A comparative evaluation of loading times and exposures for permanent prostate brachytherapy.

The loading of needles for loose seed implantation of the prostate gland results requires a significant amount of effort and some radiation exposure to members of the medical staff. This study was performed to quantify the time spent and exposure levels associated with implant preparation, as well as to investigate any improvement in the time or exposure burden due to the introduction of a new loading device. The movements and radiation exposures for two single, highly experienced dosimetrists were monitored for ten conventionally loaded iodine implant cases. These same cases were reloaded with dummy sources using the sleeved system to determine time savings, if any. Two of these ten cases were then loaded with live sources using the sleeved system to determine relative exposure to the loading staff between the two methods. The results were then analyzed to generate per-seed and per-needle loading time and exposure burdens. Formulas are presented that may be used to determine the average time to load implants and the resultant staff exposure, both with the conventional technique and with the sleeved method. On the average, it takes an experienced loader 48 min to prepare an implant for the operating room, receiving a hand dose of about 10 mrem and a whole body dose of about 1 mrem. The sleeved system reduced these values by at least half. The time and exposure burden associated with the preparation of iodine loose seed implants has been characterized. The use of the sleeved needles resulted in significant time and exposure reductions for the medical staff.

Sector analysis of prostate implants.

The analysis of treatment plans generated following prostate implants (post plans) is an essential part of the patient's treatment regimen. The results are used to determine the adequacy of the individual implant and, just as importantly, to provide an evaluation of the institution's brachytherapy technique. Compiled post plan results can be used to compare data from different institutions and help determine guidelines that should be established as dosimetric goals. Sector analysis, or spatial dose mapping, is a novel method of analyzing brachytherapy results that has been developed for this purpose. The display of isodose curves provides spatial information pertaining to the dosimetric evaluation of post plans but is an unwieldy tool; ill suited to the creation of general conclusions for comparative efforts. Dose-volume histogram (DVH) analysis is an excellent tool for examining dosimetric results, but the spatial information is lost. Sector analysis bridges the gap between isodose curves and DVH analysis in post plan analysis. To perform sector analysis we divide the gland into three regions in the cranial-caudal direction (base, midgland, and apex) and four regions on each transverse slice (anterior, posterior, left and right). This gives twelve sectors, each identified by its location in the cranial-caudal direction and position on the transverse slice, e.g., posterior midgland. DVH analysis is performed for each region separately and compiled for display. We present an example of the use of this technique wherein we have analyzed a sequential series of 118 implants performed by a single practitioner (BRP) at two institutions over a calendar year. The implants were performed using two different techniques at the two institutions. Sector analysis was used to compare the results of the implants at the two institutions.

CT and MRI derived source localization error in a custom prostate phantom using automated image coregistration.

Dosimetric evaluation of completed brachytherapy implant procedures is crucial in developing proper technique. Additionally, accurate dosimetry may be useful in predicting the success of an implant. Accurate definition of the prostate gland and localization of the implanted radioactive sources are critical to attain meaningful dosimetric data. MRI is recognized as a superior imaging modality in delineating the prostate gland. More importantly, MRI can be used for source localization in postimplant prostates. However, the MRI derived source localization error bears further investigation. We present a useful tool in determining the source localization error as well as permitting the fusion, or coregistration, of selected data from multiple imaging modalities. We constructed a custom prostate phantom of hydrocolloid material precisely implanted with I-125 seeds. We obtained CT, the accepted modality, and MRI scans of the phantom. Subsequently, we developed an automated algorithm that employs a sequential translation of data sets to initially maximize coregistration and minimize error between data sets. This was followed by a noniterative solution for the necessary rotation transformation matrix using the Orthogonal Procrustes Solution. We applied this algorithm to CT and MRI scans of the custom phantom. CT derived source locations had source localization errors of 1.59 mm +/- 0.64. MRI derived source locations produced similar results (1.67 mm +/- 0.76). These errors may be attributed to the image digitization process.

Use of image coregistration in salvage prostate brachytherapy.

