In-vivo dosimetry for gynaecological brachytherapy: physical and clinical considerations.

INTRODUCTION

The study aimed to estimate the dosimetric uncertainty using diodes (PTW/Germany) for a high-dose rate Iridum-192 source under clinical conditions. Finally, the role of in-vivo dosimetry for cervix cancer patients was evaluated.

MATERIAL AND METHODS

First, diode calibration and factors influencing diode response were investigated and phantom studies compared doses measured and computed by the treatment planning system. Based on that, the uncertainty for diode measurements was estimated to be 7% (1 sigma). Secondly, 55 applications of patients with cervix carcinoma were evaluated. Doses in rectum and bladder were measured and compared to the computed doses and differences were calculated. If the differences exceeded 10% the corresponding shift in probe position was evaluated. Additionally, the in-vivo dosimetry data were compared to doses at the ICRU 38 [ICRU Report No. 38, dose and volume specification for reporting intracavitary therapy in gynaecology. In: Chassagne D, Dutreix A, Almond P, Burgers J, Busch M, Joslin C editors. International commissioning on radiation units and measurements. Bethesda: 1985.] reference points for rectum and bladder.

RESULTS

In patients, in-vivo dosimetry resulted in differences between calculated and measured doses ranging from -31 to+90% (mean 11%) for the rectum and from -27 to+26% (mean 4%) for the bladder. Shifts in probe position of 2.5mm for the rectal probe and 3.5mm for the bladder probe caused dose differences exceeding 10%. The dose at the ICRU rectum reference point was underestimated by the calculated doses at probe position ranging from -61 to 156% (mean 29%). The dose to the ICRU bladder reference point was underestimated by the calculated dose ranging from 12 to 162% (mean 58%).

CONCLUSION

The study shows that diode accuracy and reproducibility is sufficient for clinical applications. For accurate in-vivo dosimetry geometric conditions are of utmost importance. It is recommended that in-vivo dosimetry should be performed in addition to computation.