Abstract
Delivery quality assurance (DQA) of tomotherapy plans is routinely performed with silver halide film which has a limited range due to the effects of saturation. DQA plans with dose values exceeding this limit require the dose of the entire plan to be scaled downward if film is used, to evaluate the dose distribution in two dimensions. The potential loss of fidelity between scaled and unscaled DQA plans as a function of dose scaling is investigated. Three treatment plans for 12 Gy fractions designed for SBRT of the lung were used to create DQA procedures that were scaled between 100% and 10%. The dose was measured with an ionization chamber array and compared to values from the tomotherapy treatment planning system. Film and cylindrical ion chamber measurements were also made for one patient for scaling factors of 50% to 10% to compare with the ionization chamber array measurements. The array results show the average gamma pass rate is ≥99% from 100% to 30% scaling. The average gamma pass rate falls to 93.6% and 51.1% at 20% and 10% scaling, respectively. Film analysis yields similar pass rates. Cylindrical ion chambers did not exhibit significant variation with dose scaling, but only represent points in the low gradient region of the dose distribution. Scaling the dose changes the mechanics of the radiation delivery, as well as the signal‐to‐noise ratio. Treatment plans which exhibit parameters that differ significantly from those common to DQA plans studied in this paper may exhibit different behavior. Dose scaling should be limited to the smallest degree possible. Planar information, such as that from film or a detector array, is required. The results show that it is not necessary to perform both a scaled and unscaled DQA plan for the treatment plans considered here.PACS numbers: 87.55.km, 87.55.Qr
Highlights
Patient-specific quality assurance measurements are commonly performed for IMRT treatments delivered using the TomoTherapy Hi·ART (TomoTherapy, Inc., Madison WI) treatment unit
Two-dimensional analysis of the dose distribution is most commonly performed with Kodak EDR2 film (Kodak, Rochester, NY) which saturates above approximately 6 Gy.[1,2] This implies that to deliver a delivery quality assurance (DQA) plan for a treatment fraction which delivers a dose above 6 Gy, the fraction dose would need to be scaled downward in order to make a planar EDR2 film measurement useful
The 100% scaled DQA plan for Patient 1 was performed after the dose rate for the tomotherapy unit had been adjusted for clinical purposes
Summary
Patient-specific quality assurance measurements are commonly performed for IMRT treatments delivered using the TomoTherapy Hi·ART (TomoTherapy, Inc., Madison WI) treatment unit. These measurements are referred to by TomoTherapy as delivery quality assurance (DQA) measurements. The TomoTherapy treatment planning system (TPS) gives users the option of scaling DQA plans during their creation. 85 Whitmore et al.: Tomotherapy DQA dose scaling leaves remain open while other parameters are held constant. This changes the mechanics of the delivery, and raises the question of when a scaled DQA plan adequately mimics the unscaled DQA plan. Film and cylindrical ion chamber measurements were acquired for one patient plan to compare with the seven results
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