A112
Dosimetry for radioiodine treatment of differentiated thyroid cancer
Glenn Flux, Masud Haq, Sarah Chittenden, Susan Buckley, Cecilia Hindorf, Kate Newbold, Clive Harmer
Royal Marsden Hospital & Institute of Cancer Research, Sutton, UK
Background
Molecular radiotherapy (MRT) is a systemic treatment that selectively targets malignant tissue to which it delivers a radiation dose. It therefore has much in common with both chemotherapy and with external beam radiotherapy. In contrast to radiotherapy practice, dosimetry for MRT has not traditionally been performed. The aim of this study was to ascertain whether dosimetry for the treatment of thyroid cancer with I-131 NaI could be sufficiently accurate to determine a dose-response relationship.
Method
Twenty-three patients were administered 3000 MBq I-131 NaI following a total or near total thyroidectomy. Up to 4 SPECT scans were acquired following administration. Image quantification was performed by independent scanning of a calibration phantom and both scatter and attenuation corrections were performed. To obviate uncertainties caused by delineation of a functional target volume and by heterogeneity of uptake, dosimetry was calculated for the maximum voxel in the reconstructed data. Trapezoidal integration was used to avoid subjective curve fitting.
Results
Eighteen of the 23 patients had a successful ablation. The maximum voxel absorbed dose to thyroid remnants of patients that responded was 99 Gy 128 Gy (range 12 Gy 570 Gy). Patients with recurrence received absorbed doses of 25 Gy 17 Gy (range 7 Gy 49 Gy). An independent samples t-test indicated a significant difference between these values (p=0030). All patients that received a radiation dose to the thyroid remnant greater than 49 Gy had a successful ablation whilst recurrence was seen in 38% of those receiving less than 49 Gy.
Conclusion
Whilst the overall response rate for the treatment of thyroid remnants is 75 80%, this study demonstrates that success is dependent on the absorbed dose delivered. Routine dosimetry should be performed for all patients receiving radioiodine treatment and similar studies should be performed for other radiopharmaceuticals.
