Interplay effect of angular dependence and calibration field size of MapCHECK 2 on RapidArc quality assurance

Abstract

The purpose of this study is to investigate an effect of angular dependence and calibration field size of MapCHECK 2 on RapidArc QA for 6, 8, 10, and 15 MV. The angular dependence was investigated by comparing MapCHECK 2 measurements in MapPHAN-MC2 to the corresponding Eclipse calculations every 10° using 10× 10 cm2 and 3 × 3 cm2 fields. Fourteen patients were selected to make RapidArc plans using the four energies, and verification plans were delivered to two phantom setups: MapCHECK 2/MapPHAN phantom (MapPHAN QA) and MapCHECK 2 on an isocentric mounting fixture (IMF QA). Migration of MapCHECK 2 on IMF was simulated by splitting arcs every 10° and displacing an isocenter of each partial arc in the Eclipse system (IMFACTUAL QA). To investigate the effect of calibration field size, MapCHECK 2 was calibrated by two field sizes (10 × 10 cm2 and 3 × 3 cm2) and applied to all QA measurements. The γ test was implemented using criteria of 1%/1 mm, 2%/2 mm, and 3%/3 mm. A mean dose of all compared points for each plan was compared with respect to a mean effective field size of the RapidArc plan. The angular dependence was considerably high at gantry angles of 90° ± 10° and 270° ± 10° (for 10 × 10/3 × 3 cm2 at 90°, 30.6% ± 6.6%/33.4%± 5.8% (6 MV), 17.3% ± 5.3%/15.0% ± 6.8% (8 MV), 8.9%± 2.9%/7.8% ± 3.2% (10 MV), and 2.2% ± 2.3%/-1.3% ± 2.6% (15 MV)). For 6 MV, the angular dependence significantly deteriorated the γ passing rate for plans of large field size in MapPHAN QA (< 90% using 3%/3 mm); however, these plans passed the γ test in IMFACTUAL QA (> 95%). The different calibration field sizes did not make any significant dose difference for both MapPHAN QA and IMFACTUAL QA. For 8, 10, and 15 MV, the angular dependence does not make any clinically meaningful impact on MapPHAN QA. Both MapPHAN QA and IMFACTUAL QA presented clinically acceptable γ passing rates using 3%/3 mm. MapPHAN QA showed better passing rates than IMFACTUAL QA for the tighter criteria. The 10 × 10 cm2 calibration showed better agreement for plans of small effective field size (< 5 × 5 cm2) in MapPHAN QA. There was no statistical difference between IMF QA and IMFACTUAL QA. In conclusion, MapPHAN QA is not recommended for plans of large field size, especially for 6 MV, and MapCHECK2 should be calibrated using a field size similar to a mean effective field size of a RapidArc plan for better agreement for IMF QA.

Figure 1

MapCHECK 2 device set up for measuring IMRT validation plans: (a) MapPHAN QA using MapPHAN‐MC2 on an extended tennis‐racket‐type grid couch top, and (b) IMF QA using the isocentric mounting fixture (IMF).

Figure 2

Displacement of central axis (CAX) of MapCHECK 2 on IMF: (a) X offset (MapCHECK 2 lateral) and (b) Y offset (MapCHECK 2 longitudinal) for different collimator angles.

Figure 3

Percent dose difference $((\text{Measurement}-\text{Plan})/\text{Plan}\times 100(\mathrm{\%}))$ of MapCHECK 2 measurements with MapPHAN‐MC2. The measurements were performed for every 10° with two different field sizes ($3\times 3{\text{cm}}^2$ and $10\times 10{\text{cm}}^2$) and four different energies: (a) 6 MV, (b) 8 MV, (c) 10 MV, and (d) 15 MV. Error bars represent 1 SD of compared points around isocenter (5 points for $3\times 3{\text{cm}}^2$ and 13 points for $10\times 10{\text{cm}}^2$).

Figure 4

Mean dose error of all compared dose points with respect to mean equivalent‐square field size of RapidArc fields in MapPHAN QA for (a) 6 MV, (b) 8 MV, (c) 10 MV, and (d) 15 MV.

Figure 5

Mean dose error of all compared dose points with respect to mean equivalent‐square field size of RapidArc fields in ${\text{IMF}}_{\text{ACTUAL}}$ QA for (a) 6 MV, (b) 8 MV, (c) 10 MV, and (d) 15 MV.

Figure 6

The tongue‐and‐groove effect on the IMF QA demonstrated by continuous hot or cold spots along MLC trajectory: (a) MapCHECK 2 diode points overlaid on the HD‐MLC pattern (inner leaf width $=2.5\text{mm}$, outer leaf width $=5\text{mm}$, and diode size $=0.8\times 0.8{\text{mm}}^2$), and (b) failed points of 15 MV IMF QA using $2\mathrm{\%}/2\text{mm}$ criteria for Patient #3. (Red = measurement is high; blue = calculation is high.)