Image guidance for post-prostatectomy radiotherapy: Are we missing the mark?

Abstract

56 Background: Image-guided radiation therapy (IGRT) has been widely adopted for both definitive and post-operative prostate radiotherapy. In the postoperative setting, numerous studies of prostate bed motion have recommended tight planning margins (<10mm) if IGRT is used daily. The purpose of this analysis is to determine the effect of IGRT on the efficacy and toxicity of post-operative prostate radiotherapy. Methods: Between 1998 and 2010, 286 patients received radiation therapy after prostatectomy at Duke. Recurrent disease following radiation therapy was defined as PSA >0.2 ng/ml and rising or initiation of salvage ADT. CTCAE v 4.0 and the RTOG/LENT late morbidity scores were used to grade acute and late toxicities. Risk for biochemical failure and late Grade 2+ GI toxicity were compared between IGRT (N = 113) and non-IGRT (N = 173) patients using multivariable adjusted Cox regression controlling for age, treatment technique (3D vs IMRT), radiation dose, androgen suppression, pathologic Gleason Score, margin status, pathologic stage, and pre-radiotherapy PSA level. Results: The median margin size for patients with IGRT was 7mm (IQR 6-10mm) and 15mm (IQR 7-15mm) for those without IGRT (p < 0.001). Median follow up was 21 months (IQR 15-33 mo) for patients with IGRT and 49 months (IQR 30-73 mo) for those without IGRT (p < 0.001). On multivariate analysis, patients treated with IGRT had a greater risk of progression versus non-daily imaging (HR = 2.51, p < 0.001), as did patients who received salvage versus adjuvant radiotherapy (HR = 2.41, p = 0.005). Higher pathologic Gleason Score (HR = 1.96, p = 0.026) and pathologic stage (HR = 1.93, p = 0.003) conferred increased risk of progression, while positive margin status was protective (HR = 0.53, p = 0.002). Age, radiation dose, androgen suppression, and treatment technique did not affect biochemical outcome (p > 0.1). There were no differences in acute or late GI toxicity according to treatment technique or use of IGRT (both p > 0.1). Conclusions: The use of IGRT was associated with increased biochemical recurrence for patients receiving post-operative prostate radiotherapy. For these patients, we recommend using treatment margins of at least 10mm to address subclinical disease and organ motion.