2282 - Comparison of Thermoluminescent Dosimetry to Calculated Dosimetry in the Tumor Margins in Diffusing Alpha-Emitters Radiation Therapy

J. Feldman¹, Y. Hillman², G. Heger³, L. Epstein^3,4, R. Den⁵, A. Popovtzer¹, and L. Arazi³; ¹Hadassah Medical Center, Jerusalem 91120, Israel, ²Hadassah Medical Center, Jerusalem, Israel, ³Ben-Gurion University of the Negev, Beer-Sheva, Israel, ⁴Soreq Nuclear Research Center, Yavne, Israel, ⁵Alpha Tau Medical, Jerusalem, Israel

Purpose/Objective(s): The dose to healthy tissue surrounding the tumor in diffusing alpha-emitters radiation therapy (DaRT) was shown in previous works to amount to negligible levels (< 3 Gy) already at a distance of ~4 mm from the outermost sources, where it is dominated by beta and gamma contributions. The aim of this work was to confirm these predictions using thermoluminescent dosimetry (TLD) detectors in the tumor margins. Materials/

Methods: Twelve subjects from an ongoing clinical trial had a TLD taped to a position approximately 5 mm from a DaRT source following implantation. The TLD was made from crystalline LiF: Mg,Ti, with a 10 uGy detection threshold and a linear dose response to 5 Gy. Following the 15-21 day duration of DaRT treatment, the TLD was removed and sent for dosimetric evaluation. In addition, subjects had a CT performed immediately following the DaRT source implantation, with the location of the TLD indicated with a radiopaque marker. The beta and gamma dose at the marker position was calculated as a superposition of contributions from all sources, based on their coordinates as inferred from the CT.
Results: The actual distance of TLD from the nearest DaRT source varied between 3.5 to 8.75 mm. In all cases where the TLD was placed greater than 5.5 mm from the nearest source (10 of 12 subjects), the measured dose was less than 0.18 Gy. TLDs placed 5.5 mm and 3.5 mm from the nearest source measured a dose of 0.5 Gy and 2.3 Gy, respectively. In all cases the measurement and calculation were within error (with the measurement smaller than the calculation in 10 of 12 cases), assuming a relative placement error of <1.8 mm. In 10 of 12 cases the measurement and calculation were within error assuming a relative placement error of <1.3 mm.
Conclusion: Given the magnitude of the measured dose at 5 mm distance from the outermost source, there appears to be no safety concern for use of DaRT even in the rare cases where higher doses than predicted are delivered as all measurements are very low and have no clinical significance.