Evaluation of Dosimetry Formalisms in Intraoperative Radiation Therapy of Glioblastoma

David Santiago Ayala Alvarez; Peter G F Watson; Marija Popovic; Veng Jean Heng; Michael D C Evans; Valerie Panet-Raymond; Jan Seuntjens

doi:10.1016/j.ijrobp.2023.04.031

Evaluation of Dosimetry Formalisms in Intraoperative Radiation Therapy of Glioblastoma

Int J Radiat Oncol Biol Phys. 2023 Nov 1;117(3):763-773. doi: 10.1016/j.ijrobp.2023.04.031. Epub 2023 May 5.

Authors

David Santiago Ayala Alvarez¹, Peter G F Watson², Marija Popovic², Veng Jean Heng³, Michael D C Evans², Valerie Panet-Raymond⁴, Jan Seuntjens⁵

Affiliations

¹ Department of Physics and Medical Physics Unit, McGill University, Montreal, QC, Canada. Electronic address: david.ayalaalvarez@mail.mcgill.ca.
² Medical Physics Unit and.
³ Department of Physics and Medical Physics Unit, McGill University, Montreal, QC, Canada.
⁴ Department of Radiation Oncology, McGill University Health Centre, Montreal, QC, Canada.
⁵ Medical Physics Unit and; Princess Margaret Cancer Centre, Radiation Medicine Program, University Health Network, Toronto, ON, Canada.

PMID: 37150259
DOI: 10.1016/j.ijrobp.2023.04.031

Abstract

Purpose: The intraoperative radiotherapy in newly diagnosed glioblastoma multiforme (INTRAGO) clinical trial assesses survival in patients with glioblastoma treated with intraoperative radiation therapy (IORT) using the INTRABEAM. Treatment planning for INTRABEAM relies on vendor-provided in-water depth dose curves obtained according to the TARGeted Intraoperative radioTherapy (TARGIT) dosimetry protocol. However, recent studies have shown discrepancies between the estimated TARGIT and delivered doses. This work evaluates the effect of the choice of dosimetry formalism on organs at risk (OAR) doses.

Methods and materials: A treatment planning framework for INTRABEAM was developed to retrospectively calculate the IORT dose in 8 INTRAGO patients. These patients received an IORT prescription dose of 20 to 30 Gy in addition to external beam radiation therapy. The IORT dose was obtained using (1) the TARGIT method; (2) the manufacturer's V4.0 method; (3) the C_Q method, which uses an ionization chamber Monte Carlo (MC) calculated factor; (4) MC dose-to-water; and (5) MC dose-to-tissue. The IORT dose was converted to 2 Gy fractions equivalent dose.

Results: According to the TARGIT method, the OAR dose constraints were respected in all cases. However, the other formalisms estimated a higher mean dose to OARs and revealed 1 case where the constraint for the brain stem was exceeded. The addition of the external beam radiation therapy and TARGIT IORT doses resulted in 10 cases of OARs exceeding the dose constraints. The more accurate MC calculation of dose-to-tissue led to the highest dosimetric differences, with 3, 3, 2, and 2 cases (out of 8) exceeding the dose constraint to the brain stem, optic chiasm, optic nerves, and lenses, respectively. Moreover, the mean cumulative dose to brain stem exceeded its constraint of 66 Gy with the MC dose-to-tissue method, which was not evident with the current INTRAGO clinical practice.

Conclusions: The current clinical approach of calculating the IORT dose with the TARGIT method may considerably underestimate doses to nearby OARs. In practice, OAR dose constraints may have been exceeded, as revealed by more accurate methods.

Publication types

Evaluation Study
Research Support, Non-U.S. Gov't

MeSH terms

Breast Neoplasms*
Female
Glioblastoma* / radiotherapy
Glioblastoma* / surgery
Humans
Organs at Risk / diagnostic imaging
Organs at Risk / radiation effects
Radiometry
Radiotherapy Dosage
Retrospective Studies

Grants and funding

FDN-143257/CIHR/Canada