Impact of inadequate respiratory motion management in SBRT for oligometastatic colorectal cancer

Purpose: Stereotactic body radiotherapy (SBRT) in oligometastatic colorectal cancer (CRC) resulted in a disappointing 1-year local control rate of 54% in our experience. We aimed to determine the root cause(s). Methods: 47 oligometastatic CRC patients were treated with SBRT by helical tomotherapy to a dose of 40 or 50 Gy in 10 fractions, without specific respiratory motion management and PTV-margins of 10–10–12 mm in all patients. The local recurrences (LRs) were delineated on diagnostic PET–CT scans and co-registered with initial planning CTs. LRs were classified as in-field or marginal with respect to the initial dose distribution, and predictors for LR were determined. Results: Out of 105 irradiated metastases, LR modeling yielded 15 in-field and 15 marginal failures. Metastases in moving organs (liver and lung) exhibited a local control of 53% at 1-year (95% confidence interval (CI): 38–67%), compared to 79% for lymph nodes (95% CI: 32–95%). The first group exhibited a sixfold increased risk compared to the latter on multivariate analysis (p = 0.01). Conclusions: The nature and locations of LR indicated that dose prescription and methodology were both inadequate for liver and lung metastases. This study demonstrates the need for individual respiratory motion management and a biological effective dose of >75 Gy