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Macro-To-Micro Modeling and Dosimetry for the Salivary Gland: Application to 131I and 225Ac-PSMA Therapy

R Hobbs*, D Plyku , G Sgouros , Johns Hopkins Medical Institute, Baltimore, MD


(Thursday, 8/2/2018) 10:00 AM - 12:00 PM

Room: Davidson Ballroom A

Purpose: Salivary gland toxicity is a quality of life concern in radioiodine treatment of thyroid cancer or radiolabeled-PSMA therapy of prostate cancer. Clinically observed toxicity is inconsistent with absorbed doses (AD) to the salivary glands calculated by absorbed fraction methods. Small scale anatomical modeling and activity apportionment, the macro-to-micro methodology, has been proven to reconcile discrepancies between whole organ AD values and clinically or pre-clinically observed toxicities.

Methods: Uptake in the salivary glands has been shown to be primarily confined to the epithelial striated ducts. Dimensions of typical striated duct cells were obtained from the literature. Based on the fraction of occupancy of these cells within the salivary glands (5.1 %), a grid of striated cells placed randomly in spheres of increasing size was used to simulate decay of activity in these cells from which ratios of striated duct AD and acinar cell AD to whole organ AD were calculated for these same isotopes using GEANT4 Monte Carlo.

Results: The grid-based Monte Carlo results showed a ductal cell AD to salivary whole organ AD ratio of 3.5 - 3.3 for ¹³¹I and 14.6 for ²²�Ac dependent on salivary gland size (5 - 25 ml). Biological effective dose values for a patient population treated with ¹³¹I for the striated ducts ranged from 3 - 25 Gy, as opposed to the 1-5 Gy calculated to the whole gland.

Conclusion: This is a significant step in quantifying the discrepancy between clinically observed toxicity and predicted toxicity based on whole organ AD values using small scale dosimetry, which has been shown to explain similar discrepancies in different cases, particularly alpha-particle dosimetry. This study shows that while the salivary glands may be considered as parallel organs for external beam radiation, the physiology for radiopharaceutical uptake means that this modality, they have a more complex structure.


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