Association between incidental dose outside the prostate and tumor control after modern image-guided radiotherapy.

Marnix Witte,Floris Pos,Wilma Heemsbergen,Luca Incrocci

doi:10.1016/j.phro.2020.12.003

Abstract

Background and purposeExternal beam radiotherapy for prostate cancer deposits incidental dose to a region surrounding the target volume. Previously, an association was identified between tumor control and incidental dose for patients treated with conventional radiotherapy. We investigated whether such an association exists for patients treated using intensity modulated radiotherapy (IMRT) and tighter margins.Materials and methodsComputed tomography scans and three-dimensional treatment planning dose distributions were available from the Dutch randomized HYPRO trial for 397 patients in the standard fractionation arm (39 × 2 Gy) and 407 patients in the hypofractionation arm (19 × 3.4 Gy), mainly delivered using online image-guided IMRT. Endpoint was any treatment failure within 5 years. A mapping of 3D dose distributions between anatomies was performed based on distance to the surface of the prostate delineation. Mean mapped dose distributions were computed for patient groups with and without failure, obtaining dose difference distributions. Random patient permutations were performed to derive p values and to identify relevant regions.ResultsFor high-risk patients treated in the conventional arm, higher incidental dose was significantly associated with a higher probability of tumor control in both univariate and multivariate analysis. The locations of the excess dose mainly overlapped with the position of obturator internus muscles at about 2.5 cm from the prostate surface. No such relationship could be established for intermediate-risk patients.ConclusionsAn association was established between reduced treatment failure and the delivery of incidental dose outside the prostate for high-risk patients treated using conventionally fractionated IMRT.

Highlights

Prostate cancer is the second most frequently diagnosed cancer [1] and the sixth leading cause of cancer death in males worldwide [2]
The clinical benefit of such elective fields in the primary RT setting is questioned since previous randomized trials in the three-dimensional conventional radiotherapy (3DCRT) era did not show any benefit of adding such fields [4,5] while at the same time these are associated with increased toxicity rates
While improved diagnostic magnetic resonance imaging (MRI) and lymph node staging using prostate-specific membrane antigen (PSMA) positron emission tomography (PET) [6] may increase the detection rate of organ non-confined disease, no imaging technique is perfect and those patients clinically diagnosed with localized disease may yet suffer undetectable locoregional progression

Summary

Introduction

Prostate cancer is the second most frequently diagnosed cancer [1] and the sixth leading cause of cancer death in males worldwide [2]. A relationship was demonstrated in a prostate cancer patient trial population treated with 3DCRT, where dose delivery in the obturator region was significantly associated with freedom from failure [7]. An association was identified between tumor control and incidental dose for patients treated with conventional radiotherapy. Results: For high-risk patients treated in the conventional arm, higher incidental dose was significantly associated with a higher probability of tumor control in both univariate and multivariate analysis. The locations of the excess dose mainly overlapped with the position of obturator internus muscles at about 2.5 cm from the prostate surface No such relationship could be established for intermediate-risk patients.

Methods

Results

Conclusion