Determination of thyroid remnant geometrical parameters in the dosimetry of radioiodine ablation

Grošev, Darko (2011) Determination of thyroid remnant geometrical parameters in the dosimetry of radioiodine ablation. Doctoral thesis, Faculty of Science > Department of Physics.

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Abstract

The aim of investigation was to improve dosimetric calculations in radioiodine ablation of thyroid remnant tissue in patients with well differentiated thyroid cancer. Methods are presented for calculation of remnant mass and beta self absorbed fraction. These two geometrical parameters are needed for beta S-factor calculation (mean absorbed dose per unit cumulated activity) of the thyroid remnant. Thyroid remnant dimensions were determined from patient neck pinhole scintigrams. Computer image processing included an algorithm for segmentation of the object from its background in the following geometrical models: sphere, cylinder, ellipsoid. Measured dimensions were corrected for restricted spatial resolution of the imaging instrument: pinhole gamma camera. Calculated remnant dimensions are input data for the Monte Carlo (MC) calculation of the beta self absorbed fraction. In MC calculation the complete beta spectrum of 131I was used. MC simulation of electron and photon transport was performed in unit density medium (water). Uncertainty in remnant mass calculation is the main cause of the error in the S-factor calculation. If the appropriate correction for the resolution loss is not applied, for object dimensions ≤10 mm, measured dimensions are significantly overestimated, while the S-factor is underestimated. As a consequence, accurately calculated absorbed fraction improves the accuracy of dose calculation only if the remnant mass is accurately measured in the first place. Thyroid remnant mass measurement and beta self absorbed fraction calculation improve dosimetric protocol in radioiodine ablation because it is a step towards the patient specific dosimetry. Absorbed dose obtained from the patient specific beta S-factor may enable more reliable correlation with the outcome of the radioiodine ablation therapy.

Item Type: Thesis (Doctoral thesis)
Keywords: gamma camera ; radioiodine therapy ; internal dosimetry ; Monte Carlo simulation
Supervisor: Lončarić, Srećko
Date: 23 November 2011
Number of Pages: 122
Subjects: NATURAL SCIENCES > Physics
Divisions: Faculty of Science > Department of Physics
Depositing User: Gordana Stubičan Ladešić
Date Deposited: 21 Dec 2013 23:13
Last Modified: 21 Dec 2013 23:13
URI: http://digre.pmf.unizg.hr/id/eprint/38

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