PURPOSE: We describe a method of performing salvage prostate brachytherapy on patients whose initial implant was suboptimal. This technique uses an image correlation algorithm only previously used to fuse postimplant magnetic resonance and computed tomographic (CT) images. Here, the initial postimplant CT and the second preimplant volume study are coregistered to plan delivery of the salvage implant. MATERIALS AND METHODS: Two early-stage patients had salvage implants performed with this technique, in which only a limited number of sources were visible on the ultrasound images. The dosimetric results of the first implant were displayed on the preplan generated for the second procedure. The planned total dose then was visualized prior to salvage implant. RESULTS: The implants were performed without complication. Rectum and urethra doses remained acceptable. In each case, the improvement in coverage of the gland was dramatic (V80 coverage improved from 65.2% and 47.3% to 93.1% and 92.2%, respectively), precluding the need for further intervention. CONCLUSIONS: Coregistration of the postimplant CT scan to an ultrasound volume study can be quantifiably and reliably performed. The resulting image set can be used to guide needle placement during a second salvage implant to achieve much improved dosimetric coverage of the gland.

The American Brachytherapy Society recommendations for permanent prostate brachytherapy postimplant dosimetric analysis.

PURPOSE: The purpose of this report is to establish guidelines for postimplant dosimetric analysis of permanent prostate brachytherapy. METHODS: Members of the American Brachytherapy Society (ABS) with expertise in prostate dosimetry evaluation performed a literature review and supplemented with their clinical experience formulated guidelines for performing and analyzing postimplant dosimetry of permanent prostate brachytherapy. RESULTS: The ABS recommends that postimplant dosimetry should be performed on all patients undergoing permanent prostate brachytherapy for optimal patient care. At present, computed tomography (CT)-based dosimetry is recommended, based on availability cost and the ability to image the prostate as well as the seeds. Additional plane radiographs should be obtained to verify the seed count. Until the ideal postoperative interval for CT scanning has been determined, each center should perform dosimetric evaluation of prostate implants at a consistent postoperative interval. This interval should be reported. Isodose displays should be obtained at 50%, 80%, 90%, 100%, 150%, and 200% of the prescription dose and displayed on multiple cross-sectional images of the prostate. A dose-volume histogram (DVH) of the prostate should be performed and the D90 (dose to 90% of the prostate gland) reported by all centers. Additionally, the D80, D100, the fractional V80, V90, V100, V150 and V200 (i.e., the percentage of prostate volume receiving 80%, 90%, 100%, 150%, and 200% of the prescribed dose, respectively), the rectal, and urethral doses should be reported and ultimately correlated with clinical outcome in the research environment. On-line real-time dosimetry, the effects of dose heterogeneity, and the effects of tissue heterogeneity need further investigation. CONCLUSION: It is essential that postimplant dosimetry should be performed on all patients undergoing permanent prostate brachytherapy. Guidelines were established for the performance and analysis of such dosimetry.

Source localization from axial image sets by iterative relaxation of the nearest neighbor criterion.

The use of axial image sets has become widely used to localize interstitial brachytherapy sources. One application of this method of localization is to perform post-implant dosimetry following transperineal interstitial permanent prostate brachytherapy (TIPPB) where the target structure and the source locations are displayed on the same image. The design of an appropriate scanning sequence often results in abutting slices of an intermediate slice width (3, 4, or 5 mm). Because a single source may be imaged on more than one slice, the resultant scans always show many more source locations than actual sources implanted. The physicist is then faced with the tedious task of determining which sources appear on more than one slice and deciding which source locations to eliminate from the data set. We have developed an algorithm, similar to one employed by Roy et al., which automates this process by relaxing the nearest neighbor criterion until the number of sources is reduced to either the number of sources implanted or the number counted on a projection radiograph. This paper details this algorithm and the results of its application to phantom studies, comparing to known source locations, as well as clinical studies, comparing to orthogonal film source localization, on a series of ten patients. Phantom studies demonstrate the superiority of the algorithm over orthogonal film reconstruction, locating 100% of the sources within 5 mm of the actual location as compared to 66% for the paired radiographs. The clinical study findings are commensurate with these results, with 72% of the sources on average located within 5 mm of the corresponding source in the other data set. The positive correlation of the quality of the orthogonal film reconstruction results with the quality of the coregistration results suggests that differences in registration between the two data sets may be due primarily to the uncertainties in the orthogonal film reconstruction.

Isoseparation curves: a mechanism for optimizing off-axis dose homogeneity of intact breast irradiation.

