Eight years of IMRT quality assurance with ionization chambers and film dosimetry: experience of the Montpellier Comprehensive Cancer Center

Abstract

Background: To present the results of quality assurance (QA) in IMRT of film dosimetry and ionization chambers measurements with an eight year follow-up.

Methods: All treatment plans were validated under the linear accelerator by absolute and relative measures obtained with ionization chambers (IC) and with XomatV and EDR2 films (Kodak).

Results: The average difference between IC measured and computed dose at isocenter with the gantry angle of 0° was 0.07 ± 1.22% (average ± 1 SD) for 2316 prostate, 1.33 ± 3.22% for 808 head and neck (h&n), and 0.37 ± 0.62% for 108 measurements of prostate bed fields. Pelvic treatment showed differences of 0.49 ± 1.86% in 26 fields for prostate cases and 2.07 ± 2.83% in 109 fields of anal canal.Composite measurement at isocenter for each patient showed an average difference with computed dose of 0.05 ± 0.87% for 386 prostate, 1.49 ± 1.86% for 158 h&n, 0.37 ± 0.34% for 23 prostate bed, 0.80 ± 0.28% for 4 pelvis, and 2.31 ± 0.56% for 17 anal canal cases. On the first 250 h&n analyzed by film in absolute dose, the average of the points crossing a gamma index 3% and 3 mm was 93%. This value reached 99% for the prostate fields.

Conclusion: More than 3500 beams were found to be within the limits defined as validated for treatment between 2001 and 2008.

Figure 1

Vidar reading (ua) of an optical density wedge. (a) Reading of the OD step (b) Graph is plotted as a function of Optical Density for different digitalization table provide by the Vidar system. (Dark value is for logarithmic acquisition table, grey for PW5, and the light grey for linear table.)

Figure 2

Vidar reading (ua) for dose calibration films performed before patients QA. XOMAT-V films (a) and EDR2 films (b).

Figure 3

Dose difference between measured and calculated dose for beam by beam measurements. Results for 2319 prostate fields (a) and 808 head and neck fields (b).

Figure 4

Dose difference between measured and calculated dose for global patient verification. Results for 383 prostate cases (a) and 158 head and neck cases (b). Square dots represent verification with gantry angle at 0° and triangular dots with the gantry in the treatment position.

Figure 5

Dose difference between measured and calculated dose for beam by beam verification for head and neck cases. The dash bars represent acquisition realized with the gantry at the real treatment position and the full bars represent the values with the gantry at 0° for the same patients.

Figure 6

Gama index results for the 100 first head and neck IMRT cases with dosimetric films. Results for the 5 different beams threshold values of 3% / 3 mm (a) and 5% / 3 mm (b). Number of points that reached a gamma value < 1 for head and neck fields: Results for threshold values of 3% / 3 mm (c) and 5% / 3 mm (d).