Medical Physics Seminar – Monday, April 30, 2012
Dosimetric Characterization of Surface Applicators used with Miniature Electronic and Iridium-192 Brachytherapy Sources
Regina K. Fulkerson (student of Dr. Larry DeWerd)
Research Assistant, Department of Medical Physics, UW-School of Medicine & Public Health, Madison, WI - USA -
Skin cancer is the most common form of cancer in the United States, with over one million cases diagnosed each year.1 If detected early, non-melanoma skin cancers (basal and squamous cell carcinomas) are usually controlled with radiation therapy with or without surgical intervention.1, 2 Historically, treatment of skin cancers with radiation therapy has employed linear accelerator based electron beams or orthovoltage x-ray beams.3 Recently, specialized applicators have been developed for use with the Xoft Axxent® electronic brachytherapy system (eBxTM) as well as 192Ir high dose rate (HDR) brachytherapy sources for use in the treatment of superficial lesions. These brachytherapy-based treatments have the advantage over external beam therapies (electrons or orthovoltage x-rays) in that they do not require the use of templates and cutouts, which can be time consuming to create and cumbersome to use. Additionally, these customized devices may increase the chance of an error in setup, leading to problems with energy, percent depth dose, and output factors.2 Traditional uses of photon brachytherapy sources follow the dosimetry recommendations made in the American Association of Physicists in Medicine (AAPM) Task Group 43 report and updates
(TG-43).4, 5 However, the surface applicators of interest have not been well characterized and certain aspects of the standard brachytherapy dosimetry protocol cannot be directly applied. Additionally, the position of a source in the applicator at some distance from the target gives the applicator some attributes of a low-energy external beam, for which some constraints of the AAPM standard low-energy external beam dosimetry protocol (Task Group 61) may apply.6 Substantial uncertainty exists in the applicability of the current protocols, and no rigorous method of output verification exists for these types of surface applicators. The goal of this investigation is to fully characterize the applicators and create a standardized method of output verification that is applicable to applicators used with the miniature electronic and 192Ir HDR brachytherapy sources.
1] American Cancer Society, “Cancer Facts & Figures 2009,†Atlanta: American Cancer Society, 2009.
[2] Z. Ouhib and M. Kasper, Clinical Guide to Surface Treatment of Skin Cancer with Brachytherapy. Nucletron, 2010.
[3] N. Stas, J. Goncalves, M. Marques, and V. Castro, “Comparison of interstitial brachytherapy with silk threads versus orthovoltage treatments in non surgical face cancer recurrences,†(abstract) Radiother. Oncol. 60, 123 (2001).
[4] M. J. Rivard, B. M. Coursey, L. A. DeWerd, W. F. Hanson, M. S. Huq, G. S. Ibbott, M. G. Mitch, R. Nath, and J. F. Williamson, “Update of AAPM Task Group No. 43 report: A revised AAPM protocol for brachytherapy dose calculations,†Med. Phys. 31, 633–674 (2004).
[5] R. Nath, L. L. Anderson, G. Luxton, K. A. Weaver, J. F. Williamson, and A. S. Megooni, “Dosimetry of interstitial brachytherapy sources: Recommendations of the AAPM Radiation Therapy Committee Task Group No. 43,†Med. Phys. 22, 209–234 (1995).
[6] C. Ma, C. W. Coffey, L. A. DeWerd, C. Liu, R. Nath, S. M. Seltzer, and J. P. Seuntjens, “AAPM protocol for 40–300 kV x-ray beam dosimetry in radiotherapy and radiobiology,†Med. Phys. 28, 868–893 (2001).