(S010) A Phase III Randomized Trial of MRI-Mapped Dose-Escalated Salvage Radiotherapy Post-Prostatectomy: The MAPS Trial-Feasibility and Acute Toxicity

Amber Orman, MD, Alan Pollack, MD, PhD, Kelin Wang, PhD, Radka Stoyanova, PhD, Elizabeth Bossart, PhD, Deukwoo Kwon, PhD, Matthew Abramowitz, MD; University of Miami

PURPOSE AND OBJECTIVES: MAPS is the first phase III randomized trial of magnetic resonance imaging (MRI)-mapped dose-escalated salvage radiotherapy. In this planned feasibility analysis, we relate dosimetry to acute toxicity in the setting of dose escalation using a simultaneous incorporated hypofractionated boost (SIHB) to MRI-identified lesions in the prostate bed.

MATERIALS AND METHODS: Two intensity-modulated radiotherapy (IMRT) plans were generated for each patient treated on the MAPS protocol, regardless of actual randomization arm: one for the standard fraction radiotherapy (SFRT) arm and one for the SIHB arm. In the SFRT arm, 68 Gy in 34 fractions was prescribed to ≥ 95% of the planning target volume (PTV). In the SIHB arm, an additional 2.25 Gy daily SIHB was prescribed to the gross tumor volume (GTV) (2.25 Gy daily, total dose of 76.5 Gy in 34 fractions). The trial stipulates that no more than 35% and 55% of the rectum should receive ≥ 65 Gy and ≥ 40 Gy, respectively, and that no more than 50% and 70% of the bladder minus the clinical target volume (B-CTV) should receive ≥ 65 Gy and ≥ 40 Gy, respectively. Acute toxicities were recorded for patients at designated times per protocol according to the National Cancer Institute Common Terminology Criteria for Adverse Events, version 4.0.

RESULTS: Data from the first 14 enrolled patients were reviewed to ensure dosimetric adequacy of the protocol requirements. In all plans, at least 95% of the PTV received 68 Gy, and at least 95% of the GTV received 76.5 Gy. Dosimetric constraints were achieved for all organs at risk (OARs) except B-CTV. Five cases had > 70% of the bladder receiving ≥ 40 Gy in both plans, and one case had > 50% of the bladder receiving ≥ 65 Gy in the SIHB plan only. The only difference between the SFRT and SIHB plans, per patient or overall, was a higher percent volume of the PTV receiving 68 Gy in the SIHB plans. In 13 patients for whom full data were available, the highest toxicity recorded was grade 2 gastrointestinal toxicity: one episode in each arm.

CONCLUSIONS: This study demonstrates that even though most MRI-identified GTVs are located in close proximity to critical structures, dose escalation is achievable without exceeding rectal constraints in all cases, and bladder constraints in the majority of cases. These variations are in cases with small bladders encompassed in the CTV and are not associated with increased acute toxicity.

Proceedings of the 97th Annual Meeting of the American Radium Society - americanradiumsociety.org