A comprehensive comparison study of three different planar IMRT QA techniques using MapCHECK 2

Abstract

The purpose of this study is to determine comparability of three different planar IMRT QA techniques: patient gantry angle composite (PGAC), single gantry angle composite (SGAC), and field by field (FBF), using MapCHECK 2 device and the γ test as performance metrics; and to assess the dependency of these techniques on intensity modulation, couch attenuation, and detector position (angular dependency). Ten highly modulated head and neck (H&N) and ten moderately modulated prostate IMRT validation plans were delivered using different techniques and were intercompared using the Student's t-test. The IMRT QA measurements were evaluated by percentage of points passing the γ test for three different criteria: 1% (dose difference)/1 mm (distance to agreement (DTA)) (C1), 2%/2 mm (C2), and 3%/3 mm (C3). To investigate dependency of the IMRT validation on treatment couch, ionization chamber measurements, as well as the conventional MapCHECK 2 QAs, were performed with PGAC and PGAC-WOC (without couch; using an extended tennis racket-type insert with negligible attenuation assumed). To determine angular dependency of the MapCHECK 2, patient gantry field-by-field (PG-FBF) technique was delivered and evaluated separately for each field. The differences of γ passing rates between SGAC and FBF were statistically insignificant, while these were statistically significant when compared to PGAC. SGAC and FBF techniques showed statistically insignificant differences between different levels of intensity modulation (H&N vs. Prostate) at C2 and C3 criteria, while PGAC could not for any criteria. The treatment couch has a significant impact on γ passing rates (PGAC vs. PGAC-WOC), but an ionization chamber-based IMRT validations showed clinically insignificant dose errors (< 2%) in all cases. This study showed that the MapCHECK 2 device has large angular dependency, especially at gantry angles of 90° and 270°, which dramatically affected the γ passing rates of PGAC. With proper consideration of couch attenuation and beam arrangement, the MapCHECK 2 will produce clinically comparable QA results using the three different planar IMRT QA techniques.

Figure 1

MapCHECK 2 device set up for measuring IMRT validation plans using extended tennis racket‐type grid insert.

Figure 2

Modeling of the Varian couch top with 10 cm height of solid water blocks with isocenter placed at 5 cm depth in the Eclipse treatment planning system.

Figure 3

Number of beams at certain gantry angles used for PG‐FBF analysis for (a) 6 MV and (b) 10 MV. Split beams were individually counted in the number of beams.

Figure 4

The percent dose difference between measurement using MapCHECK 2 and planning (all measurement and planning points within a $2\text{cm}\times 2\text{cm}$ area were averaged and compared) at different gantry angles for different energies (6 MV and 10 MV) and field sizes ($5\text{cm}\times 5\text{cm}$ and $10\text{cm}\times 10\text{cm}$).

Figure 5

The average γ passing rates for each beam angle delivered using PG‐FBF technique for 6 MV and 10 MV beams.