SPECT/CT-based dosimetry of salivary glands and iodine-avid lesions following 131I therapy

Abstract

The purpose was to provideuptake and radiation dose estimates to salivary glands (SG) and pathologic lesions following radioiodine therapy (RIT)of differentiated thyroid cancer patients (DTC). A group of DTCpatients (n = 25) undergoing 131I therapy joined this study with varying amounts of therapeuticactivity. SequentialSPECT/CT scanswere acquiredat 4 ± 2, 24 ± 2, and 168 ± 3h following administration of 3497-9250MBq131I. An earlier experiment withAcrylic glass body phantom (PET Phantom NEMA 2012 / IEC 2008) was conducted for system calibration includingscatter, partial volume effect and count loss correction.Dose calculation was made via IDAC-Dose 2.1 code. The absorbed dose to parotid glands was 0.04-0.97Gy/GBq (median: 0.26Gy/GBq). The median absorbed dose to submandibular glands was 0.14Gy/GBq (0.05 to 0.56Gy/GBq). The absorbed dose to thyroid residues was from 0.55 to 399.5Gy/GBq (median: 21.8Gy/GBq), and that to distal lesions ranged from 0.78 to 28.0Gy/GBq (median: 3.12Gy/GBq). 41% of the thyroid residues received dose > 80Gy, 18% between 70-80Gy, 18% between 40-70Gy, and 23% has dose < 40Gy. In contrast, 18% of the metastases exhibited a dose > 80Gy, 9% between 40-60Gy, and the dose to the vast majority of lesions (64%) was < 40Gy. It was inferred that dose estimation after RIT with SPECT/CT is feasible to apply, together with good agreement with published 124I PET/CT dose estimates. A broad and sub-effective dose range was estimated for thyroid residues and distal lesions. Moreover, the current methodology might be useful for establishing a dose-effect relationship and radiation-induced salivary glands damage after RIT.