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

Redefining bladder neck dose in low-dose-rate prostate brachytherapy-Can we improve urinary toxicity without impacting disease control?

BACKGROUND

We sought to assess the impact of bladder neck dose (BND) on patient reported urinary toxicity, and feasibility of relative urethral sparing technique in prostate brachytherapy (PB).

METHODS AND MATERIALS

We retrospectively identified bladder neck as a point dose on post-implant CT scans in patients treated with 131Cs PB. Urinary symptoms were assessed through EPIC questionnaires. Patient cohorts were identified based on mean BND as a percentage of prescription dose with toxicity assessment at each time point.

RESULTS

In our cohort of 542 patients, BND was associated with clinically significant acute urinary symptoms and chronic symptoms, as patients receiving >70% of the prescription dose had significantly worse overall EPIC scores than patients receiving ≤70% of prescription dose. There was no difference in bDFS between patients receiving BND ≤70% (96% bDFS) and >70% (94% bDFS) at a median follow up of 57 months.

CONCLUSIONS

BND has a significant impact on both acute and chronic urinary symptoms, with reduced symptoms reported with BND <70% of prescription dose. With a median follow up of 4.7 years, excellent bDFS has thus far been achieved with relative urethral and bladder neck sparing. Utilizing this constraint should improve urinary symptoms without impacting disease control.

ACR-ABS-ASTRO Practice Parameter for Transperineal Permanent Brachytherapy of Prostate Cancer

AIM/OBJECTIVES/BACKGROUND

The American College of Radiology (ACR), American Brachytherapy Society (ABS), and American Society for Radiation Oncology (ASTRO) have jointly developed the following practice parameter for transperineal permanent brachytherapy of prostate cancer. Transperineal permanent brachytherapy of prostate cancer is the interstitial implantation of low-dose rate radioactive seeds into the prostate gland for the purpose of treating localized prostate cancer.

METHODS

This practice parameter was developed according to the process described under the heading The Process for Developing ACR Practice Parameters and Technical Standards on the ACR website (https://www.acr.org/Clinical-Resources/Practice-Parameters-and-Technical-Standards) by the Committee on Practice Parameters-Radiation Oncology of the Commission on Radiation Oncology, in collaboration with ABS and ASTRO.

RESULTS

This practice parameter provides a framework for the appropriate use of low-dose rate brachytherapy in the treatment of prostate cancer either as monotherapy or as part of a treatment regimen combined with external-beam radiation therapy. The practice parameter defines the qualifications and responsibilities of all involved radiation oncology personnel, including the radiation oncologist, medical physicist, dosimetrist, radiation therapist, and nursing staff. Patient selection criteria and the utilization of supplemental therapies such as external-beam radiation therapy and androgen deprivation therapy are discussed. The logistics of the implant procedure, postimplant dosimetry assessment, and best practices with regard to safety and quality control are presented.

CONCLUSIONS

Adherence to established standards can help to ensure that permanent prostate brachytherapy is delivered in a safe and efficacious manner.

Treatment of intermediate-risk prostate cancer with Cs-131: Long-term results from a single institution

PURPOSE

To evaluate our institutional outcomes utilizing Cs-131 prostate brachytherapy (PB) for the intermediate-risk (IR) group of prostate cancer patients.

METHODS AND MATERIALS

We reviewed a prospectively collected database of men treated with Cs-131 PB between 2006 and 2019. Patients with less than 24-months follow-up were excluded. Patients were classified as IR if they had one of the following factors: Gleason Score 7, prostate specific antigen >10 but <20 ng/mL, or T2b-c on clinical exam. We defined unfavorable-IR (UIR) as having either Grade Group 3, >1 IR factors, or ≥50% positive core biopsies. The Kaplan-Meier method was used to estimate actuarial event-time probabilities for biochemical freedom from disease (BFD).

RESULTS

A total of 335 patients with a median follow-up of 70.1 months (IQR 48.3-106.3 months) were identified. Androgen deprivation therapy (ADT) was used in 7.2% of patients. Favorable-IR (FIR) patients were commonly treated with PB alone (91.8%). FIR patients who underwent PB alone had a 5-year BFD of 98.1%. UIR patients were commonly treated with external beam radiotherapy plus PB (61.2%). These patients had 5-year BFD of 91.1%. The 5-year BFD for UIR patients treated without ADT was 90.9%, whereas it was 95.0% among UIR patients treated with ADT (log-rank p = 0.83).

CONCLUSIONS

FIR patients have excellent outcomes when treated with PB alone. External beam radiotherapy plus PB is a reasonable treatment approach for UIR patients. Future studies may elucidate which IR patients would benefit from treatment intensification.

Low dose rate brachytherapy for primary treatment of localized prostate cancer: A systemic review and executive summary of an evidence-based consensus statement

PURPOSE

The purpose of this guideline is to present evidence-based consensus recommendations for low dose rate (LDR) permanent seed brachytherapy for the primary treatment of prostate cancer.

