Purpose: To investigate if Apparent Diffusion Coefficient (ADC) can identify radionecrosis (RN) which is a serious potential side-effect to stereotactic radiosurgery (SRS) and can be mistaken for tumour progression on imaging. There is an urgent need to develop tools that distinguish between necrosis, progression, and immunotherapy-induced pseudoprogression.
Methods: A retrospective study of 74 patients treated for melanoma with ipilimumab (prescription: 4x infusion every 3 weeks) and longitudinal Diffusion-weighted, T1-with-contrast, and FLAIR MRI on a range of clinical systems is being undertaken. The preliminary cohort consists of 27 patients. 5 were excluded as diagnosis to treatment was >1 year. 8 patients had diffusion-weighted-imaging prior to and after SRS. The SRS to ipilimumab time was +43-170 days (mean 93). SRS prescriptions ranged from 15-21Gy/1-3 fractions according to published protocols. 7 patients were imaged <20 days prior to SRS, 1 at 118 days, and all at times from 20-200 (mean: 109) days post-SRS. Lesions were contoured by an experienced radiation oncologist. Additional ROIs were derived by contracting originals by 2mm in all directions, thus separating the tumour core from the peritumoral rim. The mean ADC value for each ROI prior to radiosurgery, was compared to its corresponding mean value after to obtain a change in ADC value for each ROI. RN and non-RN groups, identified by an experienced oncologist through examination of clinical reports, were compared with a t-test .
Results: The change in ADC observed in core (peritumoral rim excluded) ROIs from prior to post treatment was significantly different between the RN and non-RN groups (p=0.047). This exaggerated a similar, not significant trend, observed when the whole tumour ROIs were examined (p=0.140).
Conclusion: Increasing ADC in the inner-core may be predictive of RN in this study. Observed differences between the complete ROI and contraction ROIs should be considered in future investigations.
Funding Support, Disclosures, and Conflict of Interest: Research supported by the Canadian Institute of Health Research and the Ontario Institute for Cancer Research