Voxel-Based Dosimetry Predicts Local Tumor Progression Post 90Y Radiation Segmentectomy of Colorectal Liver Metastases

Background: Radiation segmentectomy (RS) is an alternative potential local curative treatment for selected colorectal liver metastases (CLMs) not amenable to ablation or limited resection. Purpose: The aim of this study was to evaluate the dosimetric response of low volume CLMs to RS in heavily pretreated patients who are not candidates for resection or percutaneous ablation. Patients and Methods: This single-center retrospective study evaluated CLMs patients treated with RS (prescribed tumor dose >190 Gy) from 2015 to 2023. RS doses to tumor(s) and margins were calculated from SPECT/CT and PET/CT images. Response and local tumor progression (LTP) were assessed using anatomic (RECIST 1.1) and metabolic (PERCIST) criteria. LTP-free survival (LTPFS) and overall survival were estimated with Kaplan-Meier methodology. Variables were assessed as predictors of LTPFS using the Cox proportional hazards model. Results: Thirty-six patients underwent 38 RS procedures to treat 57 tumors. Median time from initial diagnosis to detection of liver metastases and RS were 16.4 (interquartile range: 6.5-32.2) and 26.8 (interquartile range: 12.5-40.0) months, respectively. Median overall survival after RS was 14.3 (95% confidence interval [CI]: 10.8-30.7) months. Predictors of LTPFS included tumor number(s), mean tumor dose (MTD), and margin mean absorbed dose (MMAD). Complete radiographic (hazards ratio [HR]: 1.29e-16, 95% CI: 4.06e-17-4.07e-16, P < 0.001) and metabolic response (HR: 0.38, 95% CI: 0.15-0.95, P = 0.038) correlated with prolonged LTPFS. One-year LTPFS rate was 83.3% for tumors receiving MTD >= 400 Gy and a 5-mm surrounding MMAD >= 350 Gy (P = 0.006). No instances of LTP were observed when tumors received stereotactic irradiation over 300 Gy (at least 95% of the tumor volume received >= 300 Gy). One-year LTPFS rate for tumors receiving MTD >= 400 Gy was 68.6% versus 14.3% for those that did not reach this threshold (P = 0.013). In multivariate analysis, MTD >= 400 Gy and 5-mm MMAD >= 350 Gy were independent predictors of LTPFS (HR: 0.11; 95% CI: 0.01-0.81; P = 0.03). Conclusions: MTD >= 400 Gy, MMAD >= 350 Gy, and stereotactic tumor irradiation >= 300 Gy are associated with prolonged LTPFS after RS for CLMs.