Quality assurance of TomoDirect treatment plans using I'mRT MatriXX

Abstract

Purpose: To evaluate the performance of 2D-array I'mRT MatriXX for dose verification of TomoDirect treatment plans.

Methods: In this study, a 2D-array ion chamber device - the I'mRT MatriXX and Multicube Phantom from IBA - was used for dose verification of different TomoDirect plans. Pre-treatment megavoltage computed tomography (MVCT) was performed on the phantom setup for position correction. After the irradiation of treatment plans on the I'mRT MatriXX and Multicube Phantom, the measured doses of coronal planes were compared with those from the planning calculations for verification. The results were evaluated by comparing the absolute dose difference in the high dose region as well as the gamma analysis of the 2D-dose distributions on the coronal plane. The comparison was then repeated with the measured dose corrected for angular dependence of the MatriXX.

Results: When angular dependence is taken into account, the passing rate of gamma analysis is over 90% for all measurements using the MatriXX. If there is no angular dependence correction, the passing rate of gamma analysis worsens for treatment plans with dose contribution from the rear. The passing rate can be as low as 53.55% in extreme cases, i.e. where all doses in the treatment plan are delivered from the rear.

Conclusion: It is important to correct the measured dose for angular dependence when verifying TomoDirect treatment plans using the MatriXX. If left uncorrected, a large dose discrepancy may be introduced to the verification results.

Figure 1

I’mRT MatriXX 2D-ion chamber array.

Figure 2

I’mRT MatriXX combined with the Multicube Phantom.

Figure 3

Cylindrical target outlined on the I’mRT MatriXX.

Figure 4

Beams setting simulating static beam at gantry 0°.

Figure 5

Different treatment plans for pre-treatment verification study. (a) Lateral opposing treatment plan (Gantry 90° and 270°). (b)Treatment plan with 11 fields irradiated from the front (Gantry 285°, 300°, 315°, 330°, 345°, 0°, 15°, 30°, 45°, 60°, and 75°). (c) 4-field box treatment plan (Gantry 0°, 90°, 180°, and 270°). (d) AP opposing treatment plan (Gantry 0° and 180°). (e) Treatment plan with 11 fields irradiated from the rear (Gantry 105°, 120°, 135°, 150°, 165°, 180°, 195°, 210°, 225°, 240°, and 255°).

Figure 6

Snapshots taken by the MatriXX are merged into different fields according to the gantry angle.

Figure 7

Percentage difference between measured dose and calculated dose at different gantry angles.

Figure 8

Comparison between the calculated dose profile (shown in green) and the measured dose profile (shown in red) with and without angular dependence correction for different treatment plans. (a) Lateral opposing treatment plan. (b) Treatment plan with 11 beams irradiated from the front. (c) 4-field box treatment plan. (d) AP opposing treatment plan. (e) Treatment plan with 11 beams irradiated from the rear.