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 Yttrium-90 (90Y) radioembolization. It may occur 1 to 6 months after therapy, in case a significant part of the 90Y microspheres shunt to the lungs. In current clinical practice, a predicted value of lung dose greater than 30Gy 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 literature. In this study, the relationship between the lung dose value and the eventual occurrence of radiation pneumonitis after 90Y radioembolization was investigated. Methods: A total of 317 90Y liver radioembolization procedures performed during an 8-years period (Feb.2012-Sep.2020) were retrospectively analyzed. Predicted lung mean dose using 99mTc-MAA planar scintigraphy (LMDMAA) acquired during the planning phase and left lung mean dose (LMDY-90) using the 90Y PET/CT acquired after the treatment were calculated. For the lung dose computation, it was chosen to use the left lung as representative lung volume, to compensate for the scatter from the liver moving in the cranial-caudal direction due to breathing and mainly affecting the right lung. Results: Two hundred and seventy-two patients underwent 90Y procedures, of which 63% performed with glass microspheres and 37% with resin microspheres. Median injected activity was 1974MBq (range: 242-9538MBq). Median LMDMAA was 3.5Gy (range: 0.2-89.0Gy). For 14 procedures LMDMAA was >30Gy. Median LMDY-90 was 1Gy (range: 0.0-22.1Gy). No patients had a LMDY-90 >30Gy. Of the three patients with a LMDY-90 >12Gy, two patients (LMDY-90 = 22.1Gy, LMDMAA = 89Gy and LMDY-90 = 17.7Gy, LMDMAA = 34.1Gy, respectively) developed radiation pneumonitis and consequently died. A third patient with a LMDY-90 equal to 18.4Gy (LMDMAA = 29.1Gy) died 2 months after treatment, before imaging evaluation, due to progressive disease. Conclusion: The occurrence of radiation pneumonitis as a consequence of lung shunt following 90Y radioembolization is rare (<1 %). No radiation pneumonitis developed in cases with a measured LMDY-90 lower than 12Gy.