Feasibility of Conventional Abdominal Ultrasound for Monitoring Tumor Size After Stereotactic Body Radiotherapy for Hepatocellular Carcinoma

Background/Objectives: Stereotactic body radiotherapy (SBRT) is increasingly used for hepatocellular carcinoma (HCC) that is unsuitable for surgery, radiofrequency ablation (RFA), or transplantation. Although current evidence for imaging assessment after SBRT is largely based on contrast-enhanced computed tomography (CT) or magnetic resonance imaging (MRI), repeated contrast-enhanced imaging may be difficult to perform at every routine follow-up visit. Thus, we evaluated whether conventional abdominal ultrasound (US) can monitor tumor size after SBRT for HCC. Methods: We retrospectively reviewed 67 consecutive patients who underwent SBRT for HCC at our institution between January 2015 and October 2020. After excluding patients treated for local recurrence after RFA or transarterial chemoembolization, those whose lesions were not visible on pretreatment US, and those without follow-up US within one year, 32 patients with 32 nodules were analyzed. Tumor visibility and size changes on US were assessed before treatment and at <6, 6–12, and 12–18 months after SBRT. Results: The treated lesion was identified as a discrete nodule on US in 100% (15/15; 95% CI, 78.2–100%), 75.0% (18/24; 95% CI, 53.3–90.2%), and 50.0% (8/16; 95% CI, 24.7–75.3%) of examinations at <6, 6–12, and 12–18 months, respectively. In all cases in which the lesion was no longer measurable on US, contrast-enhanced CT/MRI showed complete or partial response. Local tumor progression occurred in one patient (3.1%) during a median follow-up of 24.1 months; in this patient, interval enlargement was first detected by US 3.7 months after SBRT and was subsequently confirmed by dynamic CT/MRI. Conclusions: These descriptive findings suggest that in selected patients with lesions clearly visible on pretreatment US, conventional abdominal US may provide complementary morphologic information during the first year after SBRT when used alongside periodic dynamic CT/MRI. Prospective validation is required before routine implementation.