A novel angular dependency model for MatriXX response and its application to true composite dose verification for IMRT plans

Abstract

Purpose: This paper proposes a model for the angular dependency of MatriXX response and investigates whether MatriXX, with the angular-model-based approach can be applied to true composite dose verification for IMRT plans.

Method: This model attributes the angular dependence of MatriXX response to dynamical translation of its effective measurement plane (EMP) due to the change of beam angle. Considering this mechanism, true composite dose verifications for IMRT plans specified in AAPM TG 119 report using both MatriXX and Gafchromic EBT3 films were undertook and compared to validate the applicability of MatriXX for patient specific QA of composite beam IMRT plans. Dose verifications using MatriXX with and without angular-model-based approach were performed.

Results: MatriXX with angular-model-based approach achieved gamma passing rates with 3%/3 mm and 3%/2 mm criteria better than 98.3% and 98.1% respectively for true composite dose verification of plans in AAPM TG 119 report. The 3%/3 mm and 3%/2 mm gamma passing rates using MatriXX without angular-model-based approach ranged from 85.8% to 98.2% and from 81.3% to 96.5%, respectively. The p-values from the single sided paired t-test indicated no statistical difference between the passing rates from MatriXX with angular-model-based approach and from films, and significant difference between the passing rates from uncorrected MatriXX and from films.

Conclusion: The proposed model for angular dependent MatriXX response is necessary and effective. Dose verification using MatriXX with angular-model-based approach is acceptable for true composite beam IMRT plans with required accuracy to simplify patient specific QA.

Keywords: IMRT; MatriXX; angular response; mechanism of angular dependency; patient specific QA; radiotherapy; true composite dose verification.

FIGURE 1

(a) The photo of two dimensional ionization chamber array MatriXX. (b) CT of MatriXX and the illustration of the dynamic effective measurement plane for MatriXX in the enlarged window

FIGURE 2

(a) The relation between depth offset Δd of the EMP of the MatriXX and the gantry angle and the sensitivity of Δd with respect to set up uncertainty. (b) Reproduce of the CF curve for the center PXC with DEMP and the sensitivity of the CF curve with respect to set up uncertainty. (c) Estimation of the derivative of the CF curve. (d) Pixel‐wise average dose percentage error for dose calculated with 1 mm dose gird and with 0.8 mm dose grid, and with and without the parmaeter ρ

FIGURE 3

(a) The experimental setup for the DEMP model calibration. (b) Illustration of the irradiation area on the surface of the MatriXX. When the gantry is set close to 90, the beam 2 with larger aperture has an irradiation margin (marked by Δ) of the sensitive region of MatriXX

FIGURE 4

The workflow of the determination of DEMP model parameters

FIGURE 5

The workflow of composite dose verification for testing plans using the MatriXX with DEMP model

FIGURE 6

Comparison of (a) calculated (uncorrected) and (c) measured dose distributions, (b) dose profiles with horizontal coordinate representing x direction and (d) pixels failing the 3%/3 mm passing criterion for one of the single beam calibration plans when the gantry is set at 80°. The systematic lateral displacements of the measured dose profile, from that of the calculated dose profile uncorrected by the DEMP model, are quite obvious in (b). This displacement is also illustrated in Figure 1 as Δx

FIGURE 7

Comparison of (a) calculated (corrected) and (c) measured dose distributions, (b) dose profiles and (d) pixels failing the 3%/3 mm passing criterion for one of the single beam calibration plans when the gantry is set at 80°

FIGURE 8

(a) The location of the gamma criterion failure and the comparison of profiles (position in 2D plane marked by the two bold lines) in x direction (parallel to the MLC moving direction) and y direction (perpendicular to the MLC moving direction) for two representative composite beam plans between the measurements from the MatriXX with DEMP model and the calculated dose. (b) The location of the gamma criterion failure and the comparison of profiles in x direction and y direction for two representative composite beam plans between the measurements from the EBT3 film and the calculated dose