intra-variation of the TVS films is less than 2%. For electrons, the characteristic curve is linear over a similar density range as photons but exhibit a slight energy dependence. TVS film is slightly directional dependent on the incident radiation for both photons and electrons. The perpendicular orientation results in higher optical density than the parallel orientation. The differences are within +/- 2% except in the buildup region for photons and in the exponential fall-off region of the electron beams where differences up to 4% are noted. For the CEA TLF film which is about three times faster than the TVS film, the characteristic curve is reasonably linear over the dose range of 0-15 cGy and energy independent within the experimental uncertainty (+/- 5%). Percent depth dose and isodose measurements with the TVS films are in good agreement with ion chamber results.

Biophysical Phenomena↗
PubMed1995

Harvesting backscatter electrons for radiation therapy.

PURPOSE: An innovative technique is used to harvest backscatter electrons for the treatment of superficial small lesions of skin, oral cavity, and rectum where a significant dose gradient and maximum surface dose is desired. METHODS AND MATERIALS: Backscatter electrons are harvested out of the primary electron beams from the linear accelerators. The design consists of a short cylindrical cone that fits snugly over a long cylindrical electron cone. The short cylindrical cone has a thick circular plate of high atomic number medium (Pb) attached to the distal end, and a lateral slit of variable length and width. The width of the slit could be closed as desired by rotating the two cones and the length can be increased by lowering the short cylindrical cone. Primary electrons strike the Pb plate perpendicularly and produce backscatter electrons that pass through the lateral slit for treatment. Using film and a parallel plate ion chamber, backscattered electron dose characteristics are studied. RESULTS: The depth dose characteristic of the backscatter electron is very similar to that of the 0.2 mm Al half-value layer x-ray beam that is commonly used for the intracavitary and superficial lesions. The backscatter electron energy is nearly constant and effectively < or = 1 MeV from the clinical megavoltage beams. The backscatter electron dose rate of 0.32-0.8 Gy/min could be achieved from modern accelerators without any modification. The beam flatness is dependent on the slit size and the depth of treatment, but is satisfactory to treat small lesions. CONCLUSIONS: The measured data for backscatter electron energy, fluence, depth dose, flatness, dose rate, and absolute dose indicates that the harvested backscattered electrons are suitable for clinical use.

Electrons↗
PubMed1995

Optimum field size and choice of isodose lines in electron beam treatment.

PURPOSE: A method is provided for the optimum field size and the choice of isodose line for the dose prescription in electron beam therapy. METHODS AND MATERIALS: Electron beam dose uniformity was defined in terms of target coverage factor (TCF) which is an index of dose coverage of a given treatment volume. The TCF was studied with respect to the field size, the beam energy, and the isodose level for prescription from the measured data for various accelerators. The effect of the TCF on air gap between electron applicator/cone and the surface was investigated. Electron beams from scattering foil and scanned beam units were analyzed for the target coverage. RESULTS: A mathematical method is provided to optimize a field size for target coverage by a given isodose line in terms of TCF which is strongly dependent on the type of accelerator and the design of the collimator. For a given type of collimating system, the TCF does not depend on the type of electron beam production (scattering foil or swept scanned beam). Selection of isodose line for dose prescription is very critical for the value of the TCF and the dose coverage. The TCF is inversely proportional to the isodose value selected for the treatment and nearly linear with field size and beam energy. Air gap between applicator and the surface reduces the dose uniformity. Tertiary collimator moderately improves the lateral coverage for high energy beams. CONCLUSIONS: To adequately cover the target volume in electron beam treatment, lateral and depth coverage should be considered. The coverage at depth is strongly dependent on the choice of isodose line or beam normalization. If the dose prescription is at dmax (i.e., the 100% isodose line is selected), the choice of beam energy is not critical for depth coverage since dmax is nearly independent of energy for smaller fields. The 100% isodose line should not be chosen for treatment because of the significant constriction of this isodose line and inadequate coverage at depth. For a higher TCF, a minimum air gap between the cone to the surface of the patient is desired. If such is not possible, then a tertiary collimator at the skin is required. Whenever, a tertiary collimator is used, it is advised to increase the collimator field size by a factor of 1.4.

Models, Structural↗
PubMed1995

Picture archiving and communications systems in radiation oncology (PACSRO): tools for a physician-based digital image review system.

Digital imaging is becoming more and more important in the diagnosis, staging, and treatment of patients in radiation oncology. In order to facilitate the most efficient interface of this technology to physicians and other users of this information, a medical image display system (MID) has been developed at the Fox Chase Cancer Center (FCCC). The system runs on 20 personal computers situated in physicians offices as well as a modified system located in the radiation oncology conference room. Access to CT, MRI, and EPID information is achieved through an Ethernet connection to the hospital picture archiving and communications system (PACS). Over a 1-year period a total of 503 patients and 3845 images have been stored on the system. Physician approval using the MID system (without conventional films) was performed on 106 patients. Of these, 22%, 16%, 11%, 10%, and 9% consisted of breast, prostate, pelvic, lung, and head and neck patients, respectively. Digital images sent from a variety of image sources to the MID system take up to 15 s to process and format while image access and display can take 2-5 s, dependent upon image size and speed of the host computer.