The purpose of this paper is to determine whether using off-axis isoseparation curves to optimize the collimator rotation angle improves dose homogeneity. Eleven intact breast irradiation patients underwent computerized tomography (CT) treatment planning with 1 cm abutting slices. Central plane treatment planning, using 6 MV photons, tissue inhomogeneity corrections, and isocentric opposed tangent treatment fields, was performed. Collimators were rotated to match chest wall slope through the use of a beam's-eye-view setting. Patient separations were measured from the apex of the breast to the posterior field border on each axial CT slice. Sagittal-plane isoseparation curves were constructed from these measurements. Using these curves, the collimator rotation that minimized off-axis separation differences was determined. A comparison of off-axis dose inhomogeneity was performed for patients with a > or =10 degrees difference between this optimized collimator angle and the angle determined by chest wall slope. These comparative treatment plans differed only with respect to collimator angle rotation. The mean optimal collimator rotation angle differed significantly from the mean rotation angle which matched the chest wall slope (5.4 degrees vs. 11 degrees, respectively, P < 0.001). Four of the 11 patients had rotation angle differences of 10 degrees. In these patients, the optimization of collimator angle reduced the percentage of breast volume to "that" received > or =110% of the prescribed dose. For the patient with the largest breast size to the patient with the smallest breast size the decreases were, respectively, 5% (15% to 10%), 3% (24% to 21%), 1% (4% to 3%), and 1% (0.9% to 0%). The mean reduction in dose inhomogeneity was greatest in the inferior breast quadrants. At 6 cm and 4 cm off axis, the mean reductions in the percentages of the breast tissue to "that" received 110% of the prescribed dose were respectively 15.1% and 5.3 %. Optimizing the collimator angle through the use of isoseparation curves decreases dose inhomogeneity. The greatest improvements are in the inferior quadrants of the intact breast. The improved dose homogeneity may have clinical relevance in the treatment of patients with large breast sizes.

Centralized multiinstitutional postimplant analysis for interstitial prostate brachytherapy.

PURPOSE: To investigate the feasibility and utility of performing centralized postimplant analysis for transperineal interstitial permanent prostate brachytherapy (TIPPB) by conducting a pilot study that compares the results obtained from 125I implants conducted at five different institutions. METHODS AND MATERIALS: Dose-volume histogram (DVH) analysis was performed on 10 postimplant CT scans from each of five institutions. This analysis included the total implanted activity of 125I, ultrasound, and CT volumes of the prostate, target-volume ratios, dose homogeneity quantifiers, prostate dose coverage indices, and rectal doses. As a result of the uncertainty associated with the delineation of the prostatic boundaries on a CT scan, the contours were redrawn by a single, study center physician, and a repeat DVH analysis was performed. This provided the basis for comparison between institutions in terms of implant technique and quality. RESULTS: By comparing total activity to preimplant ultrasound volume we clearly demonstrated that differences exist in implant technique among these five institutions. The difficulty associated with determining glandular boundaries on CT scans was apparent, based upon the variability in prostate volumes drawn by the various investigators compared to those drawn by the study center physician. This made no difference, of course, in the TVR or homogeneity quantifiers that are independent of target location. Furthermore, this variability made surprisingly little difference in terms of dose coverage of the prostate gland. Rectal doses varied between institutions according to the various implant techniques. CONCLUSIONS: Centralized, outcome-based evaluation of transperineal interstitial permanent prostate brachytherapy is viable and appropriate. Such an approach could be reasonably used in the conduct of multiinstitutional trials used to study the efficacy of the procedure.

Intraobserver and interobserver variability of MR imaging- and CT-derived prostate volumes after transperineal interstitial permanent prostate brachytherapy.

PURPOSE: To evaluate the relative accuracy and precision of magnetic resonance (MR) imaging and computed tomography (CT) in the assessment of postimplantation prostate volume by determining intraobserver, interobserver, and intermodality variations. MATERIALS AND METHODS: CT and MR images of 41 consecutive patients, after transperineal interstitial permanent prostate brachytherapy, were evaluated by two physicians to determine interobserver and intermodality variability in prostate volume measurements. Repeat evaluation in five randomly selected patients was used to determine intraobserver variability. RESULTS: Observer 1 versus 2 CT-determined mean prostate volume difference was statistically significant (-8.5 cm3 +/- 9.74 [standard deviation], P < .001); observer 1 versus 2 MR-determined mean prostate volume difference was not significant (1.9 cm3 +/- 11.7, P = .492). CT intraobserver range of dimensional errors was 3.5 and 11.4 times that of MR imaging. Observer 1 CT and MR volumes were significantly different (P = .001); observer 2 CT and MR volumes were not significantly different (P = .079). CONCLUSION: With both CT and MR imaging, variation is less when evaluations are conducted by one observer. Variation in one observer may be further reduced by using MR imaging in place of CT.

