Room: AAPM ePoster Library
Purpose: common approach to obtain transmission correction for planar imaging-based dosimetry with MIRD schema is to measure the transmission factor with a flood source of the same isotope of interest (I-131). However, preparing a liquid I-131 filled flood source presents operational challenges. The feasibility of obtaining transmission factors with a Co-57 solid flood source, which is widely available and easily handled, was investigated as a sub-study of a phase III clinical trial for a novel I-131 labeled antibody (I-131 Apamistamab, Iomab-B) conditioning agent for relapsed and refractory acute myeloid leukemia prior to bone marrow transplant.
Methods: factors (?) were obtained from the anterior images of an I-131 and a Co-57 flood source with and without the positioning patients for both the abdomen and pelvis regions. The ratio of ?I-131 to ?Co-57 is defined as Transmission Ratio (TR). A phantom study was performed following the same procedure using tissue-equivalent phantoms with different thicknesses (12-34 cm, 2 cm increments) to demonstrate the relationship between TR and thickness (t). Results for 11 patients were summarized and the TR vs.t relationship was established as a look-up curve. Dose calculations for three organs (liver, spleen, marrow) were performed retrospectively with TR corrected ?Co-57. The results were compared with dosimetry calculated with ?I-131.
Results: the phantom study data fitted with an exponential and a line showed equivalent goodness-of-fit, a linear relationship was used to represent the relationship between TR and t. Liver, spleen and marrow dose calculated with ?Co-57 show median differences from dose calculated with ?I-131 by 1.5% [-4.3, 5.8%], 1.8% [-5.1, 5.7%], and -2.5% [-8.7, 5.2%].
Conclusion: calculations with Co-57 measured transmission factors for I-131 labeled antibody dosimetry show the feasibility to replace I-131 flood source with Co-57 solid flood source. This could simplify the performance of planar dosimetry scans in clinic.
Funding Support, Disclosures, and Conflict of Interest: Qing Liang is an employee of Actinium Pharmaceuticals.