High Dose Rate Prostate Brachytherapy Research Articles

Rapid multi-catheter segmentation for magnetic resonance image-guided catheter-based interventions.

Magnetic resonance imaging (MRI) is the gold standard for delineating cancerous lesions in soft tissue. Catheter-based interventions require the accurate placement of multiple long, flexible catheters at the target site. The manual segmentation of catheters in MR images is a challenging and time-consuming task. There is a need for automated catheter segmentation to improve the efficiency of MR-guided procedures. To develop and assess a machine learning algorithm for the detection of multiple catheters in magnetic resonance images used during catheter-based interventions. In this work, a 3D U-Net was trained to retrospectively segment catheters in scans acquired during clinical MR-guided high dose rate (HDR) prostate brachytherapy cases. To assess confidence in segmentation, multiple AI models were trained. On clinical test cases, average segmentation results were used to plan the brachytherapy delivery. Dosimetric parameters were compared to the original clinical plan. Data was obtained from 35 patients who underwent HDR prostate brachytherapy for focal disease with a total of 214 image volumes. 185 image volumes from 30 patients were used for training using a five-fold cross validation split to divide the data for training and validation. To generate confidence measures of segmentation accuracy, five trained models were generated. The remaining five patients (29 volumes) were used to test the performance of the trained model by comparison to manual segmentations of three independent observers and assessment of dosimetric impact on the final clinical brachytherapy plans. The network successfully identified 95% of catheters in the test set at a rate of 0.89s per volume. The multi-model method identified the small number of cases where AI segmentation of individual catheters was poor, flagging the need for user input. AI-based segmentation performed as well as segmentations by independent observers. Plan dosimetry using AI-segmented catheters was comparable to the original plan. The vast majority of catheters were accurately identified by AI segmentation, with minimal impact on plan outcomes. The use of multiple AI models provided confidence in the segmentation accuracy and identified catheter segmentations that required further manual assessment. Real-time AI catheter segmentation can be used during MR-guided insertions to assess deflections and for rapid planning of prostate brachytherapy.

Dosimetric benefits and preclinical performance of steerable needles in HDR prostate brachytherapy

Prostate cancer patients with an enlarged prostate and/or excessive pubic arch interference (PAI) are generally considered non-eligible for high-dose-rate (HDR) brachytherapy (BT). Steerable needles have been developed to make these patients eligible again. This study aims to validate the dosimetric impact and performance of steerable needles within the conventional clinical setting.HDR BT treatment plans were generated, needle implantations were performed in a prostate phantom, with prostate volume > 55 cm3 and excessive PAI of 10 mm, and pre- and post-implant dosimetry were compared considering the dosimetric constraints: prostate V100 > 95 % (13.50 Gy), urethra D0.1cm3 < 115 % (15.53 Gy) and rectum D1cm3 < 75 % (10.13 Gy).The inclusion of steerable needles resulted in a notable enhancement of the dose distribution and prostate V100 compared to treatment plans exclusively employing rigid needles to address PAI. Furthermore, the steerable needle plan demonstrated better agreement between pre- and post-implant dosimetry (prostate V100: 96.24 % vs. 93.74 %) compared to the rigid needle plans (79.13 % vs. 72.86 % and 87.70 % vs. 81.76 %), with no major changes in the clinical workflow and no changes in the clinical set-up.The steerable needle approach allows for more flexibility in needle positioning, ensuring a highly conformal dose distribution, and hence, HDR BT is a feasible treatment option again for prostate cancer patients with an enlarged prostate and/or excessive PAI.

Surgical management of rectal cancer with synchronous treatment of prostate cancer.

To assess the safety and efficacy of synchronous treatments for rectal (RC) and prostate (PC) cancers. Single-center retrospective study (2007-2021) of patients treated with neoadjuvant radiotherapy (RT) and total mesorectal excision (TME) for RC with synchronous PC treatment. The endpoints were 30-day postoperative severe complications, R0 resection rates, 3-year disease-free survival (DFS) and 3-year overall survival (OS). Among the 16 patients, 15 (93.7%) received neoadjuvant pelvic RT (40-50.4 Gray) followed by either transperineal high dose rate prostate brachytherapy (62.5%), prostate external RT boost (25.0%), or androgen deprivation therapy (ADT) alone (6.3%). One (6.3%) patient received neoadjuvant rectal brachytherapy and ADT. Pelvic RT was combined with chemotherapy in 87.5% of cases. TME was performed in all patients with low anterior resection (87.5%) or abdominoperineal resection (12.5%), primarily using minimally invasive surgery (87.5%). The R0 resection rate was 93.8%. Six (37.5%) patients experienced 30-day Clavien-Dindo grade IIIb complications, including one (7.1%) anastomotic leak. After a median follow-up of 39.0months, 63.6% of diverting ileostomies were reversed. Three-year DFS from RC was 71.4% (CI 40.2-88.3) and 3-year OS was 84.4% (CI 95% 50.4-95.9). No PC recurrence or death occurred. Synchronous management of RC and PC with pelvic RT followed by curative prostate RT doses and TME showed acceptable morbidity and oncologic results. Prostate brachytherapy, the most commonly used treatment modality, allowed avoidance of prostatectomy and additional external RT to the rectum. PC should not limit the curative intent of RC, as all recurrences were from rectal origin.