Comparison of MRI- and CT-based post-implant dosimetric analysis of transperineal interstitial permanent prostate brachytherapy.

The purpose of this work was to investigate how a recently developed MRI-based post-implant dosimetric analysis technique for ultrasound guided transperineal interstitial permanent prostate brachytherapy (TIPPB) compared with the currently accepted CT-based technique. The study was based upon 3-mm MRI and CT scans of 15 patients who had received either 125I or 103Pd implantation. All images were acquired on post-operative day 1 and within 1 hr of each other. Prostate volumes were determined by the same physician. Sources were digitized and calculations performed using an in-house treatment planning system with a nearest neighbor seed sorting routine and AAPM TG43 formalism. Prostate volume, geometric source distribution spread (rcom), dose volume histogram (DVH), and tumor control probability (TCP) calculations were performed from both image sets. Differences in source localization were evaluated by comparing source spread and prescription isodose volumes. Differences in dosimetric analysis were evaluated through prostate-specific DVH and TCP comparisons. Prostate volume as determined from MRI was larger than that of CT by an average of +9.1% (R = 0.70). Calculated rcom was smaller by an average of -0.9 mm (R = 0.81). Isodose volumes at 80, 90, 100, and 150% of the prescription dose differed by an average of +2.5, +2.9, -2.9, and +4.8%, respectively (R = 0.97, 0.98, 0.98, and 0.91). Percentage volume of the prostate encompassed by 80, 100, and 150% of the prescription dose differed by an average of -0.9, -0.9, and -0.1%, respectively (R = 0.34, 0.35, and 0.35). TCP differed by an average of -0.8% (R = 0.37). The results of this study further support our initial findings that MRI may be used to reliably localize the implanted sources for TIPPB. This study also demonstrated that MRI-based post-implant dosimetric analysis is possible. However, it is evident that differences in prostate localization from MRI to CT can result in significantly different assessments of prostate volume coverage. There is clearly a need to further quantify the differences between these two imaging modalities in this application and address whether greater accuracy in describing the dose-volume relationship based on improvements in visualization of the prostate gland from MRI will translate into improved correlation with treatment outcome.

Timing of computed tomography-based postimplant assessment following permanent transperineal prostate brachytherapy.

PURPOSE: To establish the rate of resolution of prostatic edema following transperineal interstitial permanent prostate brachytherapy, and to determine the results and impact of timing of the postimplant assessment on the dose-volume relationship. METHODS AND MATERIALS: A series of 19 consecutive patients with early-stage adenocarcinoma of the prostate receiving transperineal interstitial permanent prostate brachytherapy, were enrolled in this study. Twelve received 125I and seven received 103Pd. Postoperative assessment included a computed tomographic (CT) scan on postoperative days 1, 8, 30, 90, and 180. On each occasion, CT scans were performed on a GE helical unit at 3-mm abutting slices, 15-cm field of view. Prostate volumes were outlined on CT scans by a single clinician. Following digitization of the volumes and radioactive sources, volumes and dose-volume histograms were calculated. The prostate volume encompassed by the 80% and 100% reference isodose volumes was calculated. RESULTS: Preimplant transrectal ultrasound determined volumes varied from 17.5 to 38.6 cc (median 27.9 cc). Prostate volumes previously defined on 40 randomly selected postimplant CT scans were compared in a blinded fashion to a second CT-derived volume and ranged from -32% to +24%. The Pearson correlation coefficient for prostate CT volume reproducibility was 0.77 (p < 0.03). CT scan-determined volume performed on postoperative day 1 was an average of 41.4% greater than the volume determined by preimplant ultrasound. Significant decreases in average volume were seen during the first month postoperatively. Average volume decreased 14% from day 1 to day 8, 10% from day 8 to day 30, 3% from day 30 to day 90, and 2% thereafter. Coverage of the prostate volume by the 80% isodose volume increased from 85.6% on postoperative day 1 to 92.2% on postoperative day 180. The corresponding increase in the 100% reference dose coverage of the prostate volume ranged from 73.1% to 83.3% between postoperative days 1, and 180, respectively. CONCLUSIONS: Most of the prostatic edema induced by brachytherapy appears to resolve by postoperative day 30. Scans performed on postimplant day 30 appear to adequately describe the time-averaged dose coverage of the prostate. This suggests that waiting approximately 1 month to perform postimplant analysis gives the most accurate prostatic volume and, consequently, dosimetric description of the implant.

