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Purpose

High-dose-rate brachytherapy (HDR-BT) as monotherapy is one of the most used hypofractionated treatments for low and intermediate risk prostate cancer (Pca). HDR-BT is usually delivered in several fractions given in a short time period. Reducing the number of fractions to a single fraction regimen will improve patient comfort during treatment, enhance treatment accuracy and save time, costs and human resources. Because of higher accuracy and limited risk of implant displacement in the single fraction regimen, a decrease in toxicity rates could be expected too. However, there is no clear evidence which dose level will result in the best outcome. Before embarking on a clinical study we investigated the dosimetric feasibility of 3 dose levels for single fraction HDR-BT.

Materials and Methods

CT scans of a representative group of 30 patients were selected from our prostate HDR monotherapy database. Patients were divided in groups based on prostate volume (less than 40cc, 40-70cc, more than 70cc) and the number of needles used (13-16 and 17-22). The existing needle geometry of each patient was used to generate new treatment plans for three single fraction schemes: 1x19.0Gy, 1x19.5Gy and 1x20.0Gy. All plans were optimized according to the objectives in Table 1. Ninety plans were generated. The planning target volume (PTV) was the prostate without margin. The coverage of the prostate was maximized considering the dose constraints for the organs at risk (OAR). The primary end point of the study is the feasibility of above mentioned target coverage and OAR constraints. The secondary end point is to investigate the restricting factors to reach a feasible plan stratified to prostate volume, OAR position and implant geometry.

Results

The average prostate V100% for the 19.0, 19.5 and 20.0Gy schemes was 96.6%, 95.3% and 93.0%, respectively, with 83%, 57% and 33% of plans meeting the objective for the V100%. The D90% of the prostate averaged 20.3 Gy, 20.3 Gy and 20.4 Gy, respectively. Only 4 plans failed the D90% objective. The 40-70cc group showed an average prostate V100% of 96.3%, while it was 2.1% and 2.7% lower in the less than 40cc and more than 70cc groups, respectively. The number of needles had no influence on prostate coverage and urethra constraints. However, the rectum D1cc and D2cc were 5.7% and 8.6% higher in the 17-22 needle group compared to the 13-16 needle group. Also, the bladder D1cc and D2cc were higher, i.e. 3.3% and 5.3% respectively. The number of needles posterior to the urethra showed no correlation with prostate coverage nor OAR dose. The average prostate V100% was lower in patients with a larger distance between the urethra and the posterior border of the prostate, i.e. 95.7% for a distance larger than 12.5 mm versus 97.4% for a distance of 12.5 mm or less. The distance between rectum and prostate had an effect on prostate coverage too, i.e. the prostate V100% was 97.5% in patients with a prostate to rectum distance of 2 mm or more compared to 95.7% in patients with a prostate to rectum distance less than 2 mm.

Conclusions

For the 3 dose levels of 19Gy, 19.5Gy and 20Gy, single fraction HDR brachytherapy as monotherapy in PCa patients is feasible using our current implant geometry. Considering the OAR constraints, an acceptable D90% was reached in 95.6% of plans. However, target coverage depends on prostate volume, implant geometry and OAR proximity.

11 Unisex White Classic Black White Men's 13 T Authentic Vans Women's True 5 amp;C Medium Table 1 Planning objectives in single fraction HDR brachytherapy
Prostate V100% = 95% Bladder D1cc < 16.0 Gy
11 Vans Authentic Men's True White Medium 13 White 5 Black T Unisex Classic amp;C Women's D90% = 100% D2cc <15.5 Gy
Rectum D1cc <15.5Gy Urethra D0,1cc <21.0 Gy
D2cc <14.5 Gy D10% < 205 Gy
13 Unisex Classic Men's White 5 Authentic True Medium White T 11 Women's Vans Black amp;C V100% 0 cc V120% 0 cc

 

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