The influence of field size on stopping‐power ratios in‐ and out‐of‐field: quantitative data for the BrainLAB m3 micro‐multileaf collimator

Abstract

The objective of this work is to quantify the systematic errors introduced by the common assumption of invariant secondary electron spectra with changing field sizes, as relevant to stereotactic radiotherapy and other treatment modes incorporating small beam segments delivered with a linac‐based stereotactic unit. The EGSnrc/BEAMnrc Monte Carlo radiation transport code was used to construct a dosimetrically‐matched model of a Varian 600C linear accelerator with mounted BrainLAB micro‐multileaf collimator. Stopping‐power ratios were calculated for field sizes ranging from 6×6 mm2 up to the maximum (98×98 mm2), and differences between these and the reference field were computed. Quantitative stopping power data for the BrainLAB micro‐multileaf collimator has been compiled. Field size dependent differences to reference conditions increase with decreasing field size and increasing depth, but remain a fraction of a percent for all field sizes studied. However, for dosimetry outside the primary field, errors induced by the assumption of invariant electron spectra can be greater than 1%, increasing with field size. It is also shown that simplification of the Spencer‐Attix formulation by ignoring secondary electrons below the cutoff kinetic energy applied to the integration results in underestimation of stopping‐power ratios of about 0.3% (and is independent of field size and depth). This work is the first to quantify stopping powers from a BrainLAB micro‐multileaf collimator. Many earlier studies model simplified beams, ignoring collimator scatter, which is shown to significantly influence the spectrum. Importantly, we have confirmed that the assumption of unchanging electron spectra with varying field sizes is justifiable when performing (typical) in‐field dosimetry of stereotactic fields. Clinicians and physicists undertaking precise out‐of‐field measurements for the purposes of risk estimation, ought to be aware that the more pronounced spectral variation results in stopping powers (and hence doses) that differ more than for in‐field dosimetry.PACS number: 87.10.RT; 87.53.Ly; 87.56.jf; 87.56.jk

Dosimetric evaluation of photon dose calculation under jaw and MLC shielding

“…A critical point is the energy dependence of the detector response. The difference in energy spectra 27,28 should be considered in any out-of-field dose measurements relative to in-field dose levels. Studies on the stopping power ratio, between water and air, showed that 28 the variation in the out-of-field region relative to the in-field for a 6 MV beam is of more than 1% (for a 10 × 10 cm 2 field).…”

“…The difference in energy spectra 27,28 should be considered in any out-of-field dose measurements relative to in-field dose levels. Studies on the stopping power ratio, between water and air, showed that 28 the variation in the out-of-field region relative to the in-field for a 6 MV beam is of more than 1% (for a 10 × 10 cm 2 field). Moreover, when measuring the dose in a point where the energy spectrum is continuously varying (from in-field to out-of-field during the same irradiation), as in intensity modulation, it is almost impossible, to correctly compensate the measurements for the different energy response.…”

“…Energy spectra infield and out-of-field are strongly different: for 6 MV beam, in-field mean energy is about 1.5-2 MeV, while out-of-field mean energy is in the order of 100-200 keV. 27,28 This difference has impact on the stopping power ratio between water and air, which presents a difference of more than 1% for standard field sizes. 28 Semiflex ion chamber, for the above reasons, presents an energy dependent response.…”

“…27,28 This difference has impact on the stopping power ratio between water and air, which presents a difference of more than 1% for standard field sizes. 28 Semiflex ion chamber, for the above reasons, presents an energy dependent response. This can be quantified, from chamber specifications and characteristics, as a dose overestimation of about 3% in the outof-field region.…”

Dosimetry of ionising radiation in modern radiation oncology

“…In megavoltage radiation beams, the spectrum can also change significantly enough to affect dose measurement, for example in the beam penumbra or in small fields (Taylor et al 2012).…”