Daily image-guided carbon ion stereotactic radiotherapy of hepatocellular carcinoma: paving the way for MR-guided online adaptive treatment.

BACKGROUND

We aim to quantify the dosimetric benefits of daily online adaptive (DOA) carbon ion stereotactic radiotherapy (CISR) in hepatocellular carcinoma (HCC).

PATIENTS AND

METHODS

13 patients received 4 × 10.5 Gy (RBE-weighted dose) CISR to 14 HCC lesions in challenging locations (N = 10: liver dome, N = 3: < 1 cm from intestines). All patients underwent daily in-room CT, with forward calculation of the baseline plan. Additionally, four patients were shuttled to a 1.5 Tesla MR scanner in treatment position for MRI simulation. Based on visual review of all data, the physician decided to treat or to perform offline plan adaptation and treat another day. We compare the dosimetry properties of four scenarios per fraction: (1) baseline plan, (2) no adaptation, (3) occasional offline adaptation (OOA), (4) DOA.

RESULTS

Baseline planning target volume (PTV) and internal target volume (ITV) coverage by the prescribed dose (median coverage: PTV: 97%, ITV: 100%) significantly decreased for no adaptation (PTV: 87.6%, p = 0.0001; ITV: 97.7%, p = 0.007) and OOA (PTV: 87.8%, p = 0.0001; ITV: 97.7%, p = 0.007), while DOA maintained high PTV coverage (PTV: 95.7%, p = 0.17; ITV: 100%, p = 0.53). Frequent violations of organs-at-risk (OAR) dose constraints occurred for no adaptation and OOA (both 20/52 fractions, 38.5%). They affected the bowel, heart and esophagus up to 6 cm from the PTV. DOA led to considerably fewer (6/52 fractions, 11.5%) and less intense violations of OAR dose constraints.

CONCLUSIONS

Daily CT- and MR-guided CISR is feasible in the liver, and demonstrates frequent violations of initial treatment planning objectives when non-adaptive or offline adaptive approaches are followed. DOA could improve target coverage and OAR protection, while particularities of carbon ions such as overdoses far from the target should be considered.