Introduction of inverse dose optimization for ultrasound-based high-dose-rate boost brachytherapy: how we do it in
Frank-André Siebert1, Sabine Wolf1, Hagen Bertermann2
1University Hospital of Schleswig-Holstein, Campus Kiel, Clinic of Radiotherapy, Kiel, Germany.
Objectives:
To describe the introduction of inverse planning optimization for a two clinical target volume (CTV) concept in the online planning technique of temporary high-dose-rate brachytherapy for prostate cancer.
Methods And Materials:
Dose-volume constraints were defined delivering a prescription dose of 8.5Gy for CTV1 (whole prostate) and 15Gy for CTV2 (peripheral zone). A total of 38 implants of 20 patients were inversely planned using the constraints and dose indices (D90 CTV1,2; V200 CTV1,2; D2 cc rectum; D0.1 cc urethra; dose nonhomogeneity ratio; and conformal index) compared against those derived from conventional planning (CP).
Results:
The inversely planned (IP) treatment plans showed similar target volume coverage than by CP. The value of D90 CTV1 for CP was 5.62Gy and 5.63Gy for IPs. For CTV2, the D90 was also similar between both methods: 11.03Gy and 10.89Gy, respectively. Only V200 CTV2 was found to be significantly lower for CP than for IP: 5.76% vs. 8.14% (p<0.01). Values for D0.1 cc urethra were found to be: 9.57Gy and 9.02Gy, respectively. Rectal dosimetry: D2 cc Rectum was quite stable with 6.04Gy and 6.12Gy for CP and IP, respectively. The conformal index and dose nonhomogeneity ratio values for CTV1 and CTV2 for both planning types were very similar.
Conclusions:
After defining an objective second target volume CTV2 and introducing adequate IP constraints to the treatment planning system, clinically applicable treatment plans could be created by an IP approach. They feature user independency, time saving, and good preservation of the OARs.