21 Gy single fraction prostate HDR brachytherapy: 5-year results of a single institution prospective pilot study

PURPOSETo present the outcome and toxicity results of a prospective trial of 21 Gy single fraction high-dose-rate (HDR) brachytherapy for men with low- or intermediate-risk prostate cancer. METHODS AND MATERIALSPatients were treated according to an IRB-approved prospective study of single fraction HDR brachytherapy. Eligible patients had low- or intermediate-risk prostate cancer with tumor stage ≤ T2b, PSA ≤ 15, and Gleason score ≤ 7. Patients underwent trans-rectal ultrasound-guided trans-perineal implant of the prostate followed by single fraction HDR brachytherapy to a dose of 21 Gy. The primary endpoint was grade ≥ 2 urinary/GI toxicity rates. RESULTSTwenty-six patients were enrolled with a median follow up of 5.1 years and median age of 64 years. 88.5% of patients had T1 disease, 15.4% had Gleason score 6 (84.6% Gleason 7), and median pre-treatment PSA was 5.0 ng/mL. Acute and chronic grade ≥ 2 urinary toxicity rates were 38.5% and 38.5%, respectively. There were no grade ≥ 2 acute or chronic GI toxicities. Six (23.1%) patients experienced biochemical failure, six (23.1%) patients experienced radiographic local failure, and five (19.2%) patients had biopsy-proven local failure. No patients developed regional lymph node recurrence or distant metastasis. 5-year overall survival and cause-specific survival were 96.2% and 100%, respectively. CONCLUSIONS21 Gy single fraction HDR brachytherapy was associated with modestly higher-than-anticipated chronic urinary toxicity, as well as high biochemical and local failure rates. The results from this prospective pilot study do not support the use of this regimen in standard clinical practice.

A Randomized Trial Comparing Quality of Life After Low-Dose Rate or High-Dose Rate Prostate Brachytherapy Boost With Pelvic External Beam Radiation Therapy

Purpose/ObjectivesTo compare health-related Quality of Life (QoL) in urinary, bowel and sexual domains after combined external beam radiotherapy (EBRT) and either low dose rate (LDR) or high dose rate (HDR) prostate brachytherapy (BT). Materials/MethodsEligible men with intermediate or high-risk prostate cancer, treated with combined pelvic EBRT and BT were randomly assigned to either HDR (15 Gy) or LDR (110 Gy) boost. International Prostate Symptom Score (IPSS), Index of Erectile Function (IIEF), Expanded Prostate Cancer Composite (EPIC-26) were collected at baseline, 1,3,6 and 12 months, q6 months to 3 years and then annually along with PSA/testosterone. Fisher's exact test compared categorical variables and the Mann Whitney U test EPIC domain scores. ResultsFrom 01/2014 to 12/2019, a random number generator assigned 195 men: 108 to HDR and 87 to LDR. Median age was 71 years. Risk group was high in 57% and unfavorable intermediate in 43%. Androgen deprivation (used in 74%) began with 3 months neoadjuvant and continued for median 12 months. Baseline EPIC scores were similar for the LDR/HDR cohorts: 89/88 for GU and 92/93 for GI. EPIC urinary scores decreased at one month for HDR but recovered promptly to a steady state by 6 months. LDR scores reached a nadir at 3 months with slow recovery to 18 months, after which urinary QoL was similar for HDR and LDR. Bowel QOL scores fell in both cohorts reaching respective nadirs at 12 months. HDR patients recovered close to baseline and maintained higher scores than LDR patients to 5 years. The decline for LDR patients remained > the MCID out to 5 years. ConclusionsPatient experience for combined EBRT and prostate BT is improved with HDR BT. Urinary QoL improves over time to be equivalent between the 2 modalities after 18 months, but LDR patients report lasting bowel symptoms.