Clinical impact of implementing the recommendations of AAPM Task Group 43 on permanent prostate brachytherapy using 125I. American Association of Physicists in Medicine.

PURPOSE: To determine the clinical impact upon permanent interstitial prostate 125I brachytherapy after conversion to AAPM Task Group 43 (TG 43) guidelines. METHODS: The value of quantities used in the calculation of dose from two institutions, Northwest Tumor Institute (NWTI) and Memorial Sloan-Kettering Cancer Center (MSKCC), which pioneered interstitial techniques for prostate brachytherapy were compared to those recently determined and published by TG 43 of the American Association of Physicists in Medicine (AAPM). Using two different weighting schemes, the change in the commonly prescribed reference dose of 160 Gy was determined and found to be in agreement with that recently suggested. Volumes encompassed by the reference isodose surface were determined from a single source implant and a regularly distributed implant to show the effect of change in reference dose. A comparative analysis on 10 patients was performed to show how this change affected common implant quality descriptors and the effect of changing the calculation formalism without changing the reference dose. RESULTS: Both weighting schemes suggested a change in reference dose from 160 to 144 Gy. Single-source and distributed-source volumetric analysis confirmed this value. The effect on commonly used conformity and uniformity quantifiers for 10 implant patients was tabulated. CONCLUSION: Upon adopting the recommendations suggested by TG 43, institutions that perform permanent 125I prostate implants using calculation methods adapted from the NWTI or MSKCC should revise their treatment prescriptions from 160 to 144 Gy so that the doses delivered to patients remain unaffected. Institutions using other techniques to calculate dose should conduct an analysis similar to the one detailed here.

A survey of physics and dosimetry practice of permanent prostate brachytherapy in the United States.

PURPOSE: To obtain data with regard to current physics and dosimetry practice in transperineal interstitial permanent prostate brachytherapy (TIPPB) in the U.S. by conducting a survey of institutions performing this procedure with the greatest frequency. METHODS AND MATERIALS: Seventy brachytherapists with the greatest volume of TIPPB cases in 1995 in the U.S. were surveyed. The four-page comprehensive questionnaire included questions on both clinical and physics and dosimetry practice. Individuals not responding initially were sent additional mailings and telephoned. Physics and dosimetry practice summary statistics are reported. Clinical practice data is reported separately. RESULTS: Thirty-five (50%) surveys were returned. Participants included 29 (83%) from the private sector and 6 (17%) from academic programs. Among responding clinicians, 125I (89%) is used with greater frequency than 103Pd (83%). Many use both (71%). Most brachytherapists perform preplans (86%), predominately employing ultrasound imaging (85%). Commercial treatment planning systems are used more frequently (75%) than in-house systems (25%). Preplans take 2.5 h (avg.) to perform and are most commonly performed by a physicist (69%). A wide range of apparent activities (mCi) is used for both 125I (0.16-1.00, avg. 0.41) and 103Pd (0.50-1.90, avg. 1.32). Of those assaying sources (71%), the range in number assayed (1 to all) and maximum accepted difference from vendor stated activity (2-20%) varies greatly. Most respondents feel that the manufacturers criteria for source activity are sufficiently stringent (88%); however, some report that vendors do not always meet their criteria (44%). Most postimplant dosimetry imaging occurs on day 1 (41%) and consists of conventional x-rays (83%), CT (63%), or both (46%). Postimplant dosimetry is usually performed by a physicist (72%), taking 2 h (avg.) to complete. Calculational formalisms and parameters vary substantially. At the time of the survey, few institutions have adopted AAPM TG-43 recommendations (21%). Only half (50%) of those not using TG-43 indicated an intent to do so in the future. Calculated doses at 1 cm from a single 1 mCi apparent activity source permanently implanted varied significantly. For 125I, doses calculated ranged from 13.08-40.00 Gy and for 103Pd, from 3.10 to 8.70 Gy. CONCLUSION: While several areas of current physics and dosimetry practice are consistent among institutions, treatment planning and dose calculation techniques vary considerably. These data demonstrate a relative lack of consensus with regard to these practices. Furthermore, the wide variety of calculational techniques and benchmark data lead to calculated doses which vary by clinically significant amounts. It is apparent that the lack of standardization with regard to treatment planning and dose calculation practice in TIPPB must be addressed prior to performing any meaningful comparison of clinical results between institutions.