METHODS AND MATERIALS

The American Brachytherapy Society convened a task force for addressing key questions concerning ultrasound-based LDR prostate brachytherapy for the primary treatment of prostate cancer. A comprehensive literature search was conducted to identify prospective and multi-institutional retrospective studies involving LDR brachytherapy as monotherapy or boost in combination with external beam radiation therapy with or without adjuvant androgen deprivation therapy. Outcomes included disease control, toxicity, and quality of life.

RESULTS

LDR prostate brachytherapy monotherapy is an appropriate treatment option for low risk and favorable intermediate risk disease. LDR brachytherapy boost in combination with external beam radiation therapy is appropriate for unfavorable intermediate risk and high-risk disease. Androgen deprivation therapy is recommended in unfavorable intermediate risk and high-risk disease. Acceptable radionuclides for LDR brachytherapy include iodine-125, palladium-103, and cesium-131. Although brachytherapy monotherapy is associated with increased urinary obstructive and irritative symptoms that peak within the first 3 months after treatment, the median time toward symptom resolution is approximately 1 year for iodine-125 and 6 months for palladium-103. Such symptoms can be mitigated with short-term use of alpha blockers. Combination therapy is associated with worse urinary, bowel, and sexual symptoms than monotherapy. A prostate specific antigen <= 0.2 ng/mL at 4 years after LDR brachytherapy may be considered a biochemical definition of cure.

CONCLUSIONS

LDR brachytherapy is a convenient, effective, and well-tolerated treatment for prostate cancer.

Intraprostatic calcification and biochemical recurrence in men treated with cesium-131 prostate brachytherapy

PURPOSE

Baseline intraprostatic calcification (IC) has been shown to be associated with a higher rate of biochemical recurrence (BCR) in men treated with iodine-125 prostate brachytherapy (PB). We evaluated this association in a cohort of men treated with cesium-131 PB.

METHODS AND MATERIALS

We retrospectively reviewed the charts of all low- and intermediate-risk prostate cancer patients treated with cesium-131 PB +/- external beam radiotherapy (EBRT) at our institution from 2/2011 to 7/2018. Patients with < 24 months of follow up or those who received androgen deprivation therapy were excluded. Baseline IC status (defined as one or more ICs ≥ 5 mm) was determined on post-PB CT scans. Cox analysis was used to assess predictors of BCR and Kaplan-Meier survival curves were calculated.

RESULTS

Two hundred and sixteen low- and intermediate-risk prostate cancer patients treated with cesium-131 PB +/- EBRT were included. Median follow up was 56.9 months (range 24.1-111.4). Overall, 76 (35.2%) patients had baseline IC and 140 (64.8%) did not. Baseline disease characteristics did not differ significantly between groups. On univariate Cox analysis, only risk group (p = 0.047) and initial PSA (p = 0.016) were significant predictors of BCR, whereas baseline IC was not (p = 0.11). The 5-year BCR-free survival in patients with versus without baseline IC was 97.7% versus 93.8% (p = 0.405), respectively.

CONCLUSIONS

In a cohort of low- and intermediate-risk prostate cancer patients treated with cesium-131 PB, the rate of BCR in men with baseline IC was low and baseline IC was not associated with a higher risk of BCR.

First clinical implementation of GammaTile permanent brain implants after FDA clearance

PURPOSE

GammaTile cesium-131 (¹³¹Cs) permanent brain implant has received Food and Drug Administration (FDA) clearance as a promising treatment for certain brain tumors. Our center was the first institution in the United States after FDA clearance to offer the clinical use of GammaTile brachytherapy outside of a clinical trial. The purpose of this work is to aid the medical physicist and radiation oncologist in implementing this collagen carrier tile brachytherapy (CTBT) program in their practice.

METHODS

A total of 23 patients have been treated with GammaTile to date at our center. Treatment planning system (TPS) commissioning was performed by configuring the parameters for the ¹³¹Cs (IsoRay Model CS-1, Rev2) source, and doses were validated with the consensus data from the American Association of Physicists in Medicine TG-43U1S2. Implant procedures, dosimetry, postimplant planning, and target delineations were established based on our clinical experience. Radiation safety aspects were evaluated based on exposure rate measurements of implanted patients, as well as body and ring badge measurements.

RESULTS

An estimated timeframe of the GammaTile clinical responsibilities for the medical physicist, radiation oncologist, and neurosurgeon is presented. TPS doses were validated with published dose to water for ¹³¹Cs. Clinical aspects, including estimation of the number of tiles, treatment planning, dosimetry, and radiation safety considerations, are presented.

CONCLUSION

The implementation of the GammaTile program requires collaboration from multiple specialties, including medical physics, radiation oncology, and neurosurgery. This manuscript provides a roadmap for the implementation of this therapy.

