Local targeted treatment (LRT) for HCC exploits the pathophysiology of dual blood supply of hepatic tumors. It blocks the predominant arterial blood supply leading to reduction in blood flow and cell death via either ischemia, thermal/coagulation, or radiation effect which do not result in tumor shrinkage [14, 15], but show necrosis and reduction in the enhancement pattern which are not accounted in RECIST 1 or RECIST 1.1 criteria. The newer guidelines have incorporated the enhancement criteria (mRECIST) and necrotic parameters in the response assessment of HCC [16–18].

Identifying enhancement features could be difficult due to benign posttreatment inflammatory changes, or a heterogeneous nature of the tumor environment and functional imaging like diffusion-weighted magnetic resonance (DWMRI) or FDG PET/CT is recommended.

Response assessment with FDG PET/CT is done either by visual assessment of the tumor site in the pre- and post-therapy scans in comparison with blood pool uptakes or by calculating the reduction of FDG uptake at the tumor site using various semiquantitative or quantitative methods, e.g., ratio of tumor SUV to the liver or mediastinum or SUV max. Studies show better survival and event-free rates in patients who depict significant reduction in the uptakes at the tumor site [19, 20]. When compared to conventional imaging methods like CECT, FDG PET/CT showed a higher sensitivity in identifying residual viable tissue which is generally seen as a focal eccentric uptake in the periphery [21, 22] (Fig. 6.4).

In a bid to standardize the response assessment of solid tumors using FDG PET/CT, the PERCIST criteria was suggested by Wahl et al., which is adapted from the anatomical-based RECIST 1.1 principle and measures the FDG standard uptake (lean) in up to five index lesions (up to two lesions per organ) with highest FDG uptake. The response is expressed as percentage change in peak standard uptake of sum of lesions of baseline and posttreatment scans [23] (Table 6.1).

The ideal time to assess response would be at 3 months post therapy allowing for posttreatment-related changes to settle which could cause false-positive or equivocal readings.

Radiofrequency ablation is a localized treatment option for smaller tumors and those away from vessels. FDG PET/CT for this indication should be performed prior to initiation of inflammatory changes, i.e., within 6–12 h of procedure to avoid masking of the residual disease seen as a focal uptake in the periphery [25] (Fig. 6.5).

Post therapy scans in patients treated with 90 Yttrium tracers are evaluated with a bremsstrahlung imaging using the SPECT/CT scanner. Positron emissions from the Y90 radioisotope have been utilized to obtain an immediate post therapy PET/CT study. The advantage of this modality is the clear demarcation of region of radioisotope delivery and dosimetry to calculate actual dose delivered (Fig. 6.6). The presence of small extravasation of tracer into stomach or elsewhere is also identified which could have been missed on a pretreatment shunt evaluation dummy scan with colloids [26–28].