William S. Bice

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.

AAPM recommendations on dose prescription and reporting methods for permanent interstitial brachytherapy for prostate cancer: Report of Task Group 137

During the past decade, permanent radioactive source implantation of the prostate has become the standard of care for selected prostate cancer patients, and the techniques for implantation have evolved in many different forms. Although most implants use I125 or P103d sources, clinical use of C131s sources has also recently been introduced. These sources produce different dose distributions and irradiate the tumors at different dose rates. Ultrasound was used originally to guide the planning and implantation of sources in the tumor. More recently, CT and/or MR are used routinely in many clinics for dose evaluation and planning. Several investigators reported that the tumor volumes and target volumes delineated from ultrasound, CT, and MR can vary substantially because of the inherent differences in these imaging modalities. It has also been reported that these volumes depend critically on the time of imaging after the implant. Many clinics, in particular those using intraoperative implantation, perform imaging only on the day of the implant. Because the effects of edema caused by surgical trauma can vary from one patient to another and resolve at different rates, the timing of imaging for dosimetry evaluation can have a profound effect on the dose reported (to have been delivered), i.e., for the same implant (same dose delivered), CT at different timing can yield different doses reported. Also, many different loading patterns and margins around the tumor volumes have been used, and these may lead to variations in the dose delivered. In this report, the current literature on these issues is reviewed, and the impact of these issues on the radiobiological response is estimated. The radiobiological models for the biological equivalent dose (BED) are reviewed. Starting with the BED model for acute single doses, the models for fractionated doses, continuous low-dose-rate irradiation, and both homogeneous and inhomogeneous dose distributions, as well as tumor cure probability models, are reviewed. Based on these developments in literature, the AAPM recommends guidelines for dose prescription from a physics perspective for routine patient treatment, clinical trials, and for treatment planning software developers. The authors continue to follow the current recommendations on using D90 and V100 as the primary quantities, with more specific guidelines on the use of the imaging modalities and the timing of the imaging. The AAPM recommends that the postimplant evaluation should be performed at the optimum time for specific radionuclides. In addition, they encourage the use of a radiobiological model with a specific set of parameters to facilitate relative comparisons of treatment plans reported by different institutions using different loading patterns or radionuclides.

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.

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.

Third-party brachytherapy source calibrations and physicist responsibilities: Report of the AAPM Low Energy Brachytherapy Source Calibration Working Group

The AAPM Low Energy Brachytherapy Source Calibration Working Group was formed to investigate and recommend quality control and quality assurance procedures for brachytherapy sources prior to clinical use. Compiling and clarifying recommendations established by previous AAPM Task Groups 40, 56, and 64 were among the working groups charges, which also included the role of third-party handlers to perform loading and assay of sources. This document presents the findings of the working group on the responsibilities of the institutional medical physicist and a clarification of the existing AAPM recommendations in the assay of brachytherapy sources. Responsibility for the performance and attestation of source assays rests with the institutional medical physicist, who must use calibration equipment appropriate for each source type used at the institution. Such equipment and calibration procedures shall ensure secondary traceability to a national standard. For each multi-source implant, 10% of the sources or ten sources, whichever is greater, are to be assayed. Procedures for presterilized source packaging are outlined. The mean source strength of the assayed sources must agree with the manufacturers stated strength to within 3%, or action must be taken to resolve the difference. Third party assays do not absolve the institutional physicist from the responsibility to perform the institutional measurement and attest to the strength of the implanted sources. The AAPM leaves it to the discretion of the institutional medical physicist whether the manufacturers or institutional physicists measured value should be used in performing dosimetry calculations.

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.

Recommendations for permanent prostate brachytherapy with 131Cs: A consensus report from the Cesium Advisory Group