A survey of current clinical practice of permanent prostate brachytherapy in the United States.

PURPOSE: To help establish standards of care for transperineal interstitial permanent prostate brachytherapy (TIPPB) by obtaining data regarding current clinical practice among the most experienced TIPPB brachytherapists in the United States. METHODS AND MATERIALS: The 70 brachytherapists who performed the greatest number of TIPPB cases in 1995 in the U.S. were surveyed. Each received a comprehensive four page questionnaire that included sections on training and experience, patient and isotope selection criteria, manpower, technique, and follow-up. Thirty-five (50%) surveys were ultimately returned after three mailings and follow-up phone calls. The cumulative experience of the 35 respondents represented approximately 45% of the total TIPPB volume in the U.S. for 1995. Respondents included 29 from the private sector and six from academic programs. RESULTS: The median physician experience with TIPPB was reported as 4.9 years. Each performed an average of 73 TIPPB procedures in 1995 (range 40-300). This represented an increase in volume for most (74%) of the respondents. Sixty-three percent of the respondents attended a formal training course, 54% had TIPPB-specific residency training, and 31% had been proctored (16 had received two or more types of training experience). The most commonly reported selection criteria for implant alone was on Gleason score < or = 7, PSA < 15, < or = Stage T2a, and gland size < or = 60 cc, although no clear consensus was found. Fifty-four percent considered a history of TURP to be a relative contraindication, while 34% considered TURP to have no impact on patient selection. Eighty-six percent of respondents combine brachytherapy with external beam radiation in an average of 32% of their patients. Boosts were given with both 125I prescribed to 120 Gy (75%) or 103Pd to 90 Gy (50%). Sixty percent reported using a Mick applicator, 46% prefer using preloaded needles, and (11%) use both techniques. Real-time imaging was usually performed with ultrasound (94%); most included fluoroscopy (60%). Definitions of PSA control varied widely. CONCLUSIONS: TIPPB clinical practice in the U.S. demonstrates similarities in technique, but differences in patient selection and definitions of biochemical control. It is, therefore, incumbent on those beginning TIPPB programs to carefully review the specific practice details of those institutions with a broad experience.

Source localization following permanent transperineal prostate interstitial brachytherapy using magnetic resonance imaging.

PURPOSE: Dosimetric evaluation of completed brachytherapy implant procedures is crucial in developing proper technique and has prognostic implications. Accurate definition of the prostate gland and localization of the implanted radioactive sources are critical to attain meaningful dosimetric data. Methods using radiographs and CT accurately localize sources, but poorly delineate the prostate gland. MRI has been recognized as a superior imaging modality in delineating the prostate gland, but poor in localizing sources due to lack of source visibility. The purpose of this study was to optimize the visualization of sources using MRI and compare to CT derived source localization. METHODS AND MATERIALS: Multiple MRI scanning techniques were attempted until an acceptable sequence to visualize both the prostate gland and the implanted sources was found. The exams were performed using a pelvic coil only in approximately 15 min. The CT and MRI scans of 20 consecutive patients who had received TRUS-guided permanent transperineal interstitial prostate 125Iodine or 103Palladium brachytherapy were evaluated using an in-house dosimetry system. To eliminate anatomical dependence, the MRI-derived DVHs for the entire calculation volume were then compared to those derived from the CT scans. RESULTS: The differences in isodose volumes, of the calculation volumes, for all implants at all dose levels were not statistically significant at the 95% confidence level. Calculation volume isodose volumes derived from MR images were statistically similar to those derived from CT images at the prescription dose for both 125Iodine (p < 0.01) and 103Palladium (p < 0.026). CONCLUSION: This study presents the first evidence that MRI may be reliably used to identify permanently implanted 125Iodine and 103Palladium sources. Given the advantage of target definition characteristics of MRI, substantially more accurate dosimetric analysis of prostate implants is now possible. The cost of the optimized and abbreviated MR scanning sequence used in this study is comparable to a pelvic CT scan. Postimplant MRI allows more accurate volumetric and anatomically relevant evaluation of permanent prostate implants, which may provide useful clinical correlation.