Long-term biochemical outcomes using cesium-131 in prostate brachytherapy

PURPOSE

Long-term outcomes reveal equivalent biochemical outcomes with low-dose-rate (LDR) brachytherapy (BT) compared with radical prostatectomy and external-beam radiotherapy for the management of prostate cancer. Iodine-125, the most commonly used isotope, may be associated with long-term urinary consequences. Cesium-131 (¹³¹Cs) has a higher dose rate and shorter dose delivery time, predicting a shorter duration of urinary morbidity. We report our institution's high-volume experience and the most mature data to date on outcomes with ¹³¹Cs prostate BT.

METHODS AND MATERIALS

571 men (median age: 65.38 years) with low (55%)-, intermediate (36%)-, and high-risk disease (9%) received monobrachytherapy, dual-modality, or trimodality using ¹³¹Cs at a single institution. Risk groups were defined according to the National Comprehensive Cancer Network definition. Median prescription dose for definitive LDR-BT and LDR-BT boost was 115 Gy and 70 Gy, respectively. Median initial PSA was 6.1 ng/mL (IQR: 4.6-8.7).

RESULTS

Median followup time was 5 years. 5/7-year overall survival for low-, intermediate-, and high-risk patients was 96.9%/96/9%, 92.8%/89.7%, and 95.8%/87.1%, respectively (p = 0.02). 5/7-year freedom from biochemical failure for low-, intermediate-, and high-risk patients was 98.5%/96.3%, 94.1%/86.4%, and 93.2%/74.5%, respectively (p < 0.01). 5/7-year prostate cancer -specific survival was 100%/100%, 99.3%/99.3%, and 98.0%/98.0% for low-, intermediate-, and high-risk patients, respectively (p < 0.01).

CONCLUSIONS

¹³¹Cs is a viable alternative isotope for prostate brachytherapy for organ-confined disease. Long-term biochemical control and survival outcomes are excellent and on par with those attained with the use of ¹²⁵I or ¹⁰³Pd. This report therefore supports the continued use of ¹³¹Cs as an effective and comparable alternative isotope.

A nomogram to determine required seed air kerma strength in planar 131Cesium permanent seed implant brachytherapy

PURPOSE

Intraoperatively implanted Cesium-131 (131Cs) permanent seed brachytherapy is used to deliver highly localized re-irradiation in recurrent head and neck cancers. A single planar implant of uniform air kerma strength (AKS) seeds and 10 mm seed-to-seed spacing is used to deliver the prescribed dose to a point 5 mm or 10 mm perpendicular to the center of the implant plane. Nomogram tables to quickly determine the required AKS for rectangular and irregularly shaped implants were created and dosimetrically verified. By eliminating the need for a full treatment planning system plan, nomogram tables allow for fast dose calculation for intraoperative re-planning and for a second check method.

MATERIAL AND METHODS

TG-43U1 recommended parameters were used to create a point-source model in MATLAB. The dose delivered to the prescription point from a single 1 U seed at each possible location in the implant plane was calculated. Implant tables were verified using an independent seed model in MIM Symphony LDR™. Implant tables were used to retrospectively determine seed AKS for previous cases: three rectangular and three irregular.

RESULTS

For rectangular implants, the percent difference between required seed AKS calculated using MATLAB and MIM was at most 0.6%. For irregular implants, the percent difference between MATLAB and MIM calculations for individual seed locations was within 1.5% with outliers of less than 3.1% at two distal locations (10.6 cm and 8.8 cm), which have minimal dose contribution to the prescription point. The retrospectively determined AKS for patient implants using nomogram tables agreed with previous calculations within 5% for all six cases.

CONCLUSIONS

Nomogram tables were created to determine required AKS per seed for planar uniform AKS 131Cs implants. Comparison with the treatment planning system confirms dosimetric accuracy that is acceptable for use as a second check or for dose calculation in cases of intraoperative re-planning.

UK & Ireland Prostate Brachytherapy Practice Survey 2014-2016

Purpose

To document the current prostate brachytherapy practice across the UK and Ireland and compare with previously published audit results.

Material and methods

Participants from 25 centers attending the annual UK & Ireland Prostate Brachytherapy Conference were invited to complete an online survey. Sixty-three questions assessed the center’s experience and staffing, clinician’s experience, clinical selection criteria and scheduling, number of cases per modality in the preceding three years, low-dose-rate (LDR) pre- and post-implant technique and high-dose-rate (HDR) implant technique. Responses were collated, and descriptive statistical analysis performed.

