In-Vivo Dosimetry in Positron-Guided Radiation Therapy πŸ“

Author: Davide Brivio, Erno Sajo, Ryan Williams, Piotr Zygmanski πŸ‘¨β€πŸ”¬

Affiliation: University of Massachusetts Lowell, Brigham and Women's Hospital, Department of Radiation Oncology, Brigham and Women’s Hospital, Dana-Farber Cancer Institute, Harvard Medical School 🌍

Abstract:

Purpose: To develop a non-invasive 3D in-vivo dosimetry tool exploring the relationship between positron annihilations and dose per voxel using Monte Carlo simulation. The Positron Guided Radiation Therapy (PGRT) technique measures 511keV photons that are produced by the annihilation of positrons generated from pair production during radiotherapy. By establishing this relationship, PGRT can provide a dual gating and in-vivo dosimetry tool.

Methods: A 4DCT scan was taken of a QUASAR Respiratory Motion Phantom with the split cedar lung tumor insert. The Monte Carlo (MC) code GATE was used to simulate five equally weighted 3.5x3.5cm 6MV photon beams at gantry angles of 0, 45, 90, 135, and 180 degrees, incident on phase 70 of the QUASAR 4DCT where the total dose and total number of positron annihilations per voxel were scored. 3D dose and positron annihilation maps, as well as profiles of these quantities along the 0-degree central beam axis were scored.

Results: The pair production cross-section is dependent on material properties, such as mass density and atomic number. This leads to discontinuities between the number of annihilations per voxel across boundaries. In contrast, the dose exhibits little dependence on material properties within the QUASAR phantom. By normalizing the annihilations to mass density, a linear relationship between dose and annihilations is seen. A linear regression was applied to this data and used to convert annihilations to dose. The results show that the converted dose from annihilations does not depend on material, and it has a close resemblance to MC dose, with an average in-field percent error of 6.13%.

Conclusion: By normalizing annihilations to mass density, a linear relationship can be established between dose and annihilations. This relationship permits the conversion of annihilations to dose. This study supports the development of a non-invasive 3D in-vivo dosimetry tool.

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