Influence of dose calculation algorithms on the helical diode array using volumetric-modulated arc therapy for small targets

Abstract

Background: For patient-specific quality assurance (PSQA) for small targets, the dose resolution can change depending on the characteristics of the dose calculation algorithms.

Purpose: This study aimed to evaluate the influence of the dose calculation algorithms Acuros XB (AXB), anisotropic analytical algorithm (AAA), photon Monte Carlo (pMC), and collapsed cone (CC) on a helical diode array using volumetric-modulated arc therapy (VMAT) for small targets.

Materials and methods: ArcCHECK detectors were inserted with a physical depth of 2.9 cm from the surface. To evaluate the influence of the dose calculation algorithms for small targets, rectangular fields of 2×100, 5×100, 10×100, 20×100, 50×100, and 100×100 mm² were irradiated and measured using ArcCHECK with TrueBeam STx. A total of 20 VMAT plans for small targets, including the clinical sites of 19 brain metastases and one spine, were also evaluated. The gamma passing rates (GPRs) were evaluated for the rectangular fields and the 20 VMAT plans using AXB, AAA, pMC, and CC.

Results: For rectangular fields of 2×100 and 5×100 mm², the GPR at 3%/2 mm of AXB was < 50% because AXB resulted in a coarser dose resolution with narrow beams. For field sizes > 10×100 mm2, the GPR at 3%/2 mm was > 88.1% and comparable for all dose calculation algorithms. For the 20 VMAT plans, the GPRs at 3%/2 mm were 79.1 ± 15.7%, 93.2 ± 5.8%, 94.9 ± 4.1%, and 94.5 ± 4.1% for AXB, AAA, pMC, and CC, respectively.

Conclusion: The behavior of the dose distribution on the helical diode array differed depending on the dose calculation algorithm for small targets. Measurements using ArcCHECK for VMAT with small targets can have lower GPRs owing to the coarse dose resolution of AXB around the detector area.

Keywords: Acuros XB; GPR; VMAT; coarser dose resolution; small‐field irradiation.

FIGURE 1

Example of dose distributions on the ArcCHECK phantom with a field size of 2×100 mm² for (a) AXB, (b) AAA, (c) pMC, and (d) CC. Isodose lines, which include 2%, 3%, 5%, and 15% to maximum dose are shown. Red dotted circles show the detector plane of ArcCHECK. AXB, Acuros XB; AAA, anisotropic analytical algorithm; pMC, photon Monte Carlo; CC, collapsed cone.

FIGURE 2

Dose distributions and dose profiles with a field size of 2×100 mm² for (a) AXB, (b) AAA, (c) pMC, and (d) CC. Red dotted circles show the detector plane of ArcCHECK. AXB, Acuros XB; AAA, anisotropic analytical algorithm; pMC, photon Monte Carlo; CC, collapsed cone.

FIGURE 3

An example of dose distributions and dose profiles of the VMAT plan for (a) AXB, (b) AAA, (c) pMC, and (d) CC. Red dotted circles show the detector plane of ArcCHECK. AXB, Acuros XB; AAA, anisotropic analytical algorithm; pMC, photon Monte Carlo; CC, collapsed cone; VMAT, volumetric‐modulated arc therapy.

FIGURE 4

Boxplot of GPRs at (a) 5%/1 mm, (b) 3%/2 mm, and (c) 3%/3 mm for AXB, AAA, pMC, and CC. (d) Explanation of interpretation of boxplot. AXB, Acuros XB; AAA, anisotropic analytical algorithm; pMC, photon Monte Carlo; CC, collapsed cone; GPR, gamma passing rate.