Assessment and clinical relevance of pulmonary shunt on 166Ho-Scout Imaging in hepatocellular carcinoma.

Aim: Prediction of posttreatment lung mean dose (LMD) during liver radioembolization (RE) work-up is essential for risk evaluation of radiation pneumonitis, especially when treating large hepatocellular carcinoma (HCC) where the chance of arteriovenous shunting is not negligible. In case of holmium-166-([¹⁶⁶Ho])-RE, either [^99mTc]TcMAA or ¹⁶⁶Ho-microspheres can be used as scout tracers. Safety of use of ¹⁶⁶Ho-scout has been demonstrated previously, but, to our notice, evaluation of lung radiation dose due to ¹⁶⁶Ho-scout activity in case of significant lung shunting has not been described so far. Therefore, a retrospective study was conducted to evaluate the presence of pulmonary shunting in HCC patients influencing therapeutical approach and to assess lung ¹⁶⁶Ho-scout dose in these patients.

Materials and methods: Twenty-nine HCC patients referred for RE were retrospectively evaluated. All work-ups were performed with ¹⁶⁶Ho-microspheres. Scout imaging consisted of a hybrid SPECT/CT acquisition covering the thoraco-abdominal region. As mentioned in the manufacturer's instruction for use of ¹⁶⁶Ho-microspheres, the possibility of > 30 Gy lung radiation exposure in a single treatment is withheld as contra-indication for RE. In patients with lung shunt resulting in predicted posttreatment LMD > 30 Gy, lung ¹⁶⁶Ho-scout dose due to patient-specific injected activity was calculated, alongside two hypothetical scenarios: lung ¹⁶⁶Ho-scout dose related to patient's lung shunt fraction (LSF) assuming administration of leaflet prescribed maximum ¹⁶⁶Ho-scout activity and in case of 100% LSF according to patient-specific injected scout activity. Afterwards, these patients were followed for 3 months or till death.

Results: In the 29 patients, average predicted posttreatment LMD was 10.0 Gy (range 0.1-138.9 Gy), four of them revealing predicted LMD > 30 Gy. Based on patient-specific injected ¹⁶⁶Ho-scout activity (range 100-200 MBq), average lung ¹⁶⁶Ho-scout dose of 0.5 Gy (range 0.1-0.8 Gy) was calculated in these 4 patients. Assuming administration of leaflet prescribed maximum activity of 250 MBq, average lung ¹⁶⁶Ho-scout dose would be 0.9 Gy (range 0.4-1.7 Gy). In case of a 100% LSF, average lung ¹⁶⁶Ho-scout dose would be 2.2 Gy (range 1.5-2.7 Gy) due to patient-specific scout activity. In these 4 patients, RE was denied and alternative treatment was started. No pulmonary adverse events related to ¹⁶⁶Ho-scout were recorded.

Conclusion: This study supports previous reports that ¹⁶⁶Ho-scout is a safe alternative to [^99mTc]TcMAA -scout and underlines the importance of predicting posttreatment LMD when treating large HCC since 13.8% of our patient group presented arteriovenous shunting with impact on treatment planning.