Impact of Target Volume and Image Guidance on Post-Prostatectomy Patients Treated With Intensity Modulated Radiation Therapy

Abstract

<h3>Purpose/Objective(s)</h3> Post-operative radiotherapy (PORT) is utilized in select prostate cancer patients. IMRT has been associated with improved dosimetry and acute toxicity. Smaller treatment margins for PORT have been recommended when using image guidance (IG), which has also been associated with reduced toxicity. Compared to intact prostate IG-IMRT, however, PORT is delivered to a space at risk for recurrence with no grossly visible target, so using targeted conformal techniques with less margin for error may compromise tumor control. Our institution initiated IG-IMRT for PORT in 2005 and adopted larger planning target volumes (PTV) in 2012. We hypothesized that smaller PTV and surgical clip-based IG-IMRT associate with increased biochemical failure (BF). <h3>Materials/Methods</h3> Patients who underwent post-prostatectomy IG-IMRT without concurrent androgen deprivation therapy (ADT) from 1/2005 to 12/2016 were included, with division into pre-2012 and post-2012 cohorts. Demographics and tumor/treatment variables were recorded retrospectively. Oncologic outcomes included BF (initiation of ADT and/or PSA > 0.2 ng/ml and rising) and late toxicities (defined by RTOG criteria, occurring > 90 days after RT). <h3>Results</h3> 133 patients met criteria: 73 treated pre-2012 and 60 treated post-2012, with a median follow-up of 5 and 9 years, respectively. Compared to the pre-2012 cohort, patients in the post-2012 cohort were more often treated with salvage vs adjuvant RT (<i>P</i> = 0.007), and had higher pre-PORT PSA (0.38 vs 0.20, <i>P</i> = 0.01). 5-year BF did not differ between cohorts (36% vs 43%, <i>P</i> = 0.48). Pre-2012, a smaller percentage of patients underwent on-treatment alignment to surgical clips (40% vs 58%, <i>P</i> = 0.033), vs bony anatomy. Alignment to surgical clips was associated with increased BF in the pre-2012 cohort (HR 2.13, <i>P</i> = 0.041) but not in the post-2012 cohort. Notably, significantly more patients received IG with CBCT in the post-2012 cohort (82% vs 33%). Pre-2012, the median PTV was 198.63 cc versus 242.59 cc post-2012 (<i>P</i> = 0.002). Among the entire cohort on univariate analysis, however, clinical target volume (CTV), CTV-PTV ratio and PTV as continuous variables were not associated with increased risk of BF. Late toxicities were not different between the two cohorts. <h3>Conclusion</h3> This retrospective analysis reports on toxicity and oncologic outcomes of post-prostatectomy patients treated with IG-IMRT. Despite changes in tumor/treatment characteristics, dosimetric parameters, and image guidance techniques, toxicity and cancer-related outcomes remain similar over a 7-year period. Clip-based IG-IMRT in conjunction with small target volumes may be associated with higher risk of BF, and use of CBCT IG may offset this risk. Additional dosimetric and treatment analyses will be presented. These findings should be validated in prospective datasets.