Adult↗
PubMed1995

Electron-beam characteristics at extended treatment distances.

A uniform dose to the target site is required with a knowledge of delivered dose, central axis depth dose, and beam flatness for successful electron treatment at an extended source to surface distance (SSD). The central axis depth dose is shown to be nearly independent of moderate changes in the treatment distance. The delivered dose at a point could be calculated with the concept of virtual source position and an inverse square correction. In an extended SSD treatment, underdosage of the lateral tissue may occur due to reduced beam flatness. To study the changes in beam characteristics, the depth dose and beam flatness were measured at different SSDs for clinically used field sizes [(3 x 3)-(15 x 15) cm2] and beam energies ranging from 6 to 20 MeV. Our results indicate that the changes in depth dose are minimal except in the buildup region for most energies. In general, the surface dose is decreased (< or = 10%) as the SSD is increased moderately. Beam flatness was measured in terms of target coverage factor (TCF) defined as the ratio of the width of a specified isodose line to the geometrical field width. It was observed that the loss in beam flatness is significant for smaller fields, higher isodose lines, and lower energies. Variations in SSD have a minimal effect on the relative changes in beam flatness for field sizes greater than 8 x 8 cm2. The lateral loss of beam uniformity could be estimated by various parameters, such as the full width at half maximum, the homogeneity index, the uniformity index, and the TCF; however, TCF is a simpler parameter to use clinically. The beam characteristics (depth dose and TCF) at extended treatment distances are presented for electron beams.

Electrons↗
PubMed1995

Backscatter dose perturbation in kilovoltage photon beams at high atomic number interfaces.

Dose perturbations at tissue interfaces have been of significant concern since the beginning of this century. However, comprehensive studies related to the backscatter perturbation in kilovoltage beams are still limited. The dose perturbation depends on various parameters, including beam energy, field size, and the thickness, width, position, and atomic number, Z, of the inhomogeneity creating the interface with soft tissue. Using a thin window parallel plate ion chamber having relatively flat response at low energies, the dose perturbation was measured as backscatter dose perturbation factor, BSDF, at various interfaces in kilovoltage x-ray beams. The BSDF is defined as the ratio of doses with and without an interface for identical setup conditions. Results indicate that the BSDF is strongly dependent on beam energy, like the backscatter factor. Contrary to its behavior in megavoltage beams, BSDF in kilovoltage beams does depend on the field size, suggesting a contribution from scattered photons and fluorescent radiation, originating in the high-Z material. The thickness of the high-Z medium is not critical, since a fraction of a millimeter is sufficient to provide full backscatter. The interface effect with wide inhomogeneity has two distinct regions: the high dose region (BSDF > 1.0), which is very localized and disappears within a fraction of a millimeter, and the low dose region (BSDF < 1.0), which is observed up to 10 cm. The dependence of BSDF is neither a quadratic function of Z nor a cube root of beam energy, indicating that the interface effect is complex and not predominantly due to photoelectron transport.

Mathematics↗
PubMed1995

Evaluation of digitally reconstructed radiographs (DRRs) used for clinical radiotherapy: a phantom study.

Digitally reconstructed radiographs produced from a commercial CT simulator have been evaluated using an in-house test phantom. The phantom consists of a polystyrene cubic block of dimension 15 cm. It contains four test patterns to measure contrast detail, modulation transfer function, ray line divergence accuracy, and spatial distortion. A total of six CT data sets, which vary by CT slice thickness and separation between slices, as well as CT slice reconstruction area have been used to analyze digitally reconstructed radiographs produced by the system. Results show that contrast detail is independent of slice thickness and separation but dependent upon slice reconstruction area for small object diameters (< or = 1.5 mm). Half field (24-cm diameter) reconstruction images provide lower threshold contrasts than full field (48-cm diameter) scans. The modulation transfer function for each data set was calculated and the spatial frequency at which the modulation transfer function is 50% (f50) of the maximum indicates that high contrast resolution depends on slice thickness for both the full and half field reconstructions. For full field scans, f50 values were 0.19, 0.10, and 0.10 line pairs/mm for the 2 mm/2 mm (slice thickness/separation), 5 mm/3 mm, and 5 mm/5 mm data sets, respectively. Similarly, half field f50 values were 0.19, 0.10, and 0.10 line pairs/mm for these same three thickness/separation data sets, respectively. The error in the ray tracing component of the digitally reconstructed radiograph algorithm for source to skin distances between 60 and 200 cm was 1.0 mm while the spatial linearity error was < or = 2.5 mm. Errors in CT simulator collimator and table rotations were calculated by measuring the angle between a grid pattern inlayed onto two orthogonal faces of the phantom and a graphical grid superimposed onto the digitally reconstructed radiograph by the CT simulator software. Measured angular differences were < or = 1.0 degree. The largest error in shifting the CT simulator field isocenter was 2.2 mm and occurred on the 5-mm slice thickness and separation CT data sets.

