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Technical Note: Break-even dose level for hypofractionated treatment schedules.

Till Tobias Böhlen1, Jean-François Germond1, Jean Bourhis2

  • 1Institute of Radiation Physics, Lausanne University Hospital and Lausanne University, Lausanne, Switzerland.

Medical Physics
|October 5, 2021
PubMed
Summary

This study introduces a method to identify the dose level below which normal tissues benefit from hypofractionated radiotherapy. This approach helps spare normal tissues and improve therapeutic index in radiation oncology.

Keywords:
BEDLQ modelLQ-L modelhypofractionationisoeffect

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Area of Science:

  • Radiation Oncology
  • Medical Physics

Background:

  • Hypofractionated radiotherapy offers potential benefits but requires careful normal tissue sparing.
  • Understanding dose-response relationships is crucial for optimizing treatment plans.

Purpose of the Study:

  • To derive a method for determining the relative isodose line (R) below which normal tissues (NT) benefit from hypofractionation.
  • To apply this formalism to clinical scenarios for assessing therapeutic index changes.

Main Methods:

  • Utilized the linear-quadratic (LQ) model to derive biologically effective dose (BED) equations for NT under varying fractionation schedules.
  • Calculated the
  • break-even
  • isodose line (R) where NT sparing begins with hypofractionation.
  • Evaluated the LQ-linear (LQ-L) model for high doses per fraction (>6 Gy).

Main Results:

  • Developed equations to calculate BED for NT and the corresponding break-even isodose line (R).
  • For typical alpha/beta ratios (10 Gy for tumor, 3 Gy for NT), R was found to be 30%.
  • R doubled for stereotactic treatments (100% max dose correlation) compared to standard isodose line correlations (50%).
  • The break-even concept remained valid up to ~20 Gy per fraction using the LQ-L model.

Conclusions:

  • The derived formalism enables estimation of the relative isodose line (R) for differential normal tissue sparing with increased hypofractionation.
  • This method provides a compact way to assess therapeutic index changes across isoeffective treatment schedules.