EQFX - A Transformation of Previous Radiotherapy Dose to Stereotactic Fractionation

Document Type

Conference Proceeding

Publication Date

10-2024

Publication Title

International Journal of Radiation Oncology, Biology, Physics

Abstract

Purpose/Objective(s): Biologically Effective Dose (BED) and Equivalent Dose in 2Gy Fractions (EQD2) are metrics that transform dose distributions to compare radiotherapy fractionation schemes. Previous radiotherapy treatments may be converted to EQD2 and summed, giving physicians insight into how much more EQD2 may safely be delivered before reaching conventional toxic endpoints. However, fractionations have trended toward shorter treatment regimens with higher dose per fraction, and EQD2 is an abstraction for tissues that don’t receive 2 Gy per fraction. This work introduces EQFX, a metric derived from the linear quadratic model to transform dose distributions into the biological equivalent if delivered in X fractions. With this metric, dose summations may be transformed toward stereotactic endpoints. Materials/Methods: The metric essentially converts a physical dose map into a map of BED, then through that BED into the physical dose distribution at each voxel had the plan been delivered with a different number of fractions X (EQF1 for 1 fraction). The result depends only on the set number of fractions X, the physical dose delivered to a voxel, the number of fractions delivered, and the a/b ratio of the voxel. a/b ratio for the whole body was set to 3, which adheres to our clinical experience with EQD2 summations. Calculations are performed with an in-house python script utilizing pyDicom. Results: For proof of concept, a retrospective cohort of 10 previously treated patients from our Linac-based SRS program (5 single-isocenter multitarget and 5 single target plans) was collected. Patients had received a prescribed 18-35 Gy in 1, 3 or 5 fractions. RTdose DICOM files from these treatments were converted into EQF1 and EQF5 maps on the clinical CT/ MR datasets and displayed in our PACS system. Following this proof of concept, applications of this technique into the lung, prostate, breast, and spine will be explored to accompany hypofractionation and SBRT planning to these sites. Conclusion: Conversion of dose distributions into EQFX is now available for physician use at our clinic. Extensions are possible into flash radiotherapy, as dose may be calculated as being delivered with less than one whole fraction (for example, EQF0.5 or EQF0.2) based on biological effect.

Volume

120

Issue

2S

First Page

e385

Comments

ASTRO 2024: 66th Annual Meeting American Society for Radiation Oncology, September 29 - October 2, 2024, Washington, DC

DOI

10.1016/j.ijrobp.2024.07.854

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