Development of patient and catheter specific error thresholds for high dose rate prostate brachytherapy.

In-vivo source tracking has been an active topic of research in the field of high-dose rate brachytherapy in recent years to verify accuracy in treatment delivery. Although detection systems for source tracking are being developed, the allowable threshold of treatment error is still unknown and is likely patient-specific due to anatomy and planning variation. The purpose of this study was to determine patient and catheter-specific shift error thresholds for in-vivo source tracking during high-dose-rate prostate brachytherapy (HDRPBT). A module was developed in the previously described graphical processor unit multi-criteria optimization (gMCO) algorithm. The module generates systematic catheter shift errors retrospectively into HDRPBT treatment plans, performed on 50 patients. The catheter shift model iterates through the number of catheters shifted in the plan (from 1 to all catheters), the direction of shift (superior, inferior, medial, lateral, cranial, and caudal), and the magnitude of catheter shift (1-6mm). For each combination of these parameters, 200 error plans were generated, randomly selecting the catheters in the plan to shift. After shifts were applied, dose volume histogram (DVH) parameters were re-calculated. Catheter shift thresholds were then derived based on plans where DVH parameters were clinically unacceptable (prostate V100<95%, urethra D0.1cc>118%, and rectum Dmax>80%). Catheter thresholds were also Pearson correlated to catheter robustness values. Patient-specific thresholds varied between 1 to 6mm for all organs, in all shift directions. Overall, patient-specific thresholds typically decrease with an increasing number of catheters shifted. Anterior and inferior directions were less sensitive than other directions. Pearson's correlation test showed a strong correlation between catheter robustness and catheter thresholds for the rectum and urethra, with correlation values of -0.81 and -0.74, respectively (p<0.01), but no correlation was found for the prostate. It was possible to determine thresholds for each patient, with thresholds showing dependence on shift direction, and number of catheters shifted. Not every catheter combination is explorable, however, this study shows the feasibility to determine patient-specific thresholds for clinical application. The correlation of patient-specific thresholds with the equivalent robustness value indicated the need for robustness consideration during plan optimization and treatment planning.

A Novel Brachytherapy Needle Design to Reduce Urethra Dose in HDR Prostate Brachytherapy.

Ultrasound-guided high-dose-rate (HDR) prostate brachytherapy is a safe and effective treatment option for prostate cancer patients; however, some patients still experience acute and late genitourinary (GU) toxicity. It has been reported that urethra dose is associated with the incidence and severity of GU toxicity. Therefore, a technique that can further spare the urethra while ensuring adequate target coverage is highly desirable. Intensity modulated brachytherapy (IMBT) designs, such as rotating shield brachytherapy (RSBT), offer ideal dosimetry on-paper but are challenging to implement clinically due to the need for high precision in moving mechanisms and the use of non-conventional HDR sources, such as 153Gd, electronic, and 169Yb. In this study, we propose a novel clinically implementable solution based on the direction modulated brachytherapy (DMBT) design concept, which has no moving parts and works effectively with the commercially available 192Ir sources. The widely used GammaMedPlus (GMP) 192Ir sources, with outer diameters of 0.9 mm, was simulated using the GEANT4 Monte Carlo (MC) simulation code. The novel DMBT needle concept consists of a 14-gauge nitinol needle with an outer diameter of 2.1 mm and a crust thickness of 0.1 mm, which housed a platinum shield inside. A single groove, consistent with the outer diameter of each source, was incorporated inside the platinum shield to accommodate the HDR source. The maximum thickness of the shield was 0.8 mm. To evaluate the effectiveness of the DMBT needle concept in reducing urethra dose, two proof-of-concept in silico DMBT plans were created from the standard clinical plan by replacing two needles close to the urethra with the DMBT needles. The dosimetrical comparisons between the two DMBT plans, and the clinical BT plan were done by assessing the target coverage, and urethral, bladder and rectum dose. The MC results showed that the use of the novel DMBT needle could reduce the radiation dose by approximately 35% at 1 cm distance from the needle behind the platinum shield, as compared to the unshielded side. Additionally, when using the same dose volume histogram (DVH) planning criteria as the original plan, the DMBT plan reduced the maximum urethral dose at the pre-apical region by 16% and 28% for 0 mm and 2 mm planning organ at risk (PRV) margins, respectively, while maintaining equivalent V100 and D90 target coverage. The novel DMBT needle design offers a promising solution for reducing the dose to the pre-apical region of the urethra without compromising target coverage, increasing the dose to other organs-at-risk, or increasing the treatment time.