Dosimetric analysis of intact breast irradiation in off-axis planes.

PURPOSE/OBJECTIVE: The purpose of this investigation is to quantify dose inhomogeneity of intact breast irradiation in off-axis planes, and determine how dose inhomogeneity varies according to patient breast size and anatomical region of the breast. METHODS AND MATERIALS: Eleven patients treated with intact breast radiation underwent a treatment-planning computer tomography (CT) scan with 1-cm slices through the entire breast. The area of breast tissue was defined on each CT slice. Treatment planning with lung correction factors was performed using a two-dimensional treatment-planning system that calculates off-axis dose distributions on a slice-by-slice basis. Each plan utilized tangential beams with matched nondivergent posterior borders and with collimator rotation to match the chest wall slope. Dose inhomogeneity within the central plane was minimized during treatment planning by the use of a wedge on the lateral tangent field and by the differential weighting of fields. Dose was normalized at the breast and pectoralis major interface at midseparation in the central plane. Off-axis dose inhomogeneity was not considered in the optimization of the treatment plan. Dose distributions were plotted for each 1-cm slice, and the area of each isodose curve within the breast on each CT slice was calculated. The results of each slice were summed to give an approximation of dose-volume relationships. RESULTS: For the entire population, an average of 10% of the breast volume (range 1-40%) received 110% or greater of the prescribed dose. Increasing dose inhomogeneity was positively correlated with increasing breast sizes (r = 0.72, p = 0.01--Spearmen rank test). Analysis of dose as a function of location within the breast, revealed that the greatest dose inhomogeneity occurred in the lower anatomical quadrants of the breast (p = 0.003-Kruskal-Wallis test). For the group, the mean breast volume that received a 110% or greater dose was: 30% at 6 cm below central axis, 14% at 4 cm below central axis, 6% at central axis, 5% at 4 cm above central axis, and 7% at 6 cm above central axis. CONCLUSION: Our study demonstrates that a significant volume of breast tissue receives 110% or greater of the prescribed dose. This inhomogeneity is greatest in women with larger breast sizes, providing a possible explanation for the poorer cosmetic result seen in this subset of patients compared to women with small breast sizes. In addition, our results show the greatest dose inhomogeneity in the lower quadrants of the breast. Off-axis dose inhomogeneity should be considered in the planning of tumor bed boosts in women with lower quadrant tumors.

The coupling of anisotropy and radial dose functions for 103Pd and 125I for use with a commercial treatment planning system.

Many commercial treatment planning systems available today employ traditional dose calculation formulae in their interstitial brachytherapy source calculation algorithms. The 1995 AAPM report on interstitial brachytherapy source dosimetry recommended a new dose calculational formalism and presented a technique for adopting it on systems which embody traditional formalism. In order to comply with these recommendations on our system, the transformations for implementing a one dimensional isotropic point source model were modified by coupling the published anisotropy and radial dose corrections and fitting them to a fifth order polynomial. Using this approach, a more accurate dose calculation is obtained.

Moldable tissue equivalent bolus for high-energy photon and electron therapy.

In radiation therapy, there is often a need to treat irregular surfaces with electron and photon beams. These surfaces require smoothing to achieve uniform doses at depth and proper buildup of dose at the surface. The surface smoothing and dose buildup is achieved by applying bolus. To deliver a known dose, produce a known central axis depth dose, and beam flatness for successful treatment, it is necessary that water or tissue equivalent bolus material is used. This material must also be able to fill extremely irregular voids. Several moldable materials, currently or formerly used in dental clinics, were evaluated for adequacy as tissue equivalent bolus. Availability was also considered during evaluation. Polyflex, a hydrocolloid, was found to be near water equivalent for electron and photon beams. It was also inexpensive, readily available, and held up well over time.