PO58: HDR Brachytherapy Salvage for Prostate Fossa Recurrence: A Mechanistic Review

Background/Purpose After radical prostatectomy 30% of men will develop biochemical recurrence. Typical salvage is with external beam radiation (EBRT) or ADT. Treatment options for a second recurrence, after salvage EBRT are limited. Prostate fossa brachytherapy (BT) offers a dosimetrically favorable treatment option that is convenient for patients and cost effective. However, there is limited data to guide management. This report describes the technique of using HDR BT salvage for prostate fossa recurrence. Methods Eligible patients had pathologically confirmed local recurrence or elevated PSA and visible lesion on PSMA PET, with no evidence of distant metastatic disease, and were suitable candidates for BT implant. Trans-rectal ultrasound (TRUS) guided volume study was performed, extending 1cm cranial and caudal to the most distant aspects of disease. The target was delineated and a 3mm circumferential planning target volume margin was added with no expansion superiorly, inferiorly, or posteriorly. A pre-plan was created to assess normal tissue tolerances and plan for presumptive needle distribution to achieve an acceptable D90. Based on disease location and size, an appropriate perineal template was selected. TRUS guidance was used for implant insertion intra-operatively with real-time plan optimization as needed. Post-operatively, the perineal template was secured, and patients underwent CT simulation to confirm appropriate needle placement. Based on the CT scan, needle depth was adjusted, the target was contoured, and the plan was optimized. Results At a single institution 7 patients completed prostate fossa HDR BT salvage between June 2020 to January 2023. BT was utilized as definitive salvage for re-irradiation (re-RT) of a second local recurrence (n=5), or as a boost to escalate dose for initial recurrence (n=2). For definitive re-RT, prescription dose was most commonly 30Gy/5fx delivered twice daily (BID) (Figure 1a), other dose regimens used include 32Gy/4fx BID, 27Gy/2fx, and 23Gy/1fx. Boost dose was 30Gy/5fx BID if close to normal tissues or 15Gy/1fx (Figure 1b) after 46-50.4Gy EBRT. Dose constraints included rectal D1cc <75% and urethral D1cc<110% with cumulative D2cc assessed to account for prior radiation. The median pre-treatment PSA was 3.75ng/ml and median post-treatment PSA was 0.52ng/ml. Three (42%) patients had acute grade 1-2 genitourinary (GU) toxicity and one patient had a late grade 3 event involving stress incontinence requiring an artificial sphincter. Conclusion Brachytherapy salvage for prostate fossa recurrence is safe and feasible. This report describes the dose, target, and technique for successful utilization in the setting of initial salvage dose escalation and re-RT for a second recurrence. Future prospective studies are required to further elucidate associated disease outcomes. After radical prostatectomy 30% of men will develop biochemical recurrence. Typical salvage is with external beam radiation (EBRT) or ADT. Treatment options for a second recurrence, after salvage EBRT are limited. Prostate fossa brachytherapy (BT) offers a dosimetrically favorable treatment option that is convenient for patients and cost effective. However, there is limited data to guide management. This report describes the technique of using HDR BT salvage for prostate fossa recurrence. Eligible patients had pathologically confirmed local recurrence or elevated PSA and visible lesion on PSMA PET, with no evidence of distant metastatic disease, and were suitable candidates for BT implant. Trans-rectal ultrasound (TRUS) guided volume study was performed, extending 1cm cranial and caudal to the most distant aspects of disease. The target was delineated and a 3mm circumferential planning target volume margin was added with no expansion superiorly, inferiorly, or posteriorly. A pre-plan was created to assess normal tissue tolerances and plan for presumptive needle distribution to achieve an acceptable D90. Based on disease location and size, an appropriate perineal template was selected. TRUS guidance was used for implant insertion intra-operatively with real-time plan optimization as needed. Post-operatively, the perineal template was secured, and patients underwent CT simulation to confirm appropriate needle placement. Based on the CT scan, needle depth was adjusted, the target was contoured, and the plan was optimized. At a single institution 7 patients completed prostate fossa HDR BT salvage between June 2020 to January 2023. BT was utilized as definitive salvage for re-irradiation (re-RT) of a second local recurrence (n=5), or as a boost to escalate dose for initial recurrence (n=2). For definitive re-RT, prescription dose was most commonly 30Gy/5fx delivered twice daily (BID) (Figure 1a), other dose regimens used include 32Gy/4fx BID, 27Gy/2fx, and 23Gy/1fx. Boost dose was 30Gy/5fx BID if close to normal tissues or 15Gy/1fx (Figure 1b) after 46-50.4Gy EBRT. Dose constraints included rectal D1cc <75% and urethral D1cc<110% with cumulative D2cc assessed to account for prior radiation. The median pre-treatment PSA was 3.75ng/ml and median post-treatment PSA was 0.52ng/ml. Three (42%) patients had acute grade 1-2 genitourinary (GU) toxicity and one patient had a late grade 3 event involving stress incontinence requiring an artificial sphincter. Brachytherapy salvage for prostate fossa recurrence is safe and feasible. This report describes the dose, target, and technique for successful utilization in the setting of initial salvage dose escalation and re-RT for a second recurrence. Future prospective studies are required to further elucidate associated disease outcomes.