Dosimetric challenges of dynamic jaws Tomotherapy: A Monte Carlo study
(2011) International Conference on Tomotherapy — Location: Heidelberg, Germany (16.September.2011)
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- Abstract
- In its present configuration, tomotherapy uses three fixed different field widths (5, 2.5 and 1 cm), limiting the control of the longitudinal modulation. This can be overcome by allowing the jaws to move dynamically. The goals of this study are : 1) describe the implementation of the dynamic jaws feature in TomoPen, a Monte Carlo (MC) based model; 2) provide a detailed knowledge of the dosimetric issues with small field deliveries including output factors and potential linac displacements. The new TomoPen model includes a validated method for arbitrary jaw openings (Sterpin et al 2010, Radiother. Oncol. 94) and new input/output interface allowing variable couch speed and jaw openings. The model was first used to compute dose/FW in a water phantom for a 10 cm x FW field scanned to deliver a “10 x 10 cm2 field”, normalized to dose/FW obtained for a same “field” but delivered using a 10x5 cm² beam (FW is the nominal longitudinal field width). Measurements using Exradin A1SL ion chamber were performed and compared to simulations of three positions of the source: centred, 0.3 mm and 1 mm off-axis (see Figure 1). Moreover, a simple dynamic jaws delivery in the “cheese” phantom that includes two cylindrical targets longitudinally separated by 2.0 cm was simulated. The plan was delivered in two modes: 1) asymmetric jaw opening and fixed table speed; 2) symmetric jaw opening and variable table speed. Dose distributions were compared to convolution/superposition and measurements (in progress) with EDR-2 films according to recommended TomoTherapy procedures. Results are shown in figure 2 with excellent agreement (2%/1mm). The new version of TomoPen showed its ability to compute dynamic jaws-couch tomotherapy treatments. The effect of source displacements on treatment head output was also investigated, demonstrating the need of source position verification when small fields are used.
Citations
Sterpin, E., Vynckier, S., & et al. (2011). Dosimetric challenges of dynamic jaws Tomotherapy: A Monte Carlo study. International Conference on Tomotherapy, Heidelberg, Germany. https://hdl.handle.net/2078.5/58079