Postoperative Radiotherapy in Prostate Cancer: The Case of the Missing Target

Abstract

Postoperative radiotherapy increases survival in high-risk (pT3/margin positive) prostate cancer patients (pts). Despite the benefit of adjuvant radiotherapy (XRT) approximately 50% of pts in long-term follow-up relapse with the primary site of failure being local (SWOG 8794). Potential reasons for recurrence include inadequate XRT dose and inadequate clinical target volume (CTV) delineation. There are four published consensus guidelines (EORTC, FROGG, PMH, and RTOG) defining postoperative CTV in prostate cancer. We explore the possibility that inadequate CTV coverage is an important cause of local failure. This study evaluates the utility of preoperative MRI in defining postoperative prostate bed CTV. Twenty prostate cancer pts at The Ottawa Hospital receiving post-op XRT who also had pre-op staging MRI were identified. Pts underwent CT Simulation and the 4 CTV consensus definitions were applied. CTVs were expanded by 1 cm to create respective Planning Target Volumes (PTVs). Pre-op MRIs were fused with the post-op planning CT scans. MRI based prostate and gross visible tumor were contoured. 3DCRT plans were developed and Dose Volume Histograms (DVH) analyzed. Subtraction analysis was conducted to assess the adequacy of prostate and gross tumor coverage. Mean pre-op prostate volume was 45cc (range, 25-79 cc) and gross tumor was visible in 17 cases. In all 20 cases, the consensus CTVs did not fully cover the pre-resection extent of the prostate seen on the MRI. On average, 38% of the prostate volume was missed by the CTVs (mean and range, CTV-RTOG 29% [5-65%], CTV-PMH 27% [4-61%], CTV-FROGG 43% [17-67%], CTV-EORTC 52% [23-77%]). In only 1 case, the entire gross tumor was completely covered. On average, 41% of the visible gross tumor volume was missed (mean and range: CTV-RTOG 35% [0-67%], CTV-PMH 33% [0-64%], CTV-FROGG 48% [18-92%], and CTV-EORTC 46% [9-88%]). Furthermore, the PTVs did not cover the prostate in 50% of cases. On average, 7% of the prostate volume was missed by PTVs (mean and range: PTV-RTOG 3% [0-13%], PTV-PMH 3% [0-13%], PTV-FROGG 8% [0-27%], PTV-EORTC 14% [0-36%]).The prostate base and mid-zones were the predominant site of inadequate coverage. CTV-PMH and CTV-RTOG were similar with respect to prostate and tumor coverage and yielded the best overall results. CTV-EORTC provided the least overall coverage. Data on anatomic shifts of bladder and rectum associated with radical prostatectomy will also be presented. Current CTV consensus definitions do not adequately cover the prostate bed and/or gross tumor based on pre-op MRI. Additionally, the PTVs do not fully cover the prostate bed in 50% of cases. CTV definitions should be modified to incorporate pre-op imaging. Improved CTV definition should result in further increases in survival for high risk prostate cancer.