SU‐G‐TeP3‐02: Determination of Geometry‐Specific Backscatter Factors for Radiobiology Studies

Abstract

Purpose: Radiation biology research relies on an accurate radiation dose delivered to the biological target. Large field irradiations in a cabinet irradiator may use the AAPM TG‐61 protocol. This relies on an air‐kerma measurement and conversion to absorbed dose to water (Dw) on the surface of a water phantom using provided backscatter factors. Cell or small animal studies differ significantly from this reference geometry. This study aims to determine the impact of the lack of full scatter conditions in four representative geometries that may be used in radiobiology studies. Methods: MCNP6 was used to model the Dw on the surface of a full scatter phantom in a validated orthovoltage x‐ray reference beam. Dw in a cylindrical mouse, 100 mm Petri dish, 6‐well and 96‐well cell culture dishes was simulated and compared to this full scatter geometry. A reference dose rate was measured using the TG‐61 protocol in a cabinet irradiator. This nominal dose rate was used to irradiate TLDs in each phantom to a given dose. Doses were obtained based on TLDs calibrated in a NIST‐traceable beam. Results: Compared to the full scattering conditions, the simulated dose to water in the representative geometries were found to be underestimated by 12‐26%. The discrepancy was smallest with the cylindrical mouse geometry, which most closely approximates adequate lateral‐ and backscatter. TLDs irradiated in the mouse and petri dish phantoms using the TG‐61 determined dose rate showed similarly lower values of Dw. When corrected for this discrepancy, they agreed with the predicted Dw within 5%. Conclusion: Using the TG‐61 in‐air protocol and given backscatter factors to determine a reference dose rate in a biological irradiator may not be appropriate given the difference in scattering conditions between irradiation and calibration. Without accounting for this, the dose rate is overestimated and is dependent on irradiation geometry.