Commissioning of dynamic jaw delivery in tomotherapy

Abstract

Objective To evaluate the dosimetric penumbra and delivery accuracy of dynamic jaw delivery in tomotherapy. Methods The jaw positioning hardware and the beam model were updated. Mechanical alignments were verified after the upgrade of the jaw positioning hardware. PDDs and beam profiles were measured by a two-dimensional water tank and compared with the new beam model. Dose penumbras in the longitudinal direction were compared between the dynamic and fixed jaw plans for different field width. Delivery accuracy was evaluated by point dose measurements with A1SL chamber and gamma analysis on the dose distribution measured by ArcCheck detector array. Results Mechanical alignments were in tolerance and beam characteristics were tuned to match the dynamic jaw beam model. Differences in the field width between the measured results and reference data were< 0.3% for both symmetric and asymmetric profiles in the longitudinal direction. The dose penumbra in dynamic jaw delivery was reduced from 17.92 mm to 7.51 mm for 2.5 cm jaw, and from 33.73 mm to 6.97 mm for 5.0 cm jaw, close to the penumbra of the traditional 1.0 cm jaw. IMRT verification of clinical cases was performed by A1SL ion chamber and ArcCheck detector array. The mean point dose difference was 0.33%±0.73% between the calculated and meassured data. Gamma analysis of dose distributions revealed that approximately 99.8% of the points satisfied the gamma criteria of 3% dose difference and 3 mm distance-to-agreement and the mean passing rates remained 97.9% even with tightest criteria of 2%/2 mm, and 100.0% with the criteria of 4%/4 mm, respectively. Conclusions Dosimetric penumbra in the longitudinal direction is significantly improved by the dynamic jaw delivery. Both the mechanical alignment and treatment delivery are qualified, suggesting that this new treatment is accurate and reliable. Key words: Tomotherapy; Dynamic jaw delivery; Quality assurance