Investigation of dosimetric characteristics of radiochromic film in response to alpha particles emitted from Americium‐241
Authors: Diaz‐Martinez Victor D. , Cyr Mélodie , Devic Slobodan , Tomic Nada , Lewis David F. , Enger Shirin A. 
Published: May 2024

Background

In radiotherapy, it is essential to deliver prescribed doses to tumors while minimizing damage to surrounding healthy tissue. Accurate measurements of absorbed dose are required for this purpose. Gafchromic® external beam therapy (EBT) radiochromic films have been widely used in radiotherapy. While the dosimetric characteristics of the EBT3 model film have been extensively studied for photon and charged particle beams (protons, electrons, and carbon ions), little research has been done on ‐particle dosimetry. ‐emitting radionuclides have gained popularity in cancer treatment due to their high linear energy transfer, short range in tissue, and ability to spare surrounding organs at risk, thereby delivering a more localized dose distribution to the tumor. Therefore, a dose‐calibration film protocol for ‐particles is required.

Purpose

This study aimed to develop a dose‐calibration protocol for the ‐particle emitting radionuclide 241Am, using Monte Carlo (MC) simulations and measurements with unlaminated EBT3 films.

Methods

In this study, a MC‐based user code was developed using the Geant4 simulation toolkit to model and simulate an 241Am source and an unlaminated EBT3 film. Two simulations were performed: one with voxelized geometries of the EBT3 active volume composition and the other using water. The dose rate was calculated within a region of interest in the voxelized geometries. Unlaminated EBT3 film pieces were irradiated with the 241Am source at various exposure times inside a black box. Film irradiations were compared to a 6‐MV photon beam from a Varian TrueBeam machine. The simulated dose rate was used to convert the exposure times into absorbed doses to water, describing a radiochromic‐film‐based reference dosimetry protocol for ‐particles. The irradiated films were scanned and through an in‐house Python script, the normalized pixel values from the green‐color channel of scanned film images were analyzed.

Results

The 241Am energy spectra obtained from the simulations were in good agreement with IAEA and NIST databases, having differences 0.516% for the emitted ‐rays and produced characteristic x‐rays and 0.006% for the ‐particles. Due to the short range of ‐particles, there was no energy deposition in the voxels outside the active 241Am source region projected onto the film surface. Thus, the total dose rate within the voxels covering the source was 0.847 0.003 Gy/min within the sensitive layer of the film (LiPCDA) and 0.847 0.004 Gy/min in water, indicating that the active volume can be considered water equivalent for the 241Am beam quality. A novel approach was employed in ‐film dosimetry using an exponential fit for the green channel, which showed promising results by reducing the uncertainty in dose estimation within 5%. Although the statistical analysis did not reveal significant differences between the 6‐MV photon beam and the calibration curves, the dose–response curves exhibited the expected behavior.

Conclusions

The developed MC user code simulated the experimental setup for ‐dosimetry using radiochromic film with acceptable uncertainty. Unlaminated EBT3 film is suitable for the dosimetry of ‐radiation at low doses and can be used in conjunction with other unlaminated GafChromic® films for quality assurance and research purposes.