SBRT is an effective locoregional treatment option for HCC. Persistent APHE is common and does not necessarily indicate viable neoplasm; thus, standard response assessment such as mRECIST should be used with caution, particularly in the early phases post-SBRT therapy.

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Purpose To determine the long-term natural history of size change in SBRT-treated HCC to identify an imaging biomarker to help assess treatment response. Methods This was a retrospective cohort study of consecutive HCCs treated with SBRT from January 2008 to December 2016 with either 2 years post-treatment MRI follow-up or post-treatment resection histology. Size, major features for HCC, and mRECIST and LI-RADS v.2018 treatment response criteria were assessed at each post-treatment MRI. Local progression, distant progression, and survival were modeled with Kaplan Meier analyses. Results 56 HCCs met inclusion criteria. Mean baseline HCC diameter was 30 mm (range: 9-105 mm). At 3 months, 76% (N = 43) of treated HCCs decreased in size (mean reduction: 8 mm, range: 5-99 mm) and 0% (N = 0) increased in size. By 24 months, 11% (N = 5) had increased in size and were considered local progression. APHE remained in 77% (43/56) at 3 months, 38% (19/50) at 12 months, and 23% (11/47) at 24 months. mRECIST-defined viable disease was observed in 77% (43/56) at 3 months and 20% (9/47) at 24 months. LI-RADS v.2018 criteria identified viable or equivocal disease in 0% at 3 months and 10% (5/47) at 24 months. Conclusion Gradual loss of APHE and slow decrease in size are normal findings in HCCs treated with SBRT, and persistent APHE does not indicate viable disease. mRECIST is not accurate in the assessment of HCC after SBRT due to an overreliance on APHE to define viable disease. Increasing mass size or new nodular APHE at the treatment site may indicate local progression.

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