Practical Implementation of Dosimetry for Patients with Thyroid Carcinoma Undergoing I-131 Therapy

Abstract

Radioactive iodine (I-131) is used to treat patients with thyroid cancer. Currently the patient radiation dose received from these therapies is not routinely calculated but could be used to optimise treatments and improve patient outcomes [1], [2]. This work investigated methods given in literature for performing dosimetry that would practical to implement clinically, applied to the systems at Royal Surrey County Hospital (RSCH). The bone marrow is most at risk of damage from the radiation, limiting I-131 activity administered. Dosimetry for this organ requires measurements of whole body I-131 retention to calculate its residence time within the body. The impact of different whole body measurement methods was investigated using data from 39 patients, including a comparison of geometric mean and anterior measurements, background correction methods and impact of a late time point measurement at 7 days post therapy. The doses to target tissues are calculated using gamma camera images, with calibrations required to allow I-131 activity quantification. Published methods of calibration were investigated for the GE Optima 640 gamma camera at RSCH using I-131 “fast” counting mode. A non-paralysable deadtime of 6.91±0.08µs was measured using planar triple energy window (TEW) corrected images of a large cylindrical phantom with 20 to 2800MBq I-131. The optimal OSEM reconstruction using the GE Volumetrix was found to be 60 iterations, 10 subsets, no filter with TEW and CT attenuation correction using cylinders with 7.5-60mm diameters. SPECT sensitivity of 22.3±0.9cps/MBq was measured using a large cylindrical phantom and volume of interest. Partial volume correction (PVC) curves were investigated using spheres and cylinders, with spheres scanned individually and a sigmoidal fit with no plateau constraint found to be optimal. Work was performed to characterise the “fast” mode and significant differences were seen with low deadtime sensitivity and PVC curves compared to normal mode. A method for external validation of the tumour dosimetry process was developed in conjunction with the National Physical Laboratory. 13ml spheres were imaged with activities corresponding to different time points for a tumour biokinetic model. A 13% overestimation of dose was seen compared to a Monte Carlo ground truth, primarily due to errors with PVC at low volumes. Further areas for development of this dosimetry validation technique were identified and this could be used to provide assurance for centres performing dosimetry and harmonise procedures. Through this innovative work a practical method for performing dosimetry has been developed which once implemented will enable radiation doses to be calculated and treatments optimised for patients.

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jennifer Prince (Supervisor)