A novel protocol for parametrization of a beam model for small stereotactic beams

Abstract

Introduction: Most Treatment Planning Systems (TPS) utilize parameterized multi-source models to represent the radiation beam of linacs. However, small stereotactic beams require special attention due to the importance of correct spot- and field-size definition. The purpose of this work is to develop a protocol for configuring a beam model that provides accurate representation of stereotactic beams.

Material and methods: 6 MV flattening filter free (FFF) dose-profiles were measured for Millennium multileaf collimator (MLC)-collimated field sizes (2 × 2 cm²-5 × 5 cm²). They served as a reference against which corresponding TPS-profiles were optimized by configuring the Dosimetric Leaf Gap (DLG) and the Effective Spot Size (ESS). The optimized model was evaluated by comparing calculated and measured stereotactic radiotherapy (SRT)-plans. The proposed protocol was implemented on Anisotropic Analytical Algorithm (AAA) and Acuros XB in the Eclipse TPS.

Results: Improvement in the agreement between measured and modelled profiles are identified using the suggested protocol. The DLG was determined at an optimum of 0.9 mm for both models whereas the ESS was determined to be (0.5, 0) mm and (1, 0,5) mm for AAA and Acuros XB, respectively. Compared to the standardly configured clinical model, the optimized AAA (Acuros XB) model showed an average improvement in gamma pass-rates of 2% (3%) with film measurements.

Conclusions: The standard protocol for TPS commissioning was shown to produce a sub-optimal beam model for small treatment volumes. The proposed protocol in this work results in a better modelling of small stereotactic beams.

Keywords: beam‐model optimization; dosimetric leaf gap; effective spot size; linac stereotactic radiotherapy.

FIGURE 1

Schematic illustration of the OIT concept depicted from the beams eye‐view. The secondary collimators have been depicted as opaque to illustrate the placement of all field limiting devices, including the MLC leaf pairs defining the radiation field in the inline direction, with the X1/X2 jaw‐to‐MLC‐tip distance being set to 8 mm. Dashed lines represent the trajectories of measured profiles through the CAX and at OIT = 5 mm. CAX, central beam axis; OIT, off‐axis inline translation; MLC, millennium multileaf collimator.

FIGURE 2

Field width (in terms of FWHM), normalized to the average, of profiles measured at MLC‐collimated 3 cm × 3 cm field size, as function of the OIT. Black diamond markers represent measurements obtained with the IBA Razor diode and the green circles were measured using Gafchromic film. Vertical blue dotted lines and red dashed lines mark the extreme position where the profile width is expected to be the largest and smallest, respectively. FWHM, full width half maximum; MLC, millennium multileaf collimator; OIT, off‐axis inline translation.

FIGURE 3

Penumbra widths, normalized to the average, of profiles measured at MLC‐collimated 2 cm × 2 cm field sizes, as function of the OIT. MLC, millennium multileaf collimator; OIT, off‐axis inline translation.

FIGURE 4

Calculated sum of AID in the penumbra region between optimization profiles measured at OIT = 1.2 mm and corresponding profiles modelled in the TPS using the AAA algorithm for a given set of beam configuration parameters in the a: crossline and b: inline directions. AAA, Anisotropic Analytical Algorithm; AID, absolute integrated difference; OIT, off‐axis inline translation; TPS, Treatment Planning Systems.

FIGURE 5

Calculated sum of AID in the penumbra region between optimization profiles measured at OIT = 1.2 mm and corresponding profiles modelled in the TPS using the Acuros XB algorithm for a given set of beam configuration parameters in the a: crossline and b: inline directions. AID, absolute integrated difference; OIT, off‐axis inline translation; TPS, Treatment Planning Systems.

FIGURE 6

Percentage deviations in modelled output factors relative to the clinically commissioned AAA beam model for field sizes between 2 cm × 2 and 5 cm × 5 cm. Blue circles represent the deviation for the SRT‐optimized Acuros XB beam model, and red squares represent the deviation for the SRT‐optimized AAA beam model. AAA, Anisotropic Analytical Algorithm; SRT, stereotactic radiotherapy.

FIGURE 7

Ratios in percent of GPR of clinical treatment plans calculated with SRT‐optimized models against clinical treatment plans; dashed blue bars are SRT‐optimized AAA/clinical AAA, filled orange bars are optimized Acuros XB/clinical AAA. GPR‐analysis of modelled versus measured, all based on Gafchromic film measurements. The SRT‐optimized Acuros XB beam model is compared against the clinical AAA beam model in absence of a clinically commissioned Acuros XB beam model. AAA, Anisotropic Analytical Algorithm; GPR, gamma pass rate; SRT, stereotactic radiotherapy.

FIGURE 8

Ratios in percent of GPR of clinical treatment plans calculated with SRT‐optimized models against clinical treatment plans; dashed blue bars are SRT‐optimized AAA/clinical AAA, filled orange bars are optimized Acuros XB /clinical AAA. GPR‐analysis of modelled versus measured, all based on EPID measurements. The SRT‐optimized Acuros XB beam model is compared against the clinical AAA beam model in absence of a clinically commissioned Acuros XB beam model. AAA, Anisotropic Analytical Algorithm; EPID, Electronic Portal Imaging Device; GPR, gamma pass rate; SRT, stereotactic radiotherapy.

FIGURE 9

Comparison in terms of dose difference of a measured DCAT plan against: (a) clinical model, (b) optimized AAA model, and (c) optimized Acuros XB model. Measured dose was performed for plan # 4 in Table 1, using EBT3 film dosimetry. AAA, Anisotropic Analytical Algorithm; DCAT, dynamic conformal arc therapy.

FIGURE 10

Comparison in terms of dose difference of a measured VMAT plan against: (A) clinical model, (B) optimized AAA model, and (C) optimized Acuros XB model. Measured dose was performed for plan # 13 in Table 1, using EBT3 film dosimetry. AAA, Anisotropic Analytical Algorithm; VMAT, volumetrically modulated arc therapy.