Results

Eighteen (72%) centers responded with 17 performing LDR only, 1 performing HDR only, and 6 performing both LDR and HDR. Seventy-one percent of centers have > 10 years of LDR brachytherapy experience, whereas 71% centers that perform HDR brachytherapy have > 5 years of experience. Thirteen centers have 2 or more clinicians performing brachytherapy with 61% of lead consultants performing > 25 cases (LDR + HDR) in 2016. The number of implants (range), that includes LDR and HDR, performed by individual practitioners in 2016 was > 50 by 21%, 25-50 by 38%, and < 25 by 41%. Eight centers reported a decline in LDR monotherapy case numbers in 2016. Number of center’s performing HDR brachytherapy increased in last five years. Relative uniformity in patient selection is noted, and LDR pre- and post-implant dosimetry adheres to published quality guidelines, with an average post-implant D₉₀ of > 145 Gy in 69% of centers in 2014 and 2015 compared to 63% in 2016. The median CT/US volume ratios were > 0.9 ≤ 1.0 (n = 4), > 1.0 ≤ 1.1 (n = 7), and > 1.1 (n = 2).

Conclusion

There is considerable prostate brachytherapy experience in the UK and Ireland. An apparent fall in LDR case numbers is noted. Maintenance of case numbers and ongoing compliance with published quality guidelines is important to sustain high quality outcomes.

Low dose rate prostate brachytherapy

Low dose rate (LDR) prostate brachytherapy is an evidence based radiation technique with excellent oncologic outcomes. By utilizing direct image guidance for radioactive source placement, LDR brachytherapy provides superior radiation dose escalation and conformality compared to external beam radiation therapy (EBRT). With this level of precision, late grade 3 or 4 genitourinary or gastrointestinal toxicity rates are typically between 1% and 4%. Furthermore, when performed as a same day surgical procedure, this technique provides a cost effective and convenient strategy. A large body of literature with robust follow-up has led multiple expert consensus groups to endorse the use of LDR brachytherapy as an appropriate management option for all risk groups of non-metastatic prostate cancer. LDR brachytherapy is often effective when delivered as a monotherapy, although for some patients with intermediate or high-risk disease, optimal outcome are achieved in combination with supplemental EBRT and/or androgen deprivation therapy (ADT). In addition to reviewing technical aspects and reported clinical outcomes of LDR prostate brachytherapy, this article will focus on the considerations related to appropriate patient selection and other aspects of its use in the treatment of prostate cancer.

The evolution of brachytherapy for prostate cancer

Brachytherapy (BT), using low-dose-rate (LDR) permanent seed implantation or high-dose-rate (HDR) temporary source implantation, is an acceptable treatment option for select patients with prostate cancer of any risk group. The benefits of HDR-BT over LDR-BT include the ability to use the same source for other cancers, lower operator dependence, and - typically - fewer acute irritative symptoms. By contrast, the benefits of LDR-BT include more favourable scheduling logistics, lower initial capital equipment costs, no need for a shielded room, completion in a single implant, and more robust data from clinical trials. Prospective reports comparing HDR-BT and LDR-BT to each other or to other treatment options (such as external beam radiotherapy (EBRT) or surgery) suggest similar outcomes. The 5-year freedom from biochemical failure rates for patients with low-risk, intermediate-risk, and high-risk disease are >85%, 69-97%, and 63-80%, respectively. Brachytherapy with EBRT (versus brachytherapy alone) is an appropriate approach in select patients with intermediate-risk and high-risk disease. The 10-year rates of overall survival, distant metastasis, and cancer-specific mortality are >85%, <10%, and <5%, respectively. Grade 3-4 toxicities associated with HDR-BT and LDR-BT are rare, at <4% in most series, and quality of life is improved in patients who receive brachytherapy compared with those who undergo surgery.

ACR appropriateness criteria: Permanent source brachytherapy for prostate cancer

PURPOSE

To provide updated American College of Radiology (ACR) appropriateness criteria for transrectal ultrasound-guided transperineal interstitial permanent source brachytherapy.

METHODS AND MATERIALS

The ACR appropriateness criteria are evidence-based guidelines for specific clinical conditions that are reviewed every 3 years by a multidisciplinary expert panel. The guideline development and review include an extensive analysis of current medical literature from peer reviewed journals and the application of a well-established consensus methodology (modified Delphi) to rate the appropriateness of imaging and treatment procedures by the panel. In those instances where evidence is lacking or not definitive, expert opinion may be used to recommend imaging or treatment.

RESULTS

Permanent prostate brachytherapy (PPB) is a treatment option for appropriately selected patients with localized prostate cancer with low to very high risk disease. PPB monotherapy remains an appropriate and effective curative treatment for low-risk prostate cancer patients demonstrating excellent long-term cancer control and acceptable morbidity. PPB monotherapy can be considered for select intermediate-risk patients with multiparametric MRI useful in evaluation of such patients. High-risk patients treated with PPB should receive supplemental external beam radiotherapy (EBRT) along with androgen deprivation. Similarly, patients with involved pelvic lymph nodes may also be considered for such combined treatment but reported long-term outcomes are limited. Computed tomography-based postimplant dosimetry completed within 60 days of PPB is essential for quality assurance. PPB may be considered for treatment of local recurrence after EBRT but is associated with an increased risk of toxicity.