A Phase I/II Dose-Escalation Study Evaluating the Safety of 21 Gy, 23 Gy, and 25 Gy for High Dose Rate (HDR) Prostate Brachytherapy: An Interim Toxicity Report.

Single-fraction high dose rate (HDR) prostate brachytherapy has shown improved actuarial biochemical control rates from 66% to 82% with dose escalation from 19 to 20.5 Gy, respectively, but is still inferior to the reported low dose rate brachytherapy control rates of over 90%. We aimed to identify whether dose escalation to 21, 23, and 25 Gy can be safely accomplished. Patients with previously untreated, pathologically confirmed, low-risk (cT1-T2a, Gleason ≤6, PSA <10 ng/mL) or favorable intermediate risk (Gleason 3+4, percentage of positive biopsy cores <50%, ≤1 NCCN intermediate risk factor) prostate adenocarcinoma were enrolled from a single institution. PSA and toxicity assessment were performed at baseline and at routine 6-month follow-ups. From May 9, 2018 to May 12, 2022, 18 patients were enrolled. None had received prior androgen deprivation therapy, 44% had low risk disease, and 61% were ECOG 0 at baseline. Median age was 68 years (total range: 43-79), and 83% were Caucasian. Eight patients received 21 Gy, nine patients received 23 Gy, and one patient received 25 Gy, with the 25 Gy cohort still accruing. The mean prostate size and range, determined by pre-operative prostate volume study on trans-rectal ultrasound, were as follows: 21Gy cohort (mean: 41.2cc; range: 21.9-63.1cc), 23Gy Cohort (mean: 41.2cc, range 28.3-71.7cc), 25Gy cohort (65.3cc). A median of 17 catheters (range: 16-20) were implanted. At a median follow-up of 35.7 months (range: IQR 4.4 - 50.2), only one grade 3 toxicity was reported, which was an ulcerative colitis (UC) flare noted in a patient whose UC was poorly controlled, requiring multiple courses of prednisone in the 6 months prior to his brachytherapy. Regarding toxicities attributable to therapy, fourteen and four patients experienced a grade 1 and 2 genitourinary toxicity respectively; four and four patients experienced a grade 1 and 2 reproductive system toxicity respectively; one patient experienced a grade 1 GI toxicity. Two patients needed foley catheters upon discharge, with neither requiring the foley long term. There were two treatment failures in the 21Gy cohort at 1.39 and 1.67 years from date of HDR brachytherapy; date of failure was defined by the first PSA 2.0ng/mL over nadir. Of these patients, one underwent focal salvage HDR, whilst the other underwent radical prostatectomy with only 1% of the volume involved by prostate cancer. Of note, there was seminal vesicle involvement on restaging prostate biopsy for the patient that underwent radical prostatectomy, however, this was not appreciated on the final prostatectomy specimen. Both patients have no evidence of disease currently. HDR brachytherapy appears well tolerated in patients with low to favorable intermediate risk prostate cancer at 21 and 23 Gy, with more accrual needed at 25Gy. Long-term follow-up needed to assess efficacy.

Efficacy of Curvilinear Needle Implantation for Prostate HDR Brachytherapy

High-dose-rate (HDR) brachytherapy is gaining popularity due to better dose distribution, good clinical outcomes, and convenience of the procedure. Conventionally, Rectilinear (straight needle) Method (RLM) is utilized for HDR procedure. However, the efficacy of a newly developed technique, the Curvilinear (curved needle conforming the prostate shape) Method (CLM), for HDR brachytherapy has not yet been explored. In this study, efficacy of the CLM is evaluated. The CLM technique is expected to significantly improve the radiation dose distribution with small number of needles that will potentially improve the clinical outcomes. Randomly 16 prostate cancer patients were selected (median age: 68.3 yrs, Stage: T1-T2a) from an IRB approved pool of patients. Prostate and OARs (urethra, rectum and bladder) were contoured on T2-weigthed MRI for all the patients. Thirty-two HDR plans, 16 for RLM and 16 for CLM, were prepared using Oncentra treatment planning software with Ir-192 isotope and were optimized using Hybrid Inverse treatment Planning Optimization (HIPO) tool. Prescription dose was 15 Gy in single fraction. Data for relevant parameters (V100, V200, D90, D10, Dmax, etc.) for prostate and OARs were collected using the dose-volume histogram. P-value less than equal to 0.05 was considered statistically significant. Average prostate volume was 33.1cc (range 17.7-51.3cc, median 30.6cc). In CLM technique the target coverage and dose constraints for OARs were improved by 0.3%-6.4% and 1.6%-34.2%, respectively (see Table below). Certain dose constraints were difficult to meet for some cases with RLM (prostate V100 (4 pts) and V200 (5 pts); rectum Dmax (5 pts) and D12 Gy (10 pts). On average, over 44.6% (average 17.5 vs 9.7 needles) reduction of needles was achieved with CLM (p-value < 0.001). The CLM technique generated significantly improved dosimetric plans using less number of needles. Reduction in number of needles would be associated with potential decrease in toxicity and improved clinical outcomes. Future plan includes clinical implementation of the CLM technique.

