Normal tissue complication probabilities (NTCP) of urethral stricture following salvage SBRT or HDR for periurethral recurrences following prior definitive radiation therapy.

246 Background: SBRT and HDR brachytherapy have emerged as definitive salvage re-irradiation options for men with locally recurrent prostate cancer after prior radiation therapy (RT). Toxicity with re-irradiation remains a concern in this setting, particularly for salvage of periurethral recurrences. We calculated the normal tissue complication probability (NTCP) of urethral stricture following salvage re-irradiation (HDR and SBRT) after previous definitive RT (external beam or HDR) for periurethral recurrences. Methods: Two upfront definitive treatment plans were generated for 5 men with localized prostate cancer: External Beam RT (EBRT1) to a dose of 79.2 Gy in 1.8 Gy fractions and HDR monotherapy (HDR1), 13.5 Gy x 2 implants. Periurethral recurrences were virtually created on diagnostic MRI scans for each of the five men and defined as a recurrent dominant intraprostatic lesion (DIL). Three salvage RT plans were generated for each patient (constraints in Table): HDR salvage (10 Gy whole gland; 13.5 Gy to DIL; 2 implants), SBRTr with maximal rectal sparing (constraints designed for salvage following prior EBRT; 30 Gy in 5 fractions whole gland; 40 Gy in 5 fractions to DIL), and SBRTu with maximal urethral sparing (constraints designed for patients with prior HDR monotherapy; same Rx). DVH data was collected for each plan (previous RT and each salvage approach) and the equivalent uniform dose (EUD) was calculated. NTCP for the urethral stricture in the summed plans (prior definitive treatment + each salvage plan) was calculated by the Lyman-Kutcher-Burman model using the following parameters: (α/β = 5Gy; TD50= 116.7; m = 0.23; n = 0.3). Results: Prescription coverage of 90% of the DIL volume was achieved for each plan. The mean NTCP of urethral stricture for EBRT1 + HDR salvage was 55.2% (range, 52.2-59.1%) and for HDR1 + HDR salvage was 49.2% (42.9-57.5%). For EBRT1 + SBRTr and EBRT1 + SBRTu the NTCP for urethral stricture was 54.0% (50.4-61.0%) and 43.2% (39.4-48.4%), respectively. The NTCP of urethral stricture for HDR1 + SBRTr was 48.1% (41.0-57.2%) and for HDR1 + SBRTu was 37.6% (30.9-44.6%). Conclusions: In this modeling analysis of salvage RT for periurethral recurrences following previous RT (EBRT or HDR monotherapy), the NTCP for urethral stricture was numerically lowest for salvage SBRT using maximum urethral sparing dose constraints in comparison to salvage SBRT with maximum rectal sparing constraints or salvage HDR brachytherapy. This type of analysis lends insight into personalized treatment planning based on previous RT modality and previous dose to organs at risk. Expanded analysis is underway. [Table: see text]