Lung dose measured on post-radioembolization 90Y-PET/CT and incidence of radiation pneumonitis

Radiation pneumonitis is a rare but possibly fatal side effect of 90Y radioembolization. It may occur 1-6 mo after therapy, if a significant part of the 90Y microspheres shunts to the lungs. In current clinical practice, a predicted lung dose greater than 30 Gy is considered a criterion to exclude patients from treatment. However, contrasting findings regarding the occurrence of radiation pneumonitis and lung dose were previously reported in the literature. In this study, the relationship between the lung dose and the eventual occurrence of radiation pneumonitis after 90Y radioembolization was investigated. Methods: We retrospectively analyzed 317 90Y liver radioembolization procedures performed during an 8-y period (February 2012 to September 2020). We calculated the predicted lung mean dose (LMD) using 99mTc-MAA planar scintigraphy (LMDMAA) acquired during the planning phase and left LMD (LMDY-90) using the 90Y PET/CT acquired after the treatment. For the lung dose computation, we used the left lung as the representative lung volume, to compensate for scatter from the liver moving in the craniocaudal direction because of breathing and mainly affecting the right lung. Results: In total, 272 patients underwent 90Y procedures, of which 63% were performed with glass microspheres and 37% with resin microspheres. The median injected activity was 1,974 MBq (range, 242-9,538 MBq). The median LMDMAA was 3.5 Gy (range, 0.2-89.0 Gy). For 14 procedures, LMDMAA was more than 30 Gy. Median LMDY-90 was 1 Gy (range, 0.0-22.1 Gy). No patients had an LMDY-90 of more than 30 Gy. Of the 3 patients with an LMDY-90 of more than 12 Gy, 2 patients (one with an LMDY-90 of 22.1 Gy and an LMDMAA of 89 Gy; the other with an LMDY-90 of 17.7 Gy and an LMDMAA of 34.1 Gy) developed radiation pneumonitis and consequently died. The third patient, with an LMDY-90 of 18.4 Gy (LMDMAA, 29.1 Gy), died 2 mo after treatment, before the imaging evaluation, because of progressive disease. Conclusion: The occurrence of radiation pneumonitis as a consequence of a lung shunt after 90Y radioembolization is rare (<1%). No radiation pneumonitis developed in patients with a measured LMDY-90 lower than 12 Gy.