Observer preference of artificial intelligence-generated versus clinical prostate contours for ultrasound-based high dose rate brachytherapy.

For trans-rectal ultrasound (TRUS)-based high dose rate (HDR) prostate brachytherapy, prostate contouring can be challenging due to artifacts from implanted needles, bleeding, and calcifications. To evaluate the geometric accuracy and observer preference of an artificial intelligence (AI) algorithm for generating prostate contours on TRUS images with implanted needles. We conducted a retrospective study of 150 patients, who underwent HDR brachytherapy. These patients were randomly divided into training (104), validation (26) and testing (20) sets. An AI algorithm was trained/validated utilizing the TRUS image and reference (clinical) contours. The algorithm then provided contours for the test set. For evaluation, we calculated the Dice coefficient between AI and reference prostate contours. We then presented AI and reference contours to eight clinician observers, and asked observers to select their preference. Observers were blinded to the source of contours. We calculated the percentage of cases in which observers preferred AI contours. Lastly, we evaluate whether the presence of AI contours improved the geometric accuracy of prostate contours provided by five resident observers for a 10-patient subset. The median Dice coefficient between AI and reference contours was 0.92 (IQR: 0.90-0.94). Observers preferred AI contours for a median of 57.5% (IQR: 47.5, 65.0) of the test cases. For resident observers, the presence of AI contours was associated with a 0.107 (95% CI: 0.086, 0.128; p<0.001) improvement in Dice coefficient for the 10-patient subset. The AI algorithm provided high-quality prostate contours on TRUS with implanted needles. Further prospective study is needed to better understand how to incorporate AI prostate contours into the TRUS-based HDR brachytherapy workflow.

PSOR12 Presentation Time: 12:55 PM: Single Fraction High Dose Rate Prostate Brachytherapy in Intermediate-Risk Prostate Cancer

PSOR12 Presentation Time: 12:55 PM: Single Fraction High Dose Rate Prostate Brachytherapy in Intermediate-Risk Prostate Cancer

PP02 Presentation Time: 11:09 AM: Doppler Ultrasound Changes of the Neurovascular Bundle during High Dose Rate Prostate Brachytherapy

PP02 Presentation Time: 11:09 AM: Doppler Ultrasound Changes of the Neurovascular Bundle during High Dose Rate Prostate Brachytherapy

PO75: Prostate High Dose-Rate Brachytherapy as Monotherapy for Low and Intermediate Risk Prostate Cancer in a Community Setting

PO75: Prostate High Dose-Rate Brachytherapy as Monotherapy for Low and Intermediate Risk Prostate Cancer in a Community Setting

162 the Development and Assessment of a Core-Needle Biopsy Localization Methodology Implemented During Prostate High Dose-Rate Brachytherapy Treatments

162 the Development and Assessment of a Core-Needle Biopsy Localization Methodology Implemented During Prostate High Dose-Rate Brachytherapy Treatments

Analytical HDR prostate brachytherapy planning with automatic catheter and isotope selection.

