Dose distribution verification for GZP6 sources: a comparison of Monte Carlo, radiochromic film, and GZP6 treatment planning system

www.onk.ns.ac.rs/Archive Vol 20, No. 1–2, July 2012 INTRODUCTION The GZP6 high dose rate (HDR) afterloading brachytherapy unit (Nuclear Power Institute of China) (1) has recently been introduced in the Iranian radiation oncology centers. The unit has six different cobalt-60 source braids being loaded in tandem and ovoid applicators. The sources are used in the treatment of cervix, vagina, rectum, esophagus, and a number of other cancers. The unit has a treatment planning system, which presents dose distributions for different insertions in terms of isodose curves (2). In dose delivery by brachytherapy systems, the delivered dose to the target volume and the dose received by the adjacent critical organs are calculated using dose distributions presented by treatment planning system (3). The treatment planning systems commonly calculate the dose distributions by assuming each source as a point source. The source capsule, inter-pellet attenuation, and scattering are not also incorporated in the algorithms used in the treatment planning system for dose calculations (4). Since the treatment planning systems use simplified algorithms in their dose calculations, verification of the dose distributions presented by the systems may be of clinical importance (5). Evaluation of the treatment planning systems in terms of their presented dose distribution can be a part of quality assurance in the clinical practice of brachytherapy (6). There are scarce studies conducted on GZP6 unit focusing on the calculation and measurement of task group number 43 (TG-43) (7) parameters for different sources of the GZP6 unit (8, 9). In our previous study, an evaluation verifying the dose distribution around the first source of GZP6 unit was performed (10). Naseri et al. have studied dose distribution around the source number one, two, and five of the unit but their study was for a vaginal applicator and was based on relative dose evaluations (2). To our knowledge there is no other study verifying dose distributions around source braids of this unit. In this study, the dose distributions presented by GZP6 treatment planning system for the source braids No. 2 and No. 5 were evaluated based on Monte Carlo simulations and inphantom radiochromic film measurements.