On the dosimetric challenges of dynamic jaws tomotherapy: a Monte Carlo study
(2011) Belgian Hospital Physicists Association Symposium — Location: Charleroi (4.February.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. TomoPen includes now efficient arbitrary jaw openings (Sterpin et al 2010, Radiother. Oncol. 94) and an input/output interface allowing completely variable treatment settings. The model was first used to compute dose/FW in a water phantom for a 10 cm x FW beam scanned to deliver a “10 x 10 cm2 field”, normalized to the dose/5 cm obtained for a same “field” but with a 10x5 cm² beam (FW is the “light-projected” beam width). Measurements using Exradin A1SL ion chamber were performed and compared to simulations of three source positions: 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 couch speed; 2) symmetric jaw opening and variable table speed. Dose distributions were compared to convolution/superposition (agreement within 2%/1mm, see figure 2) and measurements (gamma analysis succeed with an agreement criteria of 3%/3mm) with EDR-2 films according to recommended TomoTherapy procedures. 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). On the dosimetric challenges of dynamic jaws tomotherapy: a Monte Carlo study. Belgian Hospital Physicists Association Symposium, Charleroi. https://hdl.handle.net/2078.5/58782