3D IMAGE-BASED GYNECOLOGIC BRACHYTHERAPY PHYSICS

Abstract

This paper is based on my invited talk at the JASTRO Brachytherapy Subcommittee meeting in June of 2003 in Tokyo. The talk materials are derived from the presentation of low dose-rate (LDR) gynecologic cancer therapy given at the University of California, Davis Cancer Center by Dr. Fritz Lerma of Mallinckrodt Institute of Radiology (MIR), Washington University, St. Louis. For more than 5 years, a brachytherapy program has been pushed forward at MIR and the University of California at San Francisco. In particular, Dr. Jeff Williamson of MIR, now at the Medical College of Virginia, emphasized the importance of CT imageguided brachytherapy treatment, 3D simulation of anatomy and applicators, and dose calculations based on Monte Carlo applicator simulations. The use of CT images has differentiated the treatment modalities from the past in that it allows more quantitative data analysis and provides more accurate dose distribution information. Dr. Williamson has single-handedly developed the frontier of gynecologic brachytherapy in the U.S. Dr. Lerma was his disciple and worked with him for two years during the program development period. Therefore, I believe Dr. Lerma’s materials reproduced here with his permission belong in the forefront of the brachytherapy treatment approach to gynecologic cancers in North America. This paper begins with an Introduction followed by the LDR Gynecologic brachytherapy program, and Motivations to push CT image-guided treatment planning forward at Mallinckrodt. As a practical example of 3 dimensional image-guided radiotherapy, this paper will elaborate on the accurate matching of applicator contours delineated in a CT space and Monte Carlo simulation methods. The use of CT and Monte Carlo simulations of applicator in a CT space allows treatment plan dose calculations solely on the basis of the Monte Carlo method if the full Monte Carlo calculations and dose measurements by 3 dimensional (3D) water phantom scan are in agreement. The conventional 2D single source superposition method does not take into account the presence of applicators. Therefore, the scattering and absorption of photons produced by the nearby sources are not accounted for. As a result it shows some dose deviations, in particular, near the source. At the end, this paper introduces positron emission tomography (PET) image-guided brachytherapy treatment planning . This modality is compared with the conventional 2D image-guided treatment plans. The PET images are known to reflect more physiologic information than CT images, even though the CT images provide better geometrical resolution than PET. The PET-based 3D treatment planning certainly belongs to the frontier of 3D image-guided brachytherapy treatment planning and delivery. As proved from the prostate study by magnetic resonance spectroscopy (MRS)3), the tumor cell concentration within the tumor is not necessarily uniform. In this respect, though not proven yet, the PET images may shed some light on the tumor cell distribution for the gynecologic (GYN) cancer.