ESTRO 36 Abstract Book

S513 ESTRO 36 _______________________________________________________________________________________________

comparison, a model of the total accumulated dose to the target was calculated by summing the dose contributions from each time point. This was accomplished by deformably registering each post-implant CT scan and associated dose to the day 0 CT scan, scaling the dose contribution according to the seed activity at the time of the scan. A dose evaluation volume (DEV) was defined on all scans as a 5 mm isotropic expansion of the CTV trimmed to skin and chest wall muscle. Dosimetric indices for the CTV (V100) and DEV (V90, V100, and V200) were compared between each individual postplan and the accumulated dose using either a paired t-test or a Wilcoxon signed rank test, whichever carried more power given the distribution of the data. Residuals were also calculated, defined as the difference in dosimetric indices for a given time point and the accumulated dose model. As either a positive or a negative residual represents a deviation from the model, the median of the errors (where each error is the absolute value of the residual) was also calculated for each time point. Results The residuals for the DEV V100 and V200 for all 10 patients at each time point are shown in Figures 1 and 2, respectively. A statistically significant difference was observed between the day 60 scan and the accumulated dose for the DEV V90, V100, and V200 (paired t-test); no other significant differences were found. The smallest median (range) error occurred for the day 15 CT scan (as demonstrated in Figures 1 and 2); 2.4% (0.2-7.3%) and 4.5% (0.6-15.9%) for the DEV V100 and V200, respectively. Conclusion The results of this study indicate that the day 15 scan is the most representative of the accumulated dose delivered to target volumes in PBSI. For a 10-patient cohort, the median error was found to be at a minimum for the DEV V100 and V200 for the day 15 time point when compared to the day 0, 30, and 60 scans.

Poster: Brachytherapy: Prostate

PO-0926 Interstitial HDR prostate brachytherapy: comparison of pre- and post-implant dose distribution. S. Novikov 1 , S. Kanaev 1 , N. Ilin 1 , R. Novikov 1 , M. Girshovich 1 1 Prof. N.N. Petrov Research Institute of Oncology, Radiation Oncology, St. Petersburg, Russian Federation Purpose or Objective Prospective planning of interstitial high dose rate brachytherapy (HDRBT) for prostate cancer permit high accuracy of dose delivery to the tumour and\or prostate with excellent sparing of normal organs. On line correction of post-implant changes of prostate and normal tissues volumes is the key factor of precious dose delivery. The aim of the study was to evaluate possible uncertainties in dose distribution in cases when brachytherapy procedure is based only on pre-implant planning with dose distribution after HDRB with post- implant correction of dose distributiion. Material and Methods in 70 primary patients with prostate cancer we analyzed dosimetric plans that were obtained during the first session of HDRBT. Pretreatment planning was performed according to standard procedure with calculation of the following dosimetric parameters: V100, D90 – for prostate, D2cc – for rectum and D10 – for urethra. According to standard HDRBT procedure after the end of needle insertion we performed final US 3D-scanning with post implant correction of prostate, urethra, bladder and rectal volumes and subsequent post-implant optimization of treatment plan. During the study we also performed fusion of pre-implant and post-implant images. Fusion was based on needle and base-plan topography. After that we calculated dose distribution according to the model when pre-implant plan was used in patients with post-implant prostate and normal organs volumes. Results Analysis of treatment plans with post-I mplantation correction of the contours demonstrated h igh precision and excellent dosimetric parameters: mean V100 - 94.1% (V100 more than 90% in 97.2% cases), mean D90 – 104.3% (D90 more than 100% in 95.7% observations). On the contrary, after fusion of non-corrected plans and post- implant volumes we mentioned high discrepancies between preplanned and real dose distribution: V100 was below 80% in 38.6% observations; D90 was below 80% in