Comparative assessment and QA measurement array validation of Monte Carlo and Collapsed Cone dose algorithms for small fields and clinical treatment plans

Abstract

Purpose: Many studies have demonstrated superior performance of Monte Carlo (MC) over type B algorithms in heterogeneous structures. However, even in homogeneous media, MC dose simulations should outperform type B algorithms in situations of electronic disequilibrium, such as small and highly modulated fields. Our study compares MC and Collapsed Cone (CC) dose algorithms in RayStation 12A. Under consideration are 6 MV and 6 MV flattening filter-free (FFF) photon beams, relevant for VMAT plans such as head-and-neck and stereotactic lung treatments with heterogeneities, as well as plans for multiple brain metastases in one isocenter, involving highly modulated small fields. We aim to investigate collimator angle dependence of small fields and performance differences between different combinations of ArcCHECK configuration and dose algorithm.

Methods: Several verification tests were performed, ranging from simple rectangular fields to highly modulated clinical plans. To evaluate and compare the performance of the models, the agreements between calculation and measurement are compared between MC and CC. Measurements include water tank measurements for test fields, ArcCHECK measurements for test fields and VMAT plans, and film dosimetry for small fields.

Results and conclusions: In very small or narrow fields, our measurements reveal a strong dependency of dose output to collimator angle for VersaHD with Agility MLC, reproduced by both dose algorithms. ArcCHECK results highlight a suboptimal agreement between measurements and MC calculations for simple rectangular fields when using inhomogeneous ArcCHECK images. Therefore, we advocate for the use of homogeneous phantom images, particularly for static fields, in ArcCHECK verification with MC. MC might offer performance benefits for more modulated treatment fields. In ArcCHECK results for clinical plans, MC performed comparable to CC for 6 MV. For 6 MV FFF and the preferred homogeneous phantom image, MC resulted in consistently better results (13%-64% lower mean gamma index) compared to CC.

Keywords: ArcCHECK; Collapsed Cone algorithm; Monte Carlo algorithm; small field dosimetry.

FIGURE 1

Four images of the ArcCHECK phantom. Left column: “homogeneous” artificial images without (top) and with (bottom) cavity plug. Right column: “inhomogeneous” MVCT images without (top) and with (bottom) cavity plug.

FIGURE 2

Output factors as a function of collimator angles. (a) 6 MV EBT3 film measurements of 0.6 x 0.6 cm² for 0˚ (blue curve) and 90˚(red curve) collimator angle; (b) EBT3 film measurements of 0.6 x 5.0 cm² for 0˚ (blue curve) and 90˚ (red curve); (c) comparison of measured output factors to TPS calculations (CC and MC) as a function of collimator angle for 6 MV and 6 MV FFF.

FIGURE 3

Comparison of measured and calculated dose profiles for off‐axis 0.6 × 0.6 cm² fields. The left two columns show X and Y profiles calculated with CC. The right two columns show X and Y profiles calculated with MC. Each row of plots represents a field; the label above each plot indicates the center of the field in cm. Red lines are measured data, blue lines are calculated.

FIGURE 4

ArcCHECK results for a collection of nine rectangular fields, presented as box‐and‐whiskers plots grouped by dose engine, beam energy, presence of cavity plug and image homogeneity. Top: Mean gamma values at 3%/2 mm dose criteria; bottom: gamma pass rates at 2%/1 mm dose criteria.

FIGURE 5

Cross sections of calculated dose distributions of a 10 × 10 cm² treatment field on the inhomogeneous ArcCHECK image with plug. Top: CC algorithm; bottom: MC algorithm.

FIGURE 6

Comparisons between Monte Carlo‐calculated dose and Collapsed Cone‐calculated dose for two clinical VMAT plans. Top panel: head‐and‐neck plan. Bottom panel: multiple brain metastases plan (eight metastases). Both panels contain three windows: top left is the MC dose, bottom left is CC dose, the bigger panel on the right is the dose difference in % of the maximum dose. A positive difference means the MC dose is higher than the CC dose.

FIGURE 7

ArcCHECK results for 13 clinical treatment plans, grouped by dose engine, beam energy, presence of cavity plug and image homogeneity. Top: mean gamma value at 3%/2 mm dose criteria; bottom: pass rates at 3%/2 mm dose criteria.

FIGURE 8

ArcCHECK analysis (homogeneous phantom with cavity plug) for the “multiple brain metastases 2″ treatment plan, comparing a CC and a MC calculation (no measurements). The gamma threshold is set to 0%. (a) Calculated dose distributions at the ArcCHECK diode interface, left is CC and right is MC. The grid represents the positions of the diodes, transformed from the helical configuration to a 2D rectangular grid. (b1) Global gamma analysis at 2%/2 mm. Blue dots indicate gamma‐failed points where the CC calculated dose is lower than MC. (b2) Y‐profile corresponding to the green line in B1. (c1) Local gamma analysis at 2%/2 mm. Blue dots indicate gamma‐failed points where the CC calculated dose is lower than MC. (c2) Y‐profile corresponding to the green line in C1.

FIGURE 9

Measured and calculated (Collapsed Cone) results showing the effect of the calculation mask in the Collapsed Cone algorithm for 6 MV FFF: calculated dose profiles are cut off around 7 cm off‐axis. The bottom plot shows a zoomed in region of the top plot.