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Critical Structure Dose Limit Simplification for Pandemic-Related Hypofractionation of Treatment Naive Head and Neck Cancer Patients

R Price*, T Galloway, C Ma, Fox Chase Cancer Center, Philadelphia, PA

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PO-GePV-T-320 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To demonstrate simplified critical structure dose limit determination for covid-19 related hypofractionation of head and neck radiotherapy.

Methods: Cervical cord dose limits were determined for 3 common pandemic-related dose schemes using the EQD2 formalism and α/β ratios of 1Gy and 3Gy. Dose schemes were (A) 70Gy/35fxs, (B) 60Gy/25fxs and (C) 55Gy/20fxs corresponding to target EQD2 doses of 70.00, 62.00 and 58.44Gy, respectively for α/β of 10Gy. Cord limits were based on 50Gy EQD2 from QANTEC and 50Gy physical dose representing common practice. Conversion to limits for hypofractionation was made using 50Gy EQD2, conversion following common practice limit of 50Gy physical dose, and direct scaling of physical dose for comparison.

Results: Assuming an α/β of 3Gy the physical dose required to reach the QANTEC EQD2 cord limit is 54.77Gy, 50.00Gy and 46.81 Gy for A-C, respectively and 57.04Gy, 50.0Gy and 45.68Gy for an α/β of 1Gy. Conversion to EQD2 following common practice of 50Gy physical dose as a limit for (A) results in physical dose limits of 50.00Gy, 45.82Gy and 43.00Gy for A-C (α/β of 3Gy), respectively, and 50.00Gy, 44.00Gy and 40.28Gy, respectively for α/β of 1Gy. Direct scaling of the 50Gy physical dose limit by the ratio of total prescription EQD2 dose for each scheme yields physical dose limits of 44.29Gy and 41.74Gy for B and C, respectively. The resultant EQD2 values for the scaled limits are 44.29Gy, 42.27Gy and 42.47Gy (α/β of 3Gy) and 40.48Gy, 40.92Gy and 42.95Gy (α/β of 1Gy), for A-C, respectively.

Conclusion: The published pandemic-related hypofractionation schemes result in as much as 16.5% lower EQD2 target dose as compared with scheme A. Due to this it appears direct scaling of the cord dose limit will result in safe values for use in treatment planning for these schemes.

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