Red Marrow Absorbed Dose Calculation in Thyroid Cancer Patient Using a Simplified Excel Spreadsheet

OBJECTIVES: Absorbed dose to red marrow (D(rm)) can be calculated using blood dosimetry. However, this method is laborious and invasive. Therefore, image-based dosimetry is the method of choice. Nonetheless, the commercial software is expensive. The goal of this work was to develop a simplified excel spreadsheet for image-based radioiodine red marrow dosimetry. METHODS: The serial whole-body images (acquired at 2(nd), 6(th), 24(th), 48(th), and 72(th) hours) of 29 patients from the routine pretherapeutic dosimetry protocol were retrospectively reanalyzed. The commercial OLINDA/EXM image-based dosimetry software was used to calculate the whole-body time-integrated activity coefficient (TIAC(WB)) and D(rm) [in terms of absorbed dose coefficient (d(rm))]. For the simplified excel spreadsheet, the whole-body count was obtained from the vendor-supplied software. Then, the TIAC(WB) was computed by a fitting time-activity curve using an Excel function. S factor was taken from other publications and scaled according to the patient-specific mass. A comparison of the TIAC(WB) and d(rm) from both methods was done using a non-inferiority test using a paired t-test or the Wilcoxon signed-rank test. RESULTS: The TIAC(WB) showed no significant difference between both methods (p=0.243). The calculated D(rm) from a simplified Excel spreadsheet was assumed to be statistically non-inferior to the commercial OLINDA/EXM image-based dosimetry software with the non-inferiority margin of 0.02 (p<0.05). CONCLUSION: The dose assessment from a simplified Excel spreadsheet is feasible and relatively low cost compared to the commercial OLINDA/EXM image-based dosimetry software.