Biophysical Phenomena↗
PubMed1995

Use of multiplanar reformatted radiographic and digitally reconstructed radiographic images for planning conformal radiation therapy.

A three-dimensional treatment planning system capable of gantry, collimator, and table rotations is required for a noncoplanar conformal therapy. Unfortunately, such a system is not widely available. A method in which multiplanar reformatted radiographic (MPR) and digitally reconstructed radiographic (DRR) images are used is presented for conformal treatment of brain tumors. A head phantom containing a target volume was scanned on a computed tomographic (CT) simulator. The coronal MPR images were digitized on a treatment planning system to create a conformal block of the planned treatment field. The DRR images were generated on the CT simulator with the setup parameters calculated from the treatment planning system. A second set of conformal blocks was generated based on DRR images of the fields. The accuracy of the MPR- and DRR-generated blocks was verified on the vertex field. The differences between the actual planning target volume and the field edges of the MPR and DRR blocks were within +/- 4 mm and +/- 2 mm, respectively. The authors conclude that the MPR and DRR images could be successfully used to generate conformal blocks and for treatment planning of noncoplanar beams in radiation therapy.

Brain Neoplasms↗
PubMed1994

The effect of the number of computed tomographic slices on dose distributions and evaluation of treatment planning systems for radiation therapy of intact breast.

PURPOSE: This study was undertaken to answer the following questions in breast irradiation: (a) How many calculation planes are sufficient for three-dimensional (3-D) treatment planning? (b) Is pseudo-3-D planning system sufficiently accurate for 3-D treatment planning of a breast? METHODS AND MATERIALS: We carried out dose calculations and differential dose-volume analysis on three representative patients covering the range of breast size encountered in a clinic. The breast volumes were reconstructed from computed tomography (CT) scans using three slices, five slices and the full CT scan respectively. An established 3-D dose algorithm and two pseudo-3-D commercial systems were used in the calculations. Comparison of isodose distributions were made between the central axis plane, a cephalic and a caudal plane 6 cm above or below the central axis respectively. RESULTS: When comparing isodose distributions generated with conventional two-dimensional treatment planning with 3-D dose calculations, the former underestimated the size and magnitude of the hot spots in the medial and the lateral subcutaneous (SC) regions. When comparing the three-slice with the full CT model, while the three-slice model was found to be adequate for the "small" and the "medium" size patients, the full CT model provided a more accurate representation of dose distributions for the "large" patient. Comparison of a true 3-D algorithm with pseudo-3-D algorithms showed that while the latter systems were adequate for the "small" and the "medium" patients, significant differences were noted between the true 3-D and the pseudo-3-D algorithms for the "large" patient. CONCLUSION: For patients whose breast contours vary slowly within the tangential fields, a three-slice CT scan as well as a pseudo-3-D approach appears to be adequate for clinical decision. However, for patients with large variation of contours within the tangential fields, a full scale CT scan with a true 3-D dose algorithm is more accurate than either the three-slice or the five-slice model.

Breast↗
PubMed1994

Radiographic technique for acquisition of external patient contour.

Despite advances in techniques for contour acquisition, most patients are still contoured manually using lead wire. The lead wire method has unpredictable inherent inaccuracies due to the fact that the wire must be removed from the patient to obtain the contour. A simple radiographic method for a quick and accurate contour is presented that eliminates such an operator error. The technique requires that a thin lead wire be placed on the patient's skin in the desired plane as in the manual contouring process. With the wire still in place, the simulator table is rotated 90 degrees, and then the gantry is rotated to any reasonable angle (30-45 degrees) for a simple radiograph. The source-to-film distance (SFD) and gantry angle are noted. The radiograph of the lead wire contour is digitized on a personal computer (PC) and transformed from the film coordinates to the patient coordinates using the SFD and the gantry angle as input parameters. The accuracy and efficacy of this method were tested in four test cases: a square box, and contours in the head and neck, thoracic, and pelvic regions. The accuracy of the calculated contour was within +/- 4 mm for the large-gradient contours and +/- 3 mm in the other cases. A root mean square deviation of < 2 mm was observed between this method and the CT contour. This method can be used in any clinical setting where a simulator and a digitizer with a PC are available. The transformation code is simple and could be placed on any treatment planning computer system.

Humans↗