High dose rate (HDR) brachytherapy is commonly used to treat prostate cancer. Existing HDR planning systems solve the dwell time problem for predetermined catheters and a single energy source. Additional degrees of freedom can be obtained by relaxing the catheters' pre-designation and introducing more source types, and may have a dosimetric benefit, particularly in improving conformality to spare the urethra. This study presents a novel analytical approach to solving the corresponding HDR planning problem. The catheter and dual-energy source selection problem was formulated as a constrained optimization problem with a non-convex group sparsity regularization. The optimization problem was solved using the fast-iterative shrinkage-thresholding algorithm (FISTA). Two isotopes were considered. The dose rates for the HDR 4140 Ytterbium (Yb-169) source and the Elekta Iridium (Ir-192) HDR Flexisource were modeled according to the TG-43U1 formalism and benchmarked accordingly. Twenty-two retrospective HDR prostate brachytherapy patients treated with Ir-192 were considered. An Ir-192 only (IRO), Yb-169 only (YBO), and dual-source (DS) plan with optimized catheter location was created for each patient with N catheters, where N is the number of catheters used in the clinically delivered plans. The DS plans jointly optimized Yb-169 and Ir-192 dwell times. All plans and the clinical plans were normalized to deliver a 15Gy prescription (Rx) dose to 95% of the clinical treatment volume (CTV) and evaluated for the CTV D90%, V150%, and V200%, urethra D0.1cc and D1cc, bladder V75%, and rectum V75%. Dose-volume histograms (DVHs) were generated for each structure. The DS plans ubiquitously selected Ir-192 as the only treatment source. IRO outperformed YBO in organ at risk (OARs) OAR sparing, reducing the urethra D0.1cc and D1cc by 0.98% ( ) and 1.09% ( ) of the Rx dose, respectively, and reducing the bladder and rectum V75% by 0.09 ( ) and 0.13 cubic centimeters (cc) ( ), respectively. The YBO plans delivered a more homogenous dose to the CTV, with a smaller V150% and V200% by 3.20 ( ) and 1.91 cc ( ), respectively, and a lower CTV D90% by 0.49% ( ) of the prescription dose. The IRO plans reduce the urethral D1cc by 2.82% ( ) of the Rx dose compared to the clinical plans, at the cost of increased bladder and rectal V75% by 0.57 ( ) and 0.21 cc ( ), respectively, and increased CTV V150% by a mean of 1.46 cc ( ) and CTV D90% by an average of 1.40% of the Rx dose ( ). While these differences are statistically significant, the clinical differences between the plans are minimal. The proposed analytical HDR planning algorithm integrates catheter and isotope selection with dwell time optimization for varying clinical goals, including urethra sparing. The planning method can guide HDR implants and identify promising isotopes for specific HDR clinical goals, such as target conformality or OAR sparing.

Dosimetric and workflow impact of synthetic-MRI use in prostate high-dose-rate brachytherapy

Target and organ delineation during prostate high-dose-rate (HDR) brachytherapy treatment planning can be improved by acquiring both a postimplant CT and MRI. However, this leads to a longer treatment delivery workflow and may introduce uncertainties due to anatomical motion between scans. We investigated the dosimetric and workflow impact of MRI synthesized from CT for prostate HDR brachytherapy. Seventy-eight CT and T2-weighted MRI datasets from patients treated with prostate HDR brachytherapy at our institution were retrospectively collected to train and validate our deep-learning-based image-synthesis method. Synthetic MRI was assessed against real MRI using the dice similarity coefficient (DSC) between prostate contours drawn using both image sets. The DSC between the same observer's synthetic and real MRI prostate contours was compared with the DSC between two different observers' real MRI prostate contours. New treatment plans were generated targeting the synthetic MRI-defined prostate and compared with the clinically delivered plans using target coverage and dose to critical organs. Variability between the same observer's prostate contours from synthetic and real MRI was not significantly different from the variability between different observer's prostate contours on real MRI. Synthetic MRI-planned target coverage was not significantly different from that of the clinically delivered plans. There were no increases above organ institutional dose constraints in the synthetic MRI plans. We developed and validated a method for synthesizing MRI from CT for prostate HDR brachytherapy treatment planning. Synthetic MRI use may lead to a workflow advantage and removal of CT-to-MRI registration uncertainty without loss of information needed for target delineation and treatment planning.

LDR brachytherapy offers superior tumor control to single-fraction HDR prostate brachytherapy: A prospective study.

To compare the clinical outcomes of single-fraction high-dose-rate (HDR) brachytherapy and single-fraction low-dose-rate (LDR) brachytherapy as the sole treatment for primary prostate cancer. A quasi-randomized study that allocated, from March 2008 to February 2012, 129 low and intermediate risk prostate cancer patients to one single-fraction HDR of 19 Gy (61 patients) or to a 145 Gy 125 I LDR permanent implant (68 patients. Biochemical relapse-free survival (bRFS) and overall survival (OS) were compared using the Kaplan-Meier method and Cox regression analysis. After a median follow-up of 72 months in the HDR group, 26 patients relapsed, and after a median follow-up of 84 months in the LDR group, 7 patients relapsed (p < 0.0001). The 5-year bRFS was significantly better for the LDR group than for the HDR group (93.7% and 61.1%, respectively) (p < 0.0001). The 5-year OS also was significantly better in the LDR group (95.5% vs. 89.9%) (p = 0.0436). Permanent LDR prostate implant brachytherapy offers better clinical outcomes than single-fraction HDR for prostate cancer.

