(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To derive a quantitative value of overall range uncertainty after establishing a material composition vs Hounsfield Units (HU) calibration curve.
Methods: Following the well-established stoichiometric method, a mass-density vs HU curve was generated for our CT scanner. The curve was then validated with two different methodologies: a) relative stopping power (RSP) measurements of fresh tissue samples (Taasti, 2017) and b) dose fall-off quantification of a uniform SOBP in a non-uniform mixture of biologic tissue, this unprepared sample included a combination of fat, bone, and muscle. For RSP determination, the individual tissue samples were placed in containers of known thickness, and a multi-layer ionization chamber (MLIC) was used to measure the Bragg peak pull back. For the SOBP distal fall-off quantification, the unprepared animal tissue was placed in a container on top of slabs of solid water, and a 2D array was used as measuring device. The distance between isocenter and the slabs of solid water was 70.8 mm. The path length from the surface to the distal edge of the target was 216.1 mm with a field size of 100 x 100 mm².
Results: The differences between measured and calculated RSP showed values between 1.01 % and 2.9 % in the soft tissue region. The measured and planned distal fall-off of the SOBP, show differences of 1-2 mm when going through soft tissue only, and 3.0 mm when going through soft tissue and bone, which correspond to values of range uncertainty of 0.93% and 1.38% respectively.
Conclusion: Measurements were performed to enable the clinical usage of a CT calibration curve. The results of the distal fall-off measurement resemble the results provided by IROC (spine phantom test). The verification methodology presented here provides valuable information at the time of TPS and CT commissioning for clinical proton dose calculations.
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