3141 - MRI-Guided Adaptive SBRT for Kidney and Adrenal Gland Tumors: An Analysis of the Intra-Fractional Tumor Motion and Inter-Fractional Dosimetric Impact

L. Cai, S. Ding, R. Liu, B. Liu, Y. Liu, M. Chen, M. Liu, Y. Liu, and L. He; State Key Laboratory of Oncology in South China, Guangdong Provincial Clinical Research Center for Cancer, Department of Radiation Oncology, Sun Yat-sen University Cancer Center, Guangzhou, Guangdong, China

Purpose/Objective(s): This study aims to investigate intrafraction motion of tumors and quantify the inter-fractional dosimetric impact between MRI-guided adaptive SBRT (ART) and non-ART in kidney and adrenal malignancies. Materials/

Methods: All treatment fractions were delivered to patients (pts) during free breathing with real-time MRI-guidance in 1.5 T Unity MR-Linac. At the beginning of the daily MRI scan, cine-MRI scans were acquired to assess gross tumor volume (GTV) motion. The GTV and organs at risk (OARs) were delineated on online MR images by the same radiation oncologist. Then, the online adaptative treatment plans were obtained by re-optimizing based on the contours on daily pre-treatment MRI by “adapt to shape” (ATS) workflow using the same beam parameters and optimization objectives from the reference plans. If there was a violation in an OAR dose constraint, then ART plan was optimized with sacrificing planning target volume (PTV) coverage. Non-ART plans for pts were generated by recalculating the dose from the reference plans on daily online MRI by “adapt to position” (ATP) workflow. PTV coverage metrics and OARs constraint violations were used to compare ART and non-ART plans.
Results: Thirty-six pts with 23 kidney lesions and 13 adrenal lesions were identified. The most commonly prescribed dose/fraction was 40Gy/5F. Sixteen fractions of SBRT were performed on ATS workflow in 9 pts. The median treatment time per fraction was 37min and 68min on ATP and ATS workflow (P <0.001). A total of 162 fractionated motion monitored data were collected. The 95% maximum translation of GTV in left-right, anterior-posterior and superior-inferior (SI) directions was 5.02mm, 5.35mm and 8.32mm for kidney lesions, and 3.72mm, 4.18mm, and 7.90mm for adrenal lesions. The comparation between ART and non-ART plans were performed on 45 fractions/9 pts with kidney lesions and 43 fractions/9 pts with adrenal lesions. A statistically significant (P <0.001) increase in PTV V_95% was observed for non-ART compared to ART for both kidney lesions (median 94.78% vs. 98.57%) and adrenal lesions (median 98.02% vs. 99.86%). Non-ART showed small-bowel and colon constraint violations in 20.00% (9/45) and 15.56% (7/45) of fractions, respectively, for kidney lesions. As for adrenal lesions, non-ART resulted in small-bowel and stomach constraint violations in 13.95% (6/43) and 4.65% (2/43) of fractions. No OARs constraint violation was observed after ART for both kidney and adrenal lesions.
Conclusion: The use of MR-cine in treatment planning allows for quantification of kidney and adrenal tumor motion during free breathing. The translation often exceeds commonly used 7-8mm PTV margins in SI direction, and ART can significantly improve PTV coverage while ensuring dose safety for OARs with sacrificing the time of treatment.