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New 'In Plane' Reference Profiles for TomoTherapy System Based On 310015, PTW Ion Chamber, and A16 Standard Imaging Ion Chamber

C De Souza,

C. De Souza & P. Serrano-Ojeda, Caribbean Radiation Oncology Center, Bayamon, Puerto Rico

SU-E-T-142 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

To determine a new in-plane reference profile data set for TomoTherapy system based on measurements with two different ion chambers: 310015 model, PTW/Freiburg, and A16 model, Standard Imaging/Madison.

A Virtual Water phantom was machined with holes to fit, simultaneously, the following ion chambers: A1SL model (0.056cm3), SI/Madison - Tomotherapy reference ion chamber; 310015 model (0.03cm3), PTW/ Freiburg; and A16 model (0.007cm3), SI/Madison. A TomoTherapy machine was adjusted based on A1SL ion chamber measurements to produce profiles comparable to the Gold Standard in-plane profiles. The original Gold Standard profiles were used to create the planning system beam model. A technique of irradiation named topographic profile was used for all measurements. This technique takes advantage of the automatic control of the couch motion, which allows determining the couch speed. Doing so, it was possible to irradiate the three ion chambers under the same beam/geometry, simultaneously. A Tomo-Electrometer and respective software (TERMS)were used to obtain the data.

Comparisons were performed against the in-plane Gold Standard profile data based on gamma evaluation: 1% in ion chamber response normalized, and 1% of the field width in DTA for the reference ion chamber. A gamma response less than 0.3% and a field width within 0.2% were obtained, and established as tolerance to define the tested machine as twin to the one that produced the original Gold Standard in-plane profiles, or reference profiles. As these reference profiles were obtained, the respective reference/gold standard profiles for the other two ion chambers under evaluation were produced as well.

This work presents a technique that allows producing Gold Standard in plane profiles for different ion chambers than that used as reference by TomoTherapy. As such, it expands the possibility of the Tomotherapy system commissioning for any other ion chamber appropriate for small field dosimetry.

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