Dosimetric characterization and output verification for conical brachytherapy surface applicators. Part I. Electronic brachytherapy source

• Published in: Medical physics (Lancaster) 2014-02, Vol.41 (2), p.022103-n/a
• Main Authors: Fulkerson, Regina K., Micka, John A., DeWerd, Larry A.
• Format: Article
• Language: English
• Subjects: ABSORBED RADIATION DOSES; BRACHYTHERAPY; Brachytherapy - instrumentation; Calibration; cancer; COMMISSIONING; DEPTH DOSE DISTRIBUTIONS; DOSE RATES; Dose‐volume analysis; DOSIMETRY; Dosimetry/exposure assessment; Electric Conductivity; electron beam applications; ELECTRON BEAMS; Electrons; EPITHELIOMAS; Error analysis; Field aligned currents; HDR; ionisation chambers; IONIZATION CHAMBERS; iridium; IRIDIUM 192; Iridium Radioisotopes - therapeutic use; KERMA; LINEAR ACCELERATORS; Measurement of nuclear or x‐radiation; MONTE CARLO METHOD; Monte Carlo methods; PHANTOMS; Photons; QUALITY ASSURANCE; Radiation Measurement Physics; RADIATION PROTECTION AND DOSIMETRY; Radiation therapy; radioactive sources; Radioactive sources other than neutron sources (radioactive dressings A61M36/14); RADIOLOGY AND NUCLEAR MEDICINE; Radiometry - instrumentation; Radiotherapy Dosage; Scintigraphy; skin; skin cancer; statistical distributions; surface applicator; Surface ionization; Surface treatments; Therapeutic applications, including brachytherapy; Thermoluminescent dosimeters; Thermoluminescent Dosimetry; Tubes for determining the presence, intensity, density or energy of radiation or particles; Uncertainty; VERIFICATION
• ISSN: 0094-2405; 2473-4209; 0094-2405
• DOI: 10.1118/1.4862505

Purpose: Historically, treatment of malignant surface lesions has been achieved with linear accelerator based electron beams or superficial x-ray beams. Recent developments in the field of brachytherapy now allow for the treatment of surface lesions with specialized conical applicators placed directly on the lesion. Applicators are available for use with high dose rate (HDR)192Ir sources, as well as electronic brachytherapy sources. Part I of this paper will discuss the applicators used with electronic brachytherapy sources; Part II will discuss those used with HDR 192Ir sources. Although the use of these applicators has gained in popularity, the dosimetric characteristics including depth dose and surface dose distributions have not been independently verified. Additionally, there is no recognized method of output verification for quality assurance procedures with applicators like these. Existing dosimetry protocols available from the AAPM bookend the cross-over characteristics of a traditional brachytherapy source (as described by Task Group 43) being implemented as a low-energy superficial x-ray beam (as described by Task Group 61) as observed with the surface applicators of interest. Methods: This work aims to create a cohesive method of output verification that can be used to determine the dose at the treatment surface as part of a quality assurance/commissioning process for surface applicators used with HDR electronic brachytherapy sources (Part I) and192Ir sources (Part II). Air-kerma rate measurements for the electronic brachytherapy sources were completed with an Attix Free-Air Chamber, as well as several models of small-volume ionization chambers to obtain an air-kerma rate at the treatment surface for each applicator. Correction factors were calculated using MCNP5 and EGSnrc Monte Carlo codes in order to determine an applicator-specific absorbed dose to water at the treatment surface from the measured air-kerma rate. Additionally, relative dose measurements of the surface dose distributions and characteristic depth dose curves were completed in-phantom. Results: Theoretical dose distributions and depth dose curves were generated for each applicator and agreed well with the measured values. A method of output verification was created that allows users to determine the applicator-specific dose to water at the treatment surface based on a measured air-kerma rate. Conclusions: The novel output verification methods described in this work will reduce u