PURPOSE Published clinical information on the safety and efficacy of (131)Cs implants is limited. We provide consensus recommendations for (131)Cs prostate brachytherapy based on experience to date. METHODS AND MATERIALS The Cesium Advisory Group (CAG) consists of experienced (131)Cs users. Recommendations are based on three clinical trials, one of which has completed accrual and has been published in the peer reviewed literature, and combined CAG experience of more than 1200 (131)Cs implants. RESULTS We recommend using 1.059cGyh(-1)U(-1) as the dose rate constant for the IsoRay source. The prescription for monotherapy implants is 115Gy and when combined with 45-50Gy external beam it is 85Gy. Suggested individual source strength ranges from 1.6 to 2.2U. The release criterion for (131)Cs implants is 6mRh(-1) at 1m. (131)Cs brachytherapy should be performed differently from (125)I and (103)Pd brachytherapy: source placement is further from the urethra and rectum; the prostate V(150) should be < or =45%; sufficient margins may be obtained while limiting source placement to the capsule or close to the capsule. The increased dose rate may cause degradation of postimplant quantifiers due to edema. However, large variability in the magnitude and rate of resolution of edema make determination of the most representative postoperative imaging time impossible. The CAG recommends postimplant imaging on the day of the implant. Recommended postimplant evaluation goals include prostate D(90) greater than the prescription dose; maintaining D(u)(,30)<140% of the prescription dose and keeping V(r)(,100)<0.5cm(3). CONCLUSION It was the consensus of the CAG that optimal (131)Cs implants should be performed differently from those performed with (125)I or (103)Pd. Guidelines have been established to allow for safe and effective delivery of (131)Cs prostate brachytherapy.

Characterization of an in vivo diode dosimetry system for clinical use.

An in vivo dosimetry system that uses p‐type semiconductor diodes with buildup caps was characterized for clinical use on accelerators ranging in energy from 4 to 18 MV. The dose per pulse dependence was investigated. This was done by altering the source‐surface distance, field size, and wedge for photons. The off‐axis correction and effect of changing repetition rate were also investigated. A model was developed to fit the measured two‐dimensional diode correction factors. PACS number(s): 87.66.–a, 87.52.–g

Point: Cesium-131: Ready for prime time

The first permanent prostate implants with Cs were performed in late 2004. In less than 4 years, the brachytherapy community has performed almost 3000 implants at more than 50 sites (1). Still, and appropriately so, the question lingers: do we know enough about permanent prostate brachytherapy (PPB) with Cs that it may safely be used by all brachytherapists who routinely perform PPB for early-stage disease? Or should the use of Cs be limited to only a handful of the most experienced practitioners? The decision to use any particular treatment modality clearly lies in the joined hands of the practitioner and the patient. But inherent in the practitioner’s offer of treatment is the subjective belief that the outcome will be a positive one, the treatment intent will be met within an acceptable level of risk for either failure or unacceptable treatment complications. Based on these criteria, I will present arguments that support the general use of Cs for permanent prostate brachytherapy. These arguments will outline our present level of knowledge about Cs brachytherapyd radiobiologic, radiologic and clinical; and characterize this knowledge in terms of historical perspective.

SU‐FF‐T‐131: Clinical Use of Linear Array MOSFET for Urethral Dose Verification in Prostate High Dose Rate Brachytherapy

Purpose: To investigate the use of linear array MOSFET as in vivo dosimetry detector to determine the urethral dose for a single and multiple fraction during the prostate HDR treatment. Method and Materials: Commercially available Linear Array MOSFETs with 5 individual MOSFET was inserted into the 18 gage Foley catheter right after the HDR prostate implant. Measurements were performed in 25 patients receiving total of 2400cGy HDR boost in 4 fractions with 600cGy per fraction. The urethra dose was measured right after first fraction for all the patients and also subsequent fraction in 5 patients in terms of reproducibility of urethra dose. The exact location of the MOSFET was determined using radio‐opaque marker and the point dose for each MOSFET was determined using CT‐base treatment planning.Results: A Linear Array MOSFETs was placed in such a way that the first MOSFET being slightly above the bladder neck with the average reading of 75%±18% of the prescribed dose since it is beyond the base of the prostate. The dose was increased to maximum of 128% of the total dose within the prostate gland and decreased to 40% or less of the total dose beyond the apex of the gland. There was an excellent correlation of 2.8% between the MOSFET reading and treatment planningdose calculations. The MOSFET reading comparison between first and second fraction also correlated within 2.3%. Conclusion:MOSFETs are suitable for in vivodosimetry during prostate high dose rate brachyhterpy not only to verify the dose across the urethra but also to verify that the needles are maintained in its exact same position as the first fraction. Any unexpected variation in urethra dose compared to initial treatment plan can be corrected in the subsequent fraction as a result of this dose verification procedure.