Angular dependence correction of MatriXX and its application to composite dose verification

Abstract

We measured the angular dependence of central and off-axis detectors in a 2D ionization chamber array, MatriXX, and applied correction factors (CFs) to improve the accuracy of composite dose verification of IMRT and VMAT. The MatriXX doses were measured with a 10° step for gantry angles (θ) of 0°-180°, and a 1° step for lateral angles of 90°-110° in a phantom, with a 30 × 10 cm2 field for 6 MV and 10 MV photons. The MatriXX doses were also calculated under the same conditions by the Monte Carlo (MC) algorithm. The CFs for the angular dependence of MatriXX were obtained as a function of θ from the ratios of MatriXX-measured doses to MC-calculated doses, and normalized at θ = 0°. The corrected MatriXX were validated with different fields, various simple plans, and clinical treatment plans. The dose distributions were compared with those of MC calculations and film. The absolute doses were also compared with ionization chamber and MC-calculated doses. The angular dependence of MatriXX showed over-responses of up to 6% and 4% at θ = 90° and under-responses of up to 15% and 11% at 92°, and 8% and 5% at 180° for 6 MV and 10 MV photons, respectively. At 92°, the CFs for the off-axis detectors were larger by up to 7% and 6% than those for the central detectors for 6 MV and 10 MV photons, respectively, and were within 2.5% at other gantry angles. For simple plans, MatriXX doses with angular correction were within 2% of those measured with the ionization chamber at the central axis and off-axis. For clinical treatment plans, MatriXX with angular correction agreed well with dose distributions calculated by the treatment planning system (TPS) for gamma evaluation at 3% and 3 mm. The angular dependence corrections of MatriXX were useful in improving the measurement accuracy of composite dose verification of IMRT and VMAT.

Figure 1. Combination of MatriXX and MULTICube Lite phantom: (a) Orientation normally used for the dose verification of IMRT and VMAT; (b) gantry angle sensor; (c) MULTICube Lite phantom.

Figure 2. Regions of interest (ROIs) corresponding to the sizes and locations of MatriXX detectors (column: j=1, 2, ..., 32; row: i=16, 17) were configured in the CT images. The doses of ROIs were calculated with the MC algorithm on iPlan. The dose of the central detectors was defined as the average dose of four detectors at the center (red line). In addition, the dose of the off‐axis detectors was defined as the average dose of two detectors (row: i=16, 17) at column j=1,2,...,32 (blue line).

Figure 3. Angular dependence of MatriXX detectors. CFij (θ) for the detectors (column: j=1, 16, 32; row: i=16,17) is shown as a function of gantry angle for (a) 6 MV and (b) 10 MV photons. Although not shown here, CFij (θ) for other detectors is between off‐axis detectors of columns j=1 and j=32.

Figure 4. Deviations of iPlan MC‐calculated doses and MatriXX‐measured doses without and with angular correction from PTW‐TN31010 doses at θ=0°–180° for the central axis of a 10×10 cm2 field for (a) 6 MV and (b) 10 MV photons.

Figure 5. Deviations of iPlan MC‐calculated doses and MatriXX‐measured doses without and with “central correction” or “entire correction” from PTW‐TN31010 doses at θ=90° as a function of off‐axis distance in a 10×10 cm2 field for (a) 6 MV and (b) 10 MV photons.

Figure 6. Passing rates between iPlan MC‐calculated and MatriXX dose distributions without and with “central correction” or “entire correction” at θ=0°–180° with a 10×10 cm2 field for (a) 6 MV and (b) 10 MV photons. Gamma evaluation was performed with 2% dose difference and 2 mm distance to agreement.

Figure 7. Gamma evaluation analyzed at θ=90° in which was performed with 2% dose difference and 2 mm distance to agreement by using OmniPro‐I'mRT.

Figure 8. Deviations of iPlan MC‐calculated and MatriXX‐measured doses without and with angular correction from PTW‐TN31010 doses. Dose verifications were performed for various simple plans (Table 1) with a 10×10 cm2 field for (a) 6 MV and (b) 10 MV photons.

Figure 9. Deviations of TPS‐calculated and MatriXX‐measured doses with and without angular correction from PTW‐TN31010 doses. Dose verifications were performed with the IMRT (iPlan MC) and VMAT (Eclipse AAA) plans presented in Table 2.