CONCLUSIONS

Updated appropriateness criteria for patient evaluation, selection, treatment, and postimplant dosimetry are given. These criteria are intended to be advisory only with the final responsibility for patient care residing with the treating clinicians.

Determination of prescription dose for Cs-131 permanent implants using the BED formalism including resensitization correction

PURPOSE

The current widely used biological equivalent dose (BED) formalism for permanent implants is based on the linear-quadratic model that includes cell repair and repopulation but not resensitization (redistribution and reoxygenation). The authors propose a BED formalism that includes all the four biological effects (4Rs), and the authors propose how it can be used to calculate appropriate prescription doses for permanent implants with Cs-131.

METHODS

A resensitization correction was added to the BED calculation for permanent implants to account for 4Rs. Using the same BED, the prescription doses with Au-198, I-125, and Pd-103 were converted to the isoeffective Cs-131 prescription doses. The conversion factor F, ratio of the Cs-131 dose to the equivalent dose with the other reference isotope (Fr: with resensitization, Fn: without resensitization), was thus derived and used for actual prescription. Different values of biological parameters such as α, β, and relative biological effectiveness for different types of tumors were used for the calculation.

RESULTS

Prescription doses with I-125, Pd-103, and Au-198 ranging from 10 to 160 Gy were converted into prescription doses with Cs-131. The difference in dose conversion factors with (Fr) and without (Fn) resensitization was significant but varied with different isotopes and different types of tumors. The conversion factors also varied with different doses. For I-125, the average values of Fr/Fn were 0.51/0.46, for fast growing tumors, and 0.88/0.77 for slow growing tumors. For Pd-103, the average values of Fr/Fn were 1.25/1.15 for fast growing tumors, and 1.28/1.22 for slow growing tumors. For Au-198, the average values of Fr/Fn were 1.08/1.25 for fast growing tumors, and 1.00/1.06 for slow growing tumors. Using the biological parameters for the HeLa/C4-I cells, the averaged value of Fr was 1.07/1.11 (rounded to 1.1), and the averaged value of Fn was 1.75/1.18. Fr of 1.1 has been applied to gynecological cancer implants with expected acute reactions and outcomes as expected based on extensive experience with permanent implants. The calculation also gave the average Cs-131 dose of 126 Gy converted from the I-125 dose of 144 Gy for prostate implants.

CONCLUSIONS

Inclusion of an allowance for resensitization led to significant dose corrections for Cs-131 permanent implants, and should be applied to prescription dose calculation. The adjustment of the Cs-131 prescription doses with resensitization correction for gynecological permanent implants was consistent with clinical experience and observations. However, the Cs-131 prescription doses converted from other implant doses can be further adjusted based on new experimental results, clinical observations, and clinical outcomes.

Dosimetry for 131Cs and 125I seeds in solid water phantom using radiochromic EBT film

PURPOSE

To measure the 2D dose distributions with submillimeter resolution for (131)Cs (model CS-1 Rev2) and (125)I (model 6711) seeds in a Solid Water phantom using radiochromic EBT film for radial distances from 0.06cm to 5cm. To determine the TG-43 dosimetry parameters in water by applying Solid Water to liquid water correction factors generated from Monte Carlo simulations.

METHODS

Each film piece was positioned horizontally above and in close contact with a (131)Cs or (125)I seed oriented horizontally in a machined groove at the center of a Solid Water phantom, one film at a time. A total of 74 and 50 films were exposed to the (131)Cs and (125)I seeds, respectively. Different film sizes were utilized to gather data in different distance ranges. The exposure time varied according to the seed air-kerma strength and film size in order to deliver doses in the range covered by the film calibration curve. Small films were exposed for shorter times to assess the near field, while larger films were exposed for longer times in order to assess the far field. For calibration, films were exposed to either 40kV (M40) or 50kV (M50) x-rays in air at 100.0cm SSD with doses ranging from 0.2Gy to 40Gy. All experimental, calibration and background films were scanned at a 0.02cmpixel resolution using a CCD camera-based microdensitometer with a green light source. Data acquisition and scanner uniformity correction were achieved with Microd3 software. Data analysis was performed using ImageJ, FV, IDL and Excel software packages. 2D dose distributions were based on the calibration curve established for 50kV x-rays. The Solid Water to liquid water medium correction was calculated using the MCNP5 Monte Carlo code. Subsequently, the TG-43 dosimetry parameters in liquid water medium were determined.

RESULTS

Values for the dose-rate constants using EBT film were 1.069±0.036 and 0.923±0.031cGyU(-1)h(-1) for (131)Cs and (125)I seed, respectively. The corresponding values determined using the Monte Carlo method were 1.053±0.014 and 0.924±0.016cGyU(-1)h(-1) for (131)Cs and (125)I seed, respectively. The radial dose functions obtained with EBT film measurements and Monte Carlo simulations were plotted for radial distances up to 5cm, and agreed within the uncertainty of the two methods. The 2D anisotropy functions obtained with both methods also agreed within their uncertainties.

