The application of 3-D dosimetry in brachytherapy treatment.

In the UK, prostate cancer is the second most commonly occurring cancer in men. The number of prostate cancer incidents is expected to increase due to improved screening and diagnostic techniques, whilst the mortality rate continues to fall as the variety of available treatments increases. This research focuses on the commonly used brachytherapy seed OncoSeed(TM) Model 6711 manufactured by GE Healthcare. Iodine-125 seeds are surgically implanted and left inside the prostate permanently to obtain local control of the malignant prostate cancer. Prior to the surgical procedure, a treatment plan is created on a computer planning system. There is currently no 3-D method to verify whether the planning system model matches the actual delivery of radiation dose to the prostate. This study is presented as a typical case study of what can be achieved using solid polymer dosimetry. Experimental results are presented comparing solid polymer dosimetry using Optical-CT with radiochromic films (RCF) and Monte Carlo simulations. Initial Optical-CT results have shown good agreement to Monte Carlo, radiochromic films and comparable to previously published data. The advantage of using Optical-CT includes the ability to view and extract dose profiles of any image slices within the 3-D dataset at high resolution. Combining the Optical-CT system with the new solid polymer dosimeter (PRESAGE(TM)) enabled the 3-D dosimetry of low dose-rate brachytherapy sources that was not possible with previous experimental methods of dosimetry. Other brachytherapy sources were investigated including the successive EchoSeed(TM) Model 6733 and the microSelectron-HDR Ir-192 source manufactured by Nucletron. The outcome to this work suggests the use of solid polymer dosimetry using Optical- CT is particularly useful in true 3-D experimental verification of the dosimetric characteristics of the brachytherapy seeds and other HDR sources. Future success will also rely on the further optimisation of the quality, consistency and sensitivity of the solid polymer dosimeter