Permanent prostate implants: radiobiology & dosimetry

Abstract

Localised prostate tumours can be treated with low dose rate brachytherapy. Radioactive sources (Iodine-125) are brought into the prostate. Postimplant dose calculations frequently show that the dose delivered to the prostate is lower than the prescribed dose. This problem is investigated by evaluation of the dose distribution of 130 patients four weeks after the implantation of the Iodine seeds (chapter 6). The research was focussed on two implantation techniques and two techniques for imaging of the prostate (magnetic resonance imaging and ultrasound). One implantation technique is based on the seedSelectron (afterloader) for automatically delivering of the loose seeds and the other implantation technique is based on manually insertion of RAPID Strands (chapter 5).

A biological model is described for the biological effective dose (BED) as a function of biological and physical parameters (e.g. half-life of sources, edema of the prostate, repair of (sub)lethal damage). With this model, the optimal time for postplanning is derived and the influence of the biological and physical parameters on the BED is investigated (chapter 3). This biological model is also used to determine an α/β value for prostate tumours (chapter 4).

Besides good dose coverage of the prostate with the prescribed dose, it is important to investigate changes in the quality of life (QoL) of patients after the treatment. This is measured with QoL questionnaires filled in by the patients at four time points (one before and three after the implantation of the seeds). The results of the questionnaires are correlated with the dose to the prostate, rectum, and urethra (chapter 7).

Finally, a planning study is done to investigate the feasibility of a new implantation technique based on a robotic implant device, which can be used within the limited space inside a closed MR scanner (chapter 8).