Current status and future readiness of Indian radiation oncologists to embrace prostate high-dose-rate brachytherapy: An Indian Brachytherapy Society survey.

This survey aimed to understand the practice pattern and attitude of Indian doctors towards prostate brachytherapy. A 21-point questionnaire was designed in Google form and sent to radiation oncologists practicing in India, using texts, mails, and social media. Responses were collated, and descriptive statistical analysis was performed. A total of 212 radiation oncologists from 136 centers responded to the survey questionnaire, with majority (66%) being post-specialty training > 6 years. We found that about 44.3% (n = 94) of respondents do not practice interstitial brachytherapy for any site, and majority (83.3%, n = 175) do not practice high-dose-rate (HDR) prostate brachytherapy. Only 2.8% (n = 6) of doctors preferred boost by brachytherapy compared with 38.1% (n = 80) of respondents, who favored stereotactic body radiation therapy (SBRT) boost. When asked about the indication of HDR prostate brachytherapy in Indian setting, 32.5% (n = 67) of respondents favored monotherapy, 46.1% (n = 95) of oncologists thought boost as a good indication, and 21.4% (n = 44) preferred re-irradiation/salvage setting. The most cited reason for prostate brachytherapy not being popularly practiced in India was lack of training (84.8%, n = 179). It was also noted that out of 80 respondents who practiced SBRT for prostate boost, 37 would prefer HDR brachytherapy boost if given adequate training and facilities. The present survey provided insight on practice of prostate brachytherapy in India. It is evident that majority of radiation oncologists do not practice HDR prostate brachytherapy due to lack of training and infrastructure. Indian physicians are willing to learn and start prostate brachytherapy procedures if dedicated training and workshops are organized.

PP03 Presentation Time: 9:20 AM: Patterns of Failure for Salvage HDR Prostate Brachytherapy in the Prostate PET Imaging Era

<h3>Purpose</h3> Although some patients who undergo prostate re-irradiation achieve long-term biochemical control, the majority of patients ultimately experience a biochemical recurrence. Furthermore, a small, but significant percentage of patients may experience significant toxicities. Prostate positron emission tomography (PET) may improve the management of radiorecurrent prostate cancer, by better delineating the systemic extent of disease prior to treatment. Furthermore, PET may allow for a more precise understanding of the patterns of failure after treatment. The purpose of this study is to analyze oncologic outcomes including patterns-of-failure for a contemporary cohort of patients who underwent salvage high-dose rate (HDR) prostate brachytherapy in the prostate PET imaging era. <h3>Materials and Methods</h3> We conducted a retrospective analysis of all patients, who underwent salvage HDR brachytherapy from 2017 onwards. 18F-fluciclovine and 18F-DCFPyL became available in 2017 and 2021, respectively. We utilized the Kaplan-Meier method to measure oncologic endpoints including biochemical progression-free survival (bPFS), clinical progression free survival (cPFS), and metastasis-free survival (MFS). We also implemented Cox regression analysis to identify factors associated with the development of bPFS and MFS. <h3>Results</h3> The median follow-up was 16.6 months. Of the 50 patients included in this study, 48 patients had 18F-fluciclovine (Axumin) imaging and two had 18F-DCFPyL (PSMA) imaging prior to treatment initiation. Six patients (12%) had oligometastatic disease prior to treatment and were treated with SBRT to those sites. Six patients (12%) had castrate-resistant prostate cancer at the time of salvage brachytherapy. The median brachytherapy dose was 1150 cGy in two treatments, and the entire prostate was treated. 43 (86%) patients were concurrently treated with androgen deprivation therapy. Six patients (12%) patients experienced a biochemical recurrence, and four patients (8%) experienced a local-regional recurrence by post-salvage PSMA or Auxmin imaging. Two patients (4%) experienced distant metastatic progression. No initial covariates were statistically associated with bPFS survival (Figure 1). However, increasing time to recurrence trended towards negative association (HR 0.77| CI 0.56 - 1.04, p = 0.091). The bPFS was 100% (CI 100% - 100%), 94% (CI 87% - 100%), and 90% (CI 81% - 100%) at 6 months, 12 months, and 18 months, respectively. <h3>Conclusion</h3> Patients undergoing HDR prostate brachytherapy to salvage recurrent disease in the prostate PET imaging era have durable bPFS. In this cohort, HDR salvage was equally effective regardless of initial treatment covariates (i.e. initial risk group), status of disease immediately prior to salvage (i.e. castrate resistance or oligometastases), or use of androgen deprivation therapy (ADT) during salvage. Therefore, prostate PET may identify candidates for whom HDR salvage will be effective.

