Clinically relevant investigation of flattening filter‐free skin dose

Christopher L Guy,Siyong Kim,Manju Sharma,Kishor Karki

doi:10.1120/jacmp.v17i6.6307

Abstract

As flattening filter‐free (FFF) photon beams become readily available for treatment delivery in techniques such as SBRT, thorough investigation of skin dose from FFF photon beams is necessary under clinically relevant conditions. Using a parallel‐plate PTW Markus chamber placed in a custom water‐equivalent phantom, surface‐dose measurements were taken at 2×2,3×3,4×4,6×6,8×8,10×10,20×20, and 30×30 cm2 field sizes, at 80, 90, and 100 cm source‐to‐surface distances (SSDs), and with fields defined by jaws and multileaf collimator (MLC) using multiple beam energies (6X, 6XFFF, 10X, and 10XFFF). The same set of measurements was repeated with the chamber at a reference depth of 10 cm. Each surface measurement was normalized by its corresponding reference depth measurement for analysis. The FFF surface doses at 100 cm SSD were higher than flattened surface doses by 45% at 2×2 cm2 to 13% at 20×20 cm2 for 6 MV energy. These surface dose differences varied to a greater degree as energy increased, ranging from +63% at 2×2 cm2 to −2% at 20×20 cm2 for 10 MV. At small field sizes, higher energy increased FFF surface dose relative to flattened surface dose; while at larger field sizes, relative FFF surface dose was higher for lower energies. At both energies investigated, decreasing SSD caused a decrease in the ratios of FFF‐to‐flattened surface dose. Variability with SSD of FFF‐to‐flattened surface dose differences increased with field size and ranged from 0% to 6%. The field size at which FFF and flattened beams gave the same skin dose increased with decreasing beam energy. Surface dose was higher with MLC fields compared to jaw fields under most conditions, with the difference reaching its maximum at a field size between 4×4 cm2 and 6×6 cm2 for a given energy and SSD. This study conveyed the magnitude of surface dose in a clinically meaningful manner by reporting results normalized to 10 cm depth dose instead of depth of dose maximum.PACS number(s): 87.53.Bn, 87.53.Ly, 87.55.‐x, 87.55.N‐, 87.56.N‐

Highlights

141 Guy et al: flattening filter-free (FFF) skin dose reaching the patient or phantom in flattened beams is mainly due to the flattening filter; removal of the filter greatly decreases head scatter which can lead to decreased out-of-field dose.[1]
We report skin dose measurements over a large range of field sizes at multiple source-to-surface distances (SSDs) using both fields made with collimator jaws and those made using the multileaf collimator (MLC) alone
As measurements were performed over several weeks, separate surface measurements were performed during each session to account for daily variability in beam output and were obtained for all energies using a 10 × 10 cm2 jaw field

Summary

Introduction

141 Guy et al: FFF skin dose reaching the patient or phantom in flattened beams is mainly due to the flattening filter; removal of the filter greatly decreases head scatter which can lead to decreased out-of-field dose.[1]. Investigations into the dosimetric properties of FFF beams were performed on modified linacs with the flattening filter manually moved out of the beamline.[2,3] Linacs with FFF beams have since been released by vendors and implemented clinically at many institutions. Only a single sourceto-surface distance (SSD) was used, and reported measurements were normalized by the same maximum depth dose of a 10 × 10 cm field size. The goal of this investigation is to measure skin dose of FFF beams under clinically relevant conditions. We report skin dose measurements over a large range of field sizes at multiple SSDs using both fields made with collimator jaws and those made using the MLC alone

Methods

Results

Discussion

Conclusion