Determining additional radiation precautions by visual analysis of whole-body thyroid therapy images

Objectives A risk of using I-131 radiotherapy in thyroid cancer treatment is radiation exposure to other people near the patient after treatment. This risk is mitigated by using radiation safety precautions. Frequently, a week of precautions are recommended empirically though some patients will still measure ≧ 2 mrem/hr on a 1 meter survey at one week. At our institution, patients with ≧ 2 mrem/hr are prescribed additional precaution days. However, the radiation survey at 1 meter requires technologist time and, based on our prior work, most patients treated do not require extended days of radiation safety precautions. The objective of this study was to determine if visual assessment of the pre- or post-radiotherapy scan can predict the need to survey the patient.

Methods A retrospective analysis of pre- and post-therapy studies of thyroid cancer patients from 8/1/2010 to 9/30/2012 was performed by three physicians independently. The images were assigned a category: extensive metastatic disease, oligo metastatic disease with notable physiologic uptake, oligo metastatic disease without notable physiologic, no metastatic disease +/- thyroid bed uptake with notable physiologic uptake, and no metastatic disease +/- thyroid bed uptake without notable physiologic uptake. The 1 meter radiation survey data collected at the time of the post therapy scan were compared to the visual assessment. The percentage of patients with 1 meter radiation ≧ 2 mrem/hr in each category were calculated for both pre- and post-therapy images. In addition, average meter readings for each category were calculated.

Results Sufficient data for analysis was available for 110 patients. Average mrem/hr and percentage of cases ≧ 2mrem/hr per category are in Table below.

Conclusions The presence of metastatic disease or notable physiologic uptake on visual analysis correlated well with the need for additional precautions. These two findings are independently correlated with elevated meter readings at 1 week post-therapy.