CONCLUSION

EBT film dosimetry in a Solid Water phantom is a viable method for measuring (131)Cs (model CS-1 Rev2) and (125)I (model 6711) brachytherapy seed dose distributions with submillimeter resolution. With the Solid Water to liquid water correction factors generated from Monte Carlo simulations, the measured TG-43 dosimetry parameters in liquid water for these two seed models were found to be in good agreement with those in the literature.

Radiobiological comparison of single and dual-isotope prostate seed implants

Purpose: Several isotopes are available for low dose-rate prostate brachytherapy. Currently most implants use a single isotope. However, the use of dual-isotope implants may yield an advantageous combination of characteristics such as half-life and relative biological effectiveness. However, the use of dual-isotope implants complicates treatment planning and quality assurance. Do the benefits of dual-isotope implants outweigh the added difficulty? The goal of this work was to use a linear-quadratic model to compare single and dual-isotope implants.

Materials & Methods: Ten patients were evaluated. For each patient, six treatment plans were created with single or dual-isotope combinations of 125I, 103Pd and 131Cs. For each plan the prostate, urethra, rectum and bladder were contoured by a physician. The biologically effective dose was used to determine the tumor control probability and normal tissue complication probabilities for each plan. Each plan was evaluated using favorable, intermediate and unfavorable radiobiological parameters. The results of the radiobiological analysis were used to compare the single and dual-isotope treatment plans.

Results: Iodine-125 only implants were seen to be most affected by changes in tumor parameters. Significant differences in organ response probabilities were seen at common dose levels. However, after adjusting the initial seed strength the differences between isotope combinations were minimal.

Conclusions: The objective of this work was to perform a radiobiologically based comparison of single and dual-isotope prostate seed implant plans. For all isotope combinations, the plans were improved by varying the initial seed strength. For the optimized treatment plans, no substantial differences in predicted treatment outcomes were seen among the different isotope combinations.

Changes in radiobiological parameters in 131Cs permanent prostate implants

In prostate permanent implants using 131Cs seeds, the prostatic edema developed during the implantation procedure, increases the separation between the seeds. This leads to a decrease in the prostate coverage and thus causes an edema induced dose reduction, which results in an increase in tumour cell surviving fraction (SF) with a corresponding decrease in tumour control probability (TCP). To investigate the impact of edema on the SF and the TCP, the expression of the SF of the linear quadratic (LQ) model was extended to account for the effects of edema using the exponential nature of edema resolution and the dose delivered to the edematous prostate. The SF and the TCP for edematous prostate implants were calculated for 31 patients who underwent real time 131Cs permanent seed implantation. The dose delivered to the edematous prostate was calculated to compute the SF and the TCP for these patients for edema half lives (EHL) ranging from 4 days to 34 days and for edemas of magnitudes (M0) varying from 5 to 60% of the actual prostate volume.

A reduction in the dose delivered to the edematous prostate was found with the increase of EHL and edema magnitude which results in an increase of the SF, and corresponding decrease in the TCP. The dose reductions in 131Cs implants varied from 1.1% (for EHL = 4 days and M0 = 5%) to 32.3% (for EHL = 34 days and M0 = 60%). These are higher than the dose reduction in 125I implants, which vary from 0.3% (for EHL = 4 days and M0 = 5%) to 17.5% (for EHL = 34 days and M0 = 60%). As edema half life increased from 4 days to 34 days and edema magnitude increased from 5 to 60% the SF increased by 4.57 log, and the TCP decreased by 0.80. Compensation of edema induced increase in the SF and decrease in the TCP in 131Cs seed implants should be carefully done by redefining seed positions with the guidance of post-needle plans. The presented model in this study can be used to estimate the SF or the TCP for pre plan or real time permanent prostate implants using day 0 post-implant CT images.

Edema-induced changes in tumor cell surviving fraction and tumor control probability in 131Cs permanent prostate brachytherapy implant patients

The study is designed to investigate the effect of edema on the delivered dose, tumor cell surviving fraction (SF), and tumor control probability (TCP) in the patients of prostate cancer who underwent  131Cs permanent seed implantation. The dose reduction, the SF, and the TCP for edematous prostate implants were calculated for 31 patients who underwent real‐time  131Cs permanent seed implantation for edema half‐lives (EHL), ranging from 4 days to 34 days and for edema magnitudes (M0) varying from 5% to 60% of the actual prostate volume. A dose reduction in  131Cs implants varied from 1.1% (for EHL=4 days and M0=5%) to 32.3% (for EHL=34 days and M0=60%). These are higher than the dose reduction in  125I implants, which vary from 0.3% (for EHL=4 days and M0=5%) to 17.5% (for EHL=34 days and M0=60%). As EHL increased from 4 days to 34 days and edema magnitude increased from 5% to 60%, the natural logarithmic value of SF increased by 4.57 and the TCP decreased by 0.80. Edema induced increase in the SF and decrease in the TCP in  131Cs seed implants, is significantly more pronounced in a combination of higher edema magnitude and larger edema half‐lives than for less edema magnitude and lower edema half‐lives, as compared for M0=60% and EHL=34, and M0=5% and EHL=4 days.