PHSPP04 Presentation Time: 3:00 PM: The Impact of Sublethal Damage Repair on Biological Dose for a Stepping-Source Brachytherapy Delivery in a Single-Fraction Prostate Treatment

<h3>Purpose</h3> Single-fraction (1-fx) ¹⁹²Ir prostate high-dose-rate (HDR) brachytherapy prescriptions of 19-20 Gy were derived using the biologically effective dose (BED) calculation in its simplified traditional (BED_T) form, BED_T=nd(1+d/α/β). This form of BED, while extremely convenient, was an approximation predicated on instantaneous delivery and may not be an appropriate assumption in certain key scenarios: protracted delivery times (> 15 minutes), high dose/fx (> 5 Gy), late-reacting tissues (low α/β), and/or rapid sublethal damage repair half-time (< 2x delivery time). As these are all characteristic of 1-fx prostate HDR brachytherapy, the aim of this study was to examine the difference between BED_T and BED by development of a computational framework that can perform patient-specific high-resolution volumetric BED calculations that explicitly consider the effects of dose protraction and the unique temporal sequences arising from the stepping-source delivery. <h3>Materials and Methods</h3> The Lea-Catcheside dose protraction factor, g, was calculated to modify the multi-track term of the linear-quadratic model, such that BED=nd(1+gd/α/β). BED_T corresponds to the special situation where g=1. For calculations, α/β=3.0 Gy and characteristic sublethal damage repair half-time of 0.27 hours were drawn from AAPM TG-137 radiobiological values, with assumed mono-exponential repair kinetics. For the stepping-source form of dose protraction factor, g_ss, a script was developed to sequentially calculate the dose rates as a function of time, for each voxel in the calculation grid, from all individual dwell positions based on position and angular orientation extracted from a plan DICOM file using an AAPM TG-43/43U dose-rate calculation methodology. Together with the temporal ordering of dwell positions extracted from DICOM, the script provided a matrix of g_ss values on a voxel-by-voxel basis with 1.0 × 1.0 × 1.0 mm³ resolution. A second form of the dose protraction factor g₀, a commonly-used factor in LDR brachytherapy, was also calculated which approximated the entire delivery as time-averaged uniform dose-rate. BED_T (g=1) was then compared to BED calculated with actual values of g₀ and g_ss values. <h3>Results</h3> The DICOM files from a previously-treated patient with representative dose characteristics were imported into our script: a prescription of 19 Gy in 1-fx, 140 active dwell positions in 17 channels, and delivery time of 2076.1 seconds based on 17500 U (4.3 Ci) source strength. An axial slice showing a color wash of the BED (Gy₃) ranging from 139.3-519.0 Gy₃ (physical dose: 19-38 Gy in 1-fx) is shown for g=1 (BED_T), g₀, and g_ss in Figure 1 which demonstrated significant differences. The nominal BED_T was 139.3 Gy, which was reduced by 31% using g₀ and, depending on location, reduced between 20-31% using g_ss. The portion of the PTV covered by 139.3 Gy₃ prescription was 95.9%, 59.6% and 65.7% using g=1 (BED_T), g₀, and g_ss, respectively. Using g=1 (BED_T), g₀, and g_ss, 90% of the PTV was covered by 159.4 Gy₃, 110.4 Gy₃, and 112.9 Gy₃, respectively. <h3>Conclusions</h3> A custom script, capable of recalculating BED on a voxel-by-voxel level with proper accounting of sublethal damage repair arising from protracted dose delivery was developed. Significant differences in BED (∼30%) were shown for 1-fx prostate HDR brachytherapy. Two other general trends emerged in this study: 1) g₀, while simple, provided a lower bound to volumetric g_ss values. 2) Voxels closer to an activated dwell position were more resistant to dose protraction effects in an effect we attribute to effective delivery time: the time in which the dose-rate was significantly elevated (>2 orders of magnitude).