Measurement and Evaluation of Inhomogeneity Corrections and Monitor Unit Verification for Treatment Planning

Abstract 

Heterogeneous lung and bone phantoms have been constructed for the purpose of testing monitor unit calculations at or near interfaces for different planning systems. Data have been acquired for 2 linear accelerators: a Varian 2300cd (6 and 25 MV) and an Elekta Synergy (6, 10, and 18 MV). We have reviewed Pinnacle and the correction-based, pre-AAA version of Eclipse planning systems with the intent of exploring the limits of these systems with energy and field size. Data were acquired from 2 × 2 to 10 × 10 cm² field sizes over the available range of energies. Our measurements confirm that Pinnacle predicts doses mostly to within ± 5%, even near lung-tissue interfaces over the full range of energies and field sizes tested. The Eclipse-modified Batho and equivalent TMR algorithms overpredicted doses by 10% or more in the lung and near the lung-tissue interfaces if the field size was less than 10 × 10 cm² when the energy was 18 MV or higher. At lower energies, the field size had to be at least 6 × 6 cm² for calculated doses to be within 10% of measurement. For bone-tissue interfaces, doses were generally underestimated by 5% to 10% or more by all calculation methods over the range of field sizes and energies reviewed. A second goal of this study was to review methods for hand-checking monitor units when heterogeneities are included. We evaluated the range of applicability of 2, one-dimensional (1D) inhomogeneity correction factors: the effective attenuation method and the TMR ratio method. The effective attenuation method for monitor unit checking was within ± 5% to as small as 6 × 6 cm² fields for 6 to 10 MV, useable for 4 × 4 cm² fields (within 7%) for 6 MV and close to ± 5% for 10 × 10 cm² fields in the 18- to 25-MV range. The TMR ratio method was not as good, being within about ± 5% to 7% of measurements only for 6 × 6 to 10 × 10 cm² fields at 6 MV and 10 × 10 cm² fields at the higher energies. Both simple 1D correction methods performed almost as well as Pinnacle for the bone-soft tissue cases. We recommend that if direct measurement of dose for heterogeneous treatment plans is not practiced, then one of these simple cross checks be performed to assure patient safety.

Key Words: Heterogeneities, Monitor unit checking, Effective attenuation, TMR ratio