Background

In HDR brachytherapy precision of catheter implantation is crucial for conformal treatment planning as a starting point for better optimization process.

Aim

The aim was to investigate differences between virtual and real needle position and the effect of needle displacement on dosimetric parameters as a function of prostate volume for better evaluation of “real” implant with respect to final dose distribution.

Materials/Methods

Thirty treatment plans calculated by Nucletron SWIFT™ were randomly selected. Dosimetric data including V100 for prostate gland and D10 for urethra were analyzed as a function of prostate volume and needle displacement.

Needle displacement was determined by measuring the distance between virtual and real positions of respective needles in three sectional images: at the base, apex of the prostate gland and reference image. Dosimetric parameters were determined for consecutive computer plans: virtual (before implantation), live (after implantation and renewed optimization). For the purpose of this study a new parameter, VD (Volume-Dose), was created.

Results

VD indicates the quality of “real” dose distribution with respect to “virtual” treatment plan. In order to realize the assumption of virtual plan (VD<1) for a given prostate volume, mean values of needle displacement r should not exceed r_acceptable according to the formula: r_acceptable (V_p) ∝ V_p². For larger glands (above 30cc) final dose distribution is less dependent on needle displacement than smaller ones.

Conclusions

The experiment determined maximum values of needle displacement for a given Vp parameter, allowing one to take advantage of optimization algorithms and to improve the final dose distribution.