Evaluation of inhomogeneity correction factors for 6 MV flattening filter‐free beams with brass compensators

Joshua Robinson,Geoffrey Zhang,Vladimir Feygelman,Daniel Opp

doi:10.1120/jacmp.v14i3.3990

Abstract

The 6 MV flattening filter‐free (FFF) beam has been commissioned for use with compensators at our institution. This novel combination promises advantages in mitigating tumor motion due to the reduced treatment time made possible by the greatly increased dose rate of the FFF beam. Given the different energy spectrum of the FFF beam and the beam hardening effect of the compensator, the accuracy of the treatment planning system (TPS) model in the presence of low‐density heterogeneities cannot be assumed. Therefore, inhomogeneity correction factors (ICF) for an FFF beam attenuated by brass slabs were measured and compared to the TPS calculations in this work. The ICF is the ratio of the point dose in the presence of inhomogeneity to the dose in the same point in a homogeneous medium. The ICFs were measured with an ion chamber at a number of points in a flat water‐equivalent slab phantom containing a 7.5 cm deep heterogeneity (air or 0.27 g/cm3 wood). Comparisons for the FFF beam were carried out for the field sizes from 5×5 to 20×20 cm2 with the brass slabs ranging from 0 to 5 cm in thickness. For a low‐density wood heterogeneity in a slab phantom, with the exception of the point 1 cm beyond the proximal buildup interface, the TPS handles the inhomogeneity correction with the brass‐filtered 6 MV FFF beam at the requisite 2% error level. The combinations of field sizes and compensator thicknesses when the error exceeds 2% (2.6% maximum) are not likely to be experienced in clinical practice. In terms of heterogeneity corrections, the beam model is adequate for clinical use.PACS number: 87.56.ng

Highlights

227 Robinson et al.: Inhomogeneity correction for flattening filter-free (FFF) beam with compensators of inverse planning, this technique is more robust in terms of the tumor motion effect on dosimetry.[3,5,6] It was recently shown[7] that compensator-based IMRT can be commissioned for the 6X FFF beam with sufficient accuracy in a homogeneous phantom, and could be used clinically for example for abdominal tumors
Inside and beyond the lung-type heterogeneity, the measured inhomogeneity correction factors (ICF) generally agree with calculations within 2%, save a few points
The largest error (-2.6%) occurred for an open beam, which is least relevant in the context of compensator-based IMRT

Summary

Introduction

227 Robinson et al.: Inhomogeneity correction for FFF beam with compensators of inverse planning, this technique is more robust in terms of the tumor motion effect on dosimetry.[3,5,6] It was recently shown[7] that compensator-based IMRT can be commissioned for the 6X FFF beam with sufficient accuracy in a homogeneous phantom, and could be used clinically for example for abdominal tumors. The collapsed cone convolution (CCC) algorithm as implemented in Pinnacle treatment planning system (TPS) historically has proven to be sufficiently accurate under these conditions.[8] the energy spectra of the FFF beams differ substantially from the conventional ones because the low-energy photons are no longer removed by the (absent) flattening filter. We set out in this paper to evaluate the accuracy of the inhomogeneity corrections for the commercial dose calculation algorithm when a 6X FFF beam is used with the brass filters. A special case of a cylindrical phantom with a large internal air cavity corresponding to commercial diode array dosimeter geometry was studied. In the past, this geometry proved to be a good discriminator between the different algorithms’ ability to model large low-density inhomogeneities.[9]

Methods

Results

Conclusion