Indicator for local recurrence of hepatocellular carcinoma after proton beam therapy: analysis of attenuation difference between the irradiated tumor and liver parenchyma on contrast enhancement CT.

OBJECTIVES We aimed to identify dynamic computed tomography (CT) features that can be used for prediction of local recurrence of hepatocellular carcinoma (HCC) after PBT. METHODS We retrospectively retrieved CT scans of patients with PBT-treated HCC, taken between January 2004 and December 2016. Seventeen recurrent lesions and 34 non-recurrent lesions were retrieved. The attenuation difference between irradiated tumor and irradiated parenchyma (ADHCC-IP) was compared in the two groups by using the Mann-Whitney U test. Cut-off value of ADHCC-IP was estimated by using the Youden index. RESULTS The follow-up time after PBT initiation ranged from 374 to 2402 days (median, 1069 days) in recurrent lesions, and 418 to 2923 days (median, 1091.5 days) in non-recurrent lesions (p = 0.892). The time until appearance of local recurrence after PBT initiation ranged from 189 to 2270 days (median, 497 days). ADHCC-IP of recurrent lesions (mean, -21.8 Hounsfield units (HU); from -95 to -31 HU) was significantly greater than that of non-recurrent lesions (mean, -51.7 HU; from -117 to -12 HU) at 1-2 years in portal venous phase (PVP) (p = 0.039). Five-year local tumor control rates were 0.93 and 0.56 in lesions with ADHCC-IP at 1-2 years in PVP < -55 HU and ≥ -55 HU, respectively. CONCLUSIONS The attenuation difference between irradiated HCC and irradiated liver parenchyma in PVP at 1-2 years after PBT can predict long-term local recurrence of HCC after treatment. ADVANCES IN KNOWLEDGE We identified a cut-off value for contrast enhancement of HCC after PBT that could predict future local recurrence.