Comparison of two commercial detector arrays for RapidArc quality assurance Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/v979v701g

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  • Radiation therapy is used in approximately half of all cancer treatments. The rapid advances in radiation therapy have led to techniques that are much more complex and therefore must be more carefully assessed for safety and efficacy. Accurate quality assurance methods must be implemented to provide patient safety and confidence in treatment efficacy. RapidArc is a relatively new version of Intensity Modulated Radiation Therapy (IMRT) which concomitantly fluctuates dose rate and gantry speed while dynamically altering the treatment field size. This array of electronic and mechanical variables increases the potential for errors. This examines the ability of two commercially available quality assurance devices to detect errors in five RapidArc treatment plans. The ArcCHECK (Sun Nuclear Inc.) and the 2D-ARRAY seven29 with Octavius phantom (PTW) were tested. The ArcCHECK is a hollow cylindrical phantom with diodes placed in a spiral array. The PTW device is a flat panel of ion chambers positioned within an octagonal phantom. Both devices were tested for linearity over a range of 1-300 MU of 6 MV photons delivered via a static 10x10 cm² field. To assess the sensitivity of the devices in a treatment scenario, errors were introduced including couch rotations, gantry position errors and shifts in the multi-leaf collimator (MLC). Couch rotations of 0.5°-5° were induced and unaltered treatment plans were then delivered to the two devices. Errors in gantry rotation and MLC leaf positions were introduced via a Matlab program that modifies existing DICOM treatment files in a predetermined manner. Gantry errors ranged from 0.5°-5°. MLC errors included systematic shifts, systematic opening and closing and random leaf errors of 10% or 20%. The magnitude of MLC errors ranged from 0.25-5mm. Results were analyzed using the gamma analysis method. Both devices demonstrated linearity within 1% for a static 10x10 cm² field. The ArcCHECK detected a couch rotation of 2° or less in all plans and the PTW detected a couch rotation of 4° or less in the frontal plane and only detected an error in a head and neck plan when measured in the sagittal plane. However, both devices responded with less sensitivity when subjected to gantry rotation errors. The ArcCHECK was more reliable at detecting MLC errors and demonstrated an overall higher sensitivity to errors than the PTW system. A study of the effect of these errors on dose distribution in tumor treatment volumes and organs at risk would be a logical next step.
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