Discrepant salivary gland response after radioiodine and MIBG therapies

BACKGROUND: A retrospective study using PET/CT imaging with I-124-labeled metaiodobenzylguanidine (I-124-MIBG) was performed to estimate the (radiation) absorbed dose to the salivary glands in neuroendocrine cancer patients undergoing I-131-MIBG therapy and to compare these results with those in radioiodine (I-131-iodide) therapy. METHODS: Twenty-seven patients received individual I-124-MIBG-PET/CT dosimetries, among whom 18 had not previously undergone any MIBG therapies (patient group before treatment) and 9 had already received MIBG therapies prior to the tracer dosimetries (patient group after treatment). For each patient, three or four I-124-MIBG PET/CT scans were performed at approximately 4 and 24 hours, as well as at approximately 48 or/and >= 96 hours after tracer injection. The absorbed doses per administered I-131-MIBG activity to the submandibular and parotid glands were calculated based on the MIRD concept, with its assumption of a uniform glandular activity distribution. RESULTS: The mean +/- standard deviation of the (self-) absorbed dose per activity averaged over both patient groups and salivary gland types was 0.53 +/- 0.24 G y/GBq (median, 0.49 G y/GBq; range, 0.17-1.38 G y/GBq). The absorbed doses per activity of the patient group before treatment did not significantly deviate from those of the patient group after treatment (P= 0.67). In the patient group after treatment, the mean +/- standard deviation of the cumulative I-131-MIBG activity was 20 +/- 12 GBq (median, 16 GBq; range, 10-50 GBq). Among the patient groups, no significant absorbed dose difference was found between the submandibular and parotid glands (P> 0.24). In comparison to radioiodine therapy, the estimated absorbed dose per activity in MIBG was significantly higher (P< 0.001), on average twice as high, contradicting the relationship between the absorbed dose and clinical observation of glandular side effects. CONCLUSIONS: The discrepant salivary gland responses in MIBG and radioiodine therapies suggest a different radiotherapeutical distribution on microscopic scale within the glandular tissue and prove the clinical relevance of a microdosimetric analysis.