D Baldassari*, L Patton, T Roberts, A Tipton, C Esquivel, Texas Oncology San Antonio, San Antonio, TX

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  D Baldassari

PO-GePV-T-231 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: Assess the use of an automated software to evaluate hypofractionated plans using conventional constraints.

Methods: Eighteen hypofractionated treatment plans were retrospectively reviewed using MIM’s EQD2 Variation 1 of the LQ model. The hypofractionated doses were compared to conventional 2Gy constraints using Thames and Withers’ 2Gy Equivalent Dose (EQD2) formula. Within each treatment plan, EDQ2 doses were evaluated within MIM for accuracy and consistency.

Results: The 18 patients plans excel-generated hand calculations and the MIM-converted EQD2 doses demonstrated less than 5% difference for structures receiving more than 2Gy per fraction. In theory, EQD2 values should be greater than the doses from the hypofractionated treatment plan. Variations were observed with critical structures receiving fractional doses <2Gy. There is also disparity (>1%) between the hand calculated and derived EQD2 maximum and mean values for critical structures with a small volume. The MIM EQD2 is calculated per voxel and its handling of voxels within contours contributes to the discrepancy. A small change to the volume contributes to the discrepancy due to the software’s preferences set to use “nearest neighbor” interpolation for datasets.

Conclusion: MIM is a robust tool in evaluating hypofractionated treatment plans by converting to EQD2 doses to critical structures using traditional 2Gy/fxn constraints. Assessment of dose accumulated from hypofractionated and conventional plans with reference to one dataset is achievable, and the software makes for an easy workflow and rapid evaluation. While the EQD2 values are accurately calculated within the software, the user must exercise caution in cases when the fractional dose to a structure is ≤2Gy. Acquiring a more comprehensive understanding of the way in which the EQD2 is applied to the dose cloud would be of additional benefit and begs further investigation into the software’s handling of the EQD2 conversion and whether order of conversion matters.

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Keywords

Not Applicable / None Entered.

Taxonomy

TH- External Beam- Photons: General (most aspects)

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