Dosimetric Analysis of Catheter Displacement in High Dose Rate Prostate Brachytherapy
S Park*, M Kamrava, R Banerjee, D Demanes, UCLA School of Medicine, Los Angeles, CASU-E-T-343 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: To evaluate the dosimetric effect of catheter displacement on target coverage and doses to organs at risks (OARs) in high dose rate prostate brachytherapy.
Methods: CT simulations from 21 prostate cancer patients treated with HDR monotherapy (7.25 Gy x 6 fractions) were used. The prostate CTV and OARs were contoured on the 3D-CT. Treatment plans were optimized using an inverse planning simulated annealing algorithm and graphical optimization to ensure dosimetry objectives: target coverage (CTV D90=100-115%, V100 >97%, and V150 <35%) and OAR dose constraints (D0.1cc <85% (rectum), 80-95% (bladder), and <110% (urethra)). Craniocaudal catheter movements from 1 to 10 mm in 1 mm increments were simulated by simultaneously shifting all active dwell positions in the initial treatment plan using catheter offset in the Oncentra MasterPlan (Nucletron). Target coverage (D90 and V100) and OAR doses (D0.1cc and D1cc) were evaluated and compared to no displacement plans.
Results: The mean prostate CTV volume was 74.7±22.8 cc (range: 24.8-113.7 cc). Initial treatment plans provided target D90 = 107.7±2.0% (103.0-111.2%) and V100 = 98.5±0.8% (97.1-99.5%). The D0.1cc to OARs was 77.3±2.3% (rectum), 84.2±2.0% (bladder), and 106.8±1.6% (urethra). The mean target coverage was not significantly different for 1-2 mm shifts. Shifts between 3 and 6 mm still met most dosimetry objectives, but provided progressively less good target coverage and doses to OARs. Displacements of greater than or equal to 7 mm resulted in dosimetry that did not meet our dosimetry objectives.
Conclusion: Small changes in catheter position result in significant alterations in dosimetry and treatment delivery. The target coverage (D90 and V100) and bladder dose are reduced and the rectal and urethra doses are higher in direct proportion to the degree of displacement. It is essential to check and correct interstitial catheter positions before each HDR fraction to accurately deliver the planned radiation dose.