PACS number: 87.53.Jw

Calculating prescription doses for new sources by biologically effective dose matching

PURPOSE

In current clinical practice, single isotopes, such as (125)I or (103)Pd, are used as single sources in prostate seed implants. A mixture of two radionuclides in the seeds has been proposed for prostate cancer treatment. This study investigates a method for determining the prescription dose for these new seeds using the biological effective dose (BED).

METHODS

Ten prostate cancer cases previously treated using single radionuclide seeds were selected for this study. The BED distribution for these cases was calculated. Plans using other radionuclides were then calculated based on this BED distribution. Prescription values could then be obtained for the calculated plans. The method was verified by calculating the prescription dose for (103)Pd and (125)I and comparing to clinical values. The method was then applied to a hybrid seed that consisted of a mixture of (125)I and (103)Pd radionuclides, which deliver equal dose to 1cm from the source in water (50/50D@1 cm). A prescription BED value was also calculated.

RESULTS

A prescription BED of 110 Gy was found to correlate to a prescription dose of 145, 120, and 137 Gy for (125)I, (103)Pd, and 50/50D@1 cm hybrid seeds, respectively.

CONCLUSION

The method introduced in this article allows one to calculate the prescription dose for new and novel sources in brachytherapy. The method was verified by calculating a prescription dose for (125)I and (103)Pd radionuclides that coincides with values used clinically.

Report of a consensus meeting on focal low dose rate brachytherapy for prostate cancer

What's known on the subject? and What does the study add? Whole gland brachytherapy has been used to successfully treat prostate cancer but the protocol for focal therapy has not previously been established. The consensus findings provide guidance on patient selection for focal brachytherapy as well as recommendations for conducting therapy and patient follow-up. Low dose rate prostate brachytherapy is an effective treatment for localized prostate cancer. Recently, it has been considered for use in a focused manner whereby treatment is targeted only to areas of prostate cancer. The objective of focal brachytherapy is to provide effective cancer control for low-risk disease but with reduced genitourinary and rectal side-effects in a cost-effective way. We report on the outputs of a consensus meeting of international experts in brachytherapy and focal therapy convened to consider the feasibility and potential development of focal brachytherapy. A number of factors were considered for focal brachytherapy including optimal patient selection, disease characterization and localization, treatment protocols and outcome measures. The consensus meeting also addressed the design of a clinical trial that would assess the oncological outcomes and side-effect profiles resulting from focal brachytherapy.

Timing of postseed imaging influences rectal dose-volume parameters for cesium-131 prostate seed implants

PURPOSE

To study the influence of timing of postseed implant imaging on rectal dose-volume parameters for cesium-131 ((131)Cs) seed prostate implants.

METHODS AND MATERIALS

Fifteen patients were treated in our institution with combination (131)Cs brachytherapy followed by pelvic external beam radiation therapy for intermediate to high-risk prostate cancers. For all patients, CT scans were scheduled at 7 days (CT(7)) and again at 2 months for external beam radiation therapy simulation purpose (CT(60)) postseed implantation. Comprehensive postseed implant dosimetry was performed for both CT(7) and CT(60) scans. In each case, dose-volume histogram parameters, rectal separation (the distance between the center of posterior most seed and most anterior rectal wall), and posterior row activity (the total activity implanted within 2-4mm anterior to the posterior wall of the prostate) data were collected. The absolute rectal volumes receiving 100% and 110% prescription dose were also collected.

RESULTS

Rectal dose correlated strongly with rectal separation (p<0.001). The mean change in rectal separation between CT(7) and CT(60) scans was 1.1 (±1.7) mm, and the corresponding change in 0.1-cc rectal dose was 18 (±26.5) Gy. Posterior row activity did not correlate with rectal dose (p=0.51). The mean volume of rectum that receives between 100% and 110% of the prescription dose (RV(100) and RV(110)) increased twofold, between CT(7) and CT(60) evaluations (0.03 [±0.06] cc vs. 0.07 (±0.05) cc, respectively, p=0.06).

CONCLUSIONS

Our study has demonstrated that rectal doses after (131)Cs seed implants are influenced by the timing of postseed imaging. This may be a consequence of prostatic and periprostatic edema resolution.

