A new variable for SRS plan quality evaluation based on normal tissue sparing: The Effect of Prescription Isodose Levels

TL;DR

This study introduces the dose dropping speed (DDS), a new variable for evaluating normal tissue sparing in stereotactic radiosurgery plans, showing that 60-70% isodose levels optimize tissue preservation.

Contribution

The paper proposes DDS as a novel quantitative metric for SRS plan quality assessment based on normal tissue sparing, comparing effects of different prescription isodose levels.

Findings

DDS is lowest at 90% IDL, indicating less sparing.

DDS is highest at 60-70% IDL, indicating better sparing.

Prescriptions at 60-70% IDL provide optimal normal tissue sparing.

Abstract

Objectives: A new dosimetric variable, dose dropping speed (DDS), was proposed and used to evaluate normal tissue sparing among stereotactic radiosurgery (SRS) plans with different prescription isodose lines. Methods: Forty plans were generated for 8 intracranial SRS cases, prescribing to isodose levels (IDLs) ranging from 50% to 90% in 10% increments. Whilst maintaining similar coverage and conformity, plans at different IDLs were evaluated in terms of normal tissue sparing using the proposed DDS. The DDS was defined as the greater decay coefficient in a double exponential decay fit of the dose drop-off outside the PTV, which models the steep portion of the drop-off. Provided that the prescription dose covers the whole PTV, a greater DDS indicates better normal tissue sparing. Results: Among all plans, the DDS was found the lowest for the prescription at 90% IDL and the highest for the prescription at 60% or 70%. Beam profile slope change in penumbra and its field size dependence were explored and given as the physical basis of the findings. Conclusions: A variable was proposed for SRS plan quality evaluation. Using this measure, prescriptions at 60% and 70% IDLs were found to provide best normal tissue sparing. Advances in knowledge: A new variable was proposed based on which normal tissue sparing was quantitatively evaluated, comparing different prescription IDLs in SRS.