Author: Huixiao Chen, Zhe (Jay) Chen, MinYoung Lee, Sameer Taneja 👨🔬
Affiliation: Yale School of Medicine, Department of Therapeutic Radiology, Yale University School of Medicine 🌍
Purpose: Superficial radiation therapy uses low-energy x-rays to treat various types of cancers, including non-melanoma skin cancers, and dermatological conditions such as keloid scars, mycosis fungoides, and psoriasis. AAPM TG-61 provides reference dosimetry guidelines and a more recently circulated draft of AAPM MPPG 18a recommends dosimetric quality assurance (QA) procedures utilizing an ionization chamber (IC). This study evaluates the feasibility of using an ionization chamber array, the IC Profiler (ICP), to streamline QA tests and characterize orthovoltage dosimetry.
Methods: ICP measurements of relative output, collimator, and cone factors were measured on a Pantak Therapax DXT 300 unit using 120 kVp and 250 kVp beam energies and three filters. Collimator factors were measured for the adjustable collimator ranging from 5x5 cm2 to 20x20 cm2 and normalized to 10x10 cm2. Cones diameters ranging from 2 cm to 8 cm, normalized to 4 cm, were measured. Results were compared with conventional measurements using an Exradin Spokas air equivalent IC paired with a PTW Unidos electrometer. In addition, time linearity ranging from 1 to 5 minutes, radiation field size determination, flatness and symmetry were measured using the ICP.
Results: ICP measured collimator factors agreed to IC measurements within 3.5% for filters 1 and 2, and 6.0% for filter 3, with maximum deviations at small fields. Cone factors agreed to 1.4%. Relative output, field flatness and symmetry showed reproducibility within 1% across three measurement sets. Radiation field sizes agreed to within 0.3 cm and time linearity within 2.5%.
Conclusion: The ICP showed good agreement with IC for collimator and cone factors, repeatability of relative output, and timer linearity. In addition, the ICP can measure flatness and symmetry and determine radiation field size as part of output measurements, which streamlines the QA process. This work demonstrated the feasibility of implementing the ICP into QA workflow.