A review of low-dose-rate prostate brachytherapy--techniques and outcomes

Prostate cancer is the most common male cancer in the United States and the second leading cause of male cancer death. The main therapeutic modalities for the treatment of prostate cancer are surgery, external beam radiation therapy, hormonal therapy, and brachytherapy. In recent years, brachytherapy has been increasingly utilized for the treatment of early-stage prostate cancer. Technological advances, including improvements in imaging, planning, and postimplant quality assessment by dosimetry have led to widespread use of brachytherapy. Outcomes for prostate brachytherapy have been shown to be equivalent, in selected patients, to those of other treatment modalities for prostate cancer, including radical prostatectomy and external beam radiation therapy. Further, prostate brachytherapy has quality-of-life benefits in comparison to these other treatment modalities, particularly in the domain of sexual function. This paper describes the history of low-dose rate brachytherapy; current techniques for brachytherapy implantation and postoperative dosimetric evaluation; recent outcomes studies; recent quality-of-life analyses; and current and future prostate brachytherapy developments, including open clinical trials. As research in prostate brachytherapy continues, it is likely that this modality will play an increasingly important role in the treatment of early-stage prostate cancer patients in the future.

Dosimetric study of Cs-131, I-125, and Pd-103 seeds for permanent prostate brachytherapy

As a well-established single-modality approach for early-stage prostate cancer, transperineal interstitial permanent prostate brachytherapy (TIPPB) has gained increasing popularity due to its favorable clinical results. Currently, three isotopes, namely Cs-131, I-125, and Pd-103, are commercially available for TIPPB. This is the first study to systematically explore the dosimetric difference of these three isotopes for TIPPB. In total, 25 patients with T1-T2c prostate cancer previously implanted with I-125 seeds were randomly selected and replanned with Cs-131, I-125, and Pd-103 seeds to the prescription doses of 115, 145, and 125 Gy, respectively. The planning goals attempted were prostate V(p)100 approximately 95%, D(p)90 >or= 100%, and prostatic urethra D(u)10 <or= 150%. The dosimetric parameters, as well as the number of seeds and needles required, were analyzed and compared. The mean homogeneity index (HI) was 0.59, 0.56, and 0.46 for Cs-131, I-125, and Pd-103 plans, respectively. The average D(u)10 was 124.6%, 125.7%, and 129.7%, respectively. The average rectum V(r)100 was 0.19, 0.22, and 0.31 cc, respectively. In addition, the average number of seeds was 57.9, 63.0, and 63.7, and the average number of needles required was 31.6, 32.9, and 33.6 for Cs-131, I-125, and Pd-103 seeds, respectively. This study demonstrates that TIPPB, utilizing Cs-131 seeds, allows for better dose homogeneity, while providing comparable prostate coverage and sparing of the urethra and rectum, with a comparable number of, or fewer, seeds and needles required, compared to I-125 or Pd-103 seeds. Further biological and clinical studies associated with Cs-131 are warranted.

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 125I or 103Pd sources, clinical use of 131Cs 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 quantitles, 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.

Effect of planning margin on dosimetric quality in 131Cs permanent prostate brachytherapy

PURPOSE

To investigate the dosimetric effect of planning margin in (131)Cs prostate seed implants.

METHODS AND MATERIALS

The transrectal ultrasonography images are obtained intraoperatively in 5-mm steps from base to apex. The prostate is contoured as clinical target volume (CTV). The CTV is enlarged with 3mm expansion except the posterior. The CTV and planning target volume (PTV) are then used as planning target for treatment planning, respectively. Dose calculations are performed using VariSeed treatment planning system using AAPM TG-43 formalism. The total activity implanted, target coverage (the percent of the prostate volume covered by the prescription dose, V(100); the dose that covers 90% of the prostate volume, D(90)) for CTV and PTV, dose inhomogeneity (the percentage volume of the prostate receiving 150% of the prescription dose [V(150)]), and the critical organ dose (the dose that covers 10% of the urethra volume [UD(10)] for urethra and the dose that covers 50% of the rectum volume [RD(50)] for rectum) are compared.

RESULTS

When CTV is used as target for planning, compared with PTV as planning target, the total activity implanted is decreased by 5.6%. Integral dose is thus lower by 5.6%. Coverage for CTV (CTV(100)) is increased by 0.3%. Coverage for PTV (PTV(100)) is lower by 2.1%. CTV(150) is increased by 13.6%. PTV(150) is higher by 2.5% with a standard deviation of 10.2%. Rectum dose (RD(50)) is lower by 4.5%. Urethra dose (UD(10)) is higher by 10.0%.

CONCLUSION

It is shown that the planning margin has minimal effect on dosimetric quality because of (131)Cs's gradual dose fall-off. Thus, it is possible to reduce or even eliminate planning margin using (131)Cs. The modest benefits in reducing the planning margin, such as lower total activity (lower integral dose), dose reduction to surrounding healthy tissues and reduced likelihood of seeds migration, can be achieved while adequate coverage is maintained.