Clinical Implication of Dosimetry Formalisms for Electronic Low-Energy Photon Intraoperative Radiation Therapy

Peter G F Watson; Marija Popovic; Liheng Liang; Nada Tomic; Slobodan Devic; Jan Seuntjens

doi:10.1016/j.prro.2020.07.005

Clinical Implication of Dosimetry Formalisms for Electronic Low-Energy Photon Intraoperative Radiation Therapy

Pract Radiat Oncol. 2021 Jan-Feb;11(1):e114-e121. doi: 10.1016/j.prro.2020.07.005. Epub 2020 Aug 11.

Authors

Peter G F Watson¹, Marija Popovic², Liheng Liang³, Nada Tomic³, Slobodan Devic³, Jan Seuntjens²

Affiliations

¹ Medical Physics Unit, McGill University, Montreal, QC, Canada. Electronic address: peter.watson@mcgill.ca.
² Medical Physics Unit, McGill University, Montreal, QC, Canada.
³ Medical Physics Unit, Department of Radiation Oncology, SMBD Jewish General Hospital, McGill University, Montreal, QC, Canada.

PMID: 32795615
DOI: 10.1016/j.prro.2020.07.005

Abstract

Purpose: Intraoperative radiation therapy (IORT) using the INTRABEAM, a miniature x-ray source, has shown to be effective in treating breast cancer. However, recent investigations have suggested a significant deviation between the reported and delivered doses. In this work, the dose delivered by INTRABEAM in the TARGIT breast protocol was investigated, along with the dose from the Xoft Axxent, another source used in breast IORT.

Methods and materials: The absorbed dose from the INTRABEAM was determined from ionization chamber measurements using: (a) the manufacturer-recommended formula (Zeiss V4.0 method), (b) a Monte Carlo calculated chamber conversion factor (C_Q method), and (c) the formula consistent with the TARGIT breast protocol (TARGIT method). The dose from the Xoft Axxent was determined from ionization chamber measurements using the Zeiss V4.0 method and calculated using the American Association of Physicists in Medicine TG-43 formalism.

Results: For a nominal TARGIT prescription of 20 Gy, the dose at the INTRABEAM applicator surface ranged from 25.2 to 31.7 Gy according to the C_Q method for the largest (5 cm) and smallest (1.5 cm) diameter applicator, respectively. The Zeiss V4.0 method results were 7% to 10% lower (23.2 to 28.6 Gy). At 1 cm depth, the C_Q and Zeiss V4.0 absorbed doses were also larger than those predicted by the TARGIT method. The dose at 1 cm depth from the Xoft Axxent for a surface dose of 20 Gy was slightly less than INTRABEAM (3%-7% compared with C_Q method). An exception was for the 3 cm applicator, where the Xoft dose was appreciably lower (31%).

Conclusions: The doses delivered in the TARGIT breast protocol with INTRABEAM were significantly greater than the prescribed 20 Gy and depended on the size of spherical applicator used. Breast IORT treatments with the Xoft Axxent received less dose compared with TARGIT INTRABEAM, which could have implications for studies comparing clinical outcomes between the 2 devices.

MeSH terms

Breast Neoplasms / radiotherapy
Electronics
Humans
Monte Carlo Method
Photons
Radiometry*
Radiotherapy Dosage