In 2018, the first LI-RADS Treatment Response Assessment (TRA) algorithm was published to help radiologists assess the response of hepatocellular carcinoma after locoregional treatment. This algorithm allows imaging findings to be classified into four different diagnostic categories:

• LR-TR Nonevaluable: If treatment response cannot be evaluated due to image degradation or omission.
• LR-TR Nonviable: If probably or definitely not viable.
• LR-TR Viable: If equivocally viable.
• LR-TR Equivocal: If probably or definitely viable.

Follow-up CT or MRI scans are usually performed three months after treatment, although one-month post-treatment imaging may be useful for certain therapies.

Since 2018, studies have highlighted differences in expected post-treatment imaging patterns for lesions treated with radiation therapies. These differences arise due to the distinct pathophysiological effects of radiation on HCC and liver tissue.

1. Locoablative Therapies

These are percutaneous techniques that use chemical agents (such as ethanol) or energy to achieve tumour ablation. Energy-based therapies can utilise either thermal or non-thermal energy. Thermal energy therapies, including radiofrequency and microwave ablation, are the most commonly used locoablative therapies. Each of these has its specific indications according to the BCLC 2022 algorithm (see Fig.); nevertheless, they all induce cell death through coagulative necrosis.

Common findings in imaging tests in the first 1 to 3 months are described in Figure 4. 

2. Transcatheter therapies.

These are endovascular procedures where feeding arteries of the hepatocarcinoma are occluded by delivering different types of embolic material. There are four main types of trascatheter therapies:

• Transarterial embolisation (TAE): uses bland embolization material.
• Transarterial chemoembolisation (TACE): the most used procedure; uses a mixture of chemotherapeutic agent and lipiodol.
• Drug-eluting beads trasarterial embolisation (DEB- TACE): uses hydrogel beads loaded with a chemotherapeutic agent. Its advantage is that releases chemotherapy slowly within the tumor bed, avoiding the peak of chemotherapy (on the contrary ofTACE) and reducing toxicity.
• Transarterial radioembolisation (TARE): uses microspheres loaded with Yttrium-90 (90Y), and its main therapeutic action is through radiation.

TACE and DEB-TACE induce tumor coagulative necrosis through the combination of ischemia and chemotherapy toxicity. There are the first line treatment in patients with BCLC stage B(intermediate stage) for bridging or downstaging to liver transplant. Also, they are used in BCLC stage A with more than one nodule (up to three), in BCLC stage 0 when no other treatments are feasible and in advance stages (BCLC B or more) with palliative intention.

Findings after trascatheter therapies in follow up studies are similar that those seen in locoablative therapies (Figure 4).

In patients treated with TACE an hyperdense area of embolization is seen, because lipiodol contains yodo. This could obscure evaluation of treated lessions in CT.

3. Radiation Therapies

These include two types:

• Stereotactic Body Radiotherapy (SBRT): A type of external radiotherapy that delivers high fractional-dose radiation in fewer sessions than conventional radiotherapy. It uses a highly conformal dose with a rapid drop-off gradient to prevent damage to the surrounding healthy hepatic tissue.
• Transarterial Radioembolisation (TARE): A type of internal radiation therapy. Yttrium-90 (90Y)-loaded microspheres reach the tumour bed through arterial embolisation and release β-particle radiation, which has very short tissue penetration, thus minimising damage to the surrounding liver.

Both therapies induce cell death through two pathways:

• Immediate cell death: Direct DNA damage induces apoptosis.
• Delayed cell death: Indirect DNA damage induces senescence.

Senescent cells remain metabolically active but cannot replicate, so they will eventually die. This explains why tumours treated with radiation exhibit distinct imaging findings.

Although radiation therapies are not first-line treatments for HCC, the BCLC algorithm considers TARE as an alternative treatment in BCLC stages 0 and A, ideally for tumours ≤ 8 cm, when no other treatments are feasible. These therapies can be used with curative, downstaging, bridging, or palliative intent.

Findings after Radiation Therapies in the first 1 to 3 months of follow-up studies are described in Figure 5.