3251 - Dosimetric Outcomes of Cytoreductive Stereotactic Body Radiotherapy (SBRT) for Metastatic Renal Cell Carcinoma (mRCC): Data from the CYTOSHRINK Trial
Presenter(s)
L. E. Collie1,2, D. Schep1,2, G. Pond1, S. Siva3, C. William4, D. Gopaul5, S. C. Morgan6,7, N. H. Usmani8,9, L. Mendez10, M. Levine1, J. Wright2, S. J. Hotte11, A. K. A. Lalani1, and A. Swaminath12; 1McMaster University, Hamilton, ON, Canada, 2Juravinski Cancer Centre, Hamilton, ON, Canada, 3Sir Peter MacCallum Department of Oncology, The University of Melbourne, Melbourne, VIC, Australia, 4Durham Regional Cancer Centre, University of Toronto, Oshawa, ON, Canada, 5Grand River Regional Cancer Centre, Kitchener, ON, Canada, 6The Ottawa Hospital Cancer Centre, University of Ottawa, Ottawa, ON, Canada, 7Division of Radiation Oncology, University of Ottawa, Ottawa, ON, Canada, 8University of Alberta, Edmonton, AB, Canada, 9Cross Cancer Institute, Edmonton, AB, Canada, 10London Health Sciences Centre, London, ON, Canada, 11Division of Medical Oncology, Juravinski Cancer Centre & Escarpment Cancer Research Institute McMaster University, Hamilton, ON, Canada, 12Juravinski Cancer Centre, McMaster University, Hamilton, ON, Canada
Purpose/Objective(s):
CYTOSHRINK is a randomized phase II trial evaluating early cytoreductive SBRT to the primary kidney tumour combined with nivolumab plus ipilimumab in patients with IMDC intermediate- or poor-risk mRCC. SBRT in this setting frequently treats large target volumes in proximity to radiosensitive organs at risk (OARs), with limited prospective dosimetric guidance. We report the dosimetric outcomes of SBRT plans delivered on CYTOSHRINK to characterize treatment planning quality and feasibility of cytoreductive SBRT.Materials/Methods:
Dosimetric data were available for 39 of 43 patients randomized to SBRT across participating centres. All patients underwent CT simulation with motion management and immobilization strategies per institutional policies. SBRT planning was performed according to protocol-defined coverage goals and constraints. Treatment plans were uploaded to a central repository for review. Metrics analyzed included Internal Target Volume (ITV) and Planning Target Volume (PTV) coverage (V30–40Gy), maximum PTV dose, and OAR doses to kidneys, stomach, duodenum, small and large bowel. Plans not meeting protocol objectives were qualitatively assessed.Results:
The most common prescription doses were 30 Gy (54%), 35 Gy (26%), and 40 Gy (21%), delivered in 5 fractions. Median ITV and PTV volumes were 484 cc (59-2779 cc) and 721 cc (112-3384 cc), respectively. Median PTV coverage was 95%, and median ITV coverage was 99.9% of the prescription dose. Seven plans (18%) did not meet PTV and/or ITV coverage goals; this was most commonly due to proximity of the target to bowel. Maximum PTV dose demonstrated expected heterogeneity, with a median of 125% of the prescribed dose. Median stomach, duodenum, small bowel, and large bowel maximum doses were within protocol constraints across all plans. Ipsilateral and contralateral mean kidney doses were acceptable, with a median of 24.7 Gy and 3.0 Gy across patients, respectively. Dosimetric outcomes are summarized in Table 1. Only one patient did not complete SBRT, due to anemia related to disease progression. Of the 39 patients who received SBRT, 24 (62%) completed at least 4 cycles of nivolumab/ipilimumab as scheduled.Conclusion:
Cytoreductive SBRT on the CYTOSHRINK trial achieved high target coverage. Despite challenging anatomy and large tumour volumes, OAR tolerances were respected. This prospective data supports the deliverability and feasibility of cytoreductive SBRT in mRCC. Table 1: Summary of Key Dosimetric Outcomes| Parameter | Goal | Median | Range |
| PTV Coverage (V30-40Gy, %) | =90 | 95 | 54.9-100 |
| ITV Coverage (V30-40Gy, %) | =99 | 99.9 | 70.2-100 |
| PTV Maximum Dose (% of Prescribed Dose) | =140 | 125 | 106-139.5 |
| Ipsilateral Kidney Dmean (Gy) | ALARA | 24.7 | 3.0-38.7 |
| Contralateral Kidney Dmean (Gy) | <11.3 | 3.0 | 0.9-8.7 |
| Stomach Dmax (Gy) | <32 | 12.5 | 0.21-31.9 |
| Duodenum Dmax (Gy) | <32 | 18.5 | 7.4-31.9 |
| Small Bowel Dmax (Gy) | <35 | 25.0 | 0.7-34.9 |
| Large Bowel Dmax (Gy) | <38 | 31.2 | 11.0-37.4 |