Urethra low-dose tunnels: validation of and class solution for generating urethra-sparing dose plans using inverse planning simulated annealing for prostate high-dose-rate brachytherapy.

PURPOSE: Urethral dose is related to severity of genitourinary toxicity in patients treated with brachytherapy for prostate cancer. This work describes a dose planning method that uses inverse planning to create a low-dose tunnel around the urethra and presents a class solution to achieve this additional dose sparing of the urethra.

METHODS: Fifteen patients on the Radiation Therapy Oncology Group (RTOG) 0321 protocol were treated for prostate cancer with a high-dose-rate brachytherapy dose boost to an external beam radiation treatment regimen. All were treated with 9.5Gy for each of the two fractions after 45Gy of the external beam radiation. The inverse-planning algorithm, inverse planning simulated annealing (IPSA), was used to create both the standard RTOG protocol (SRP) plan for treatment and the a posteriori urethra dose sparing (UDS) plan consisting of a dose tunnel along the urethra. Both plans maintained the protocol parameters: prostate V(100) (volume receiving 100% of prescribed dose)>90% and bladder and rectum V(75)<1 cm(3). In the SRP plans, the urethra surface was optimized to receive <125% of the prescription dose and in the UDS plans <100%. Dose-volume histograms for the clinical treatment volume, bladder, rectum, penile bulb, and urethra for both plans are compared using a paired sample t test with significance claimed for probability values<0.05.

RESULTS: UDS planning reduced the urethra V(100) from 88% to 58% on average (p<0.01) and the V(125) from 3.3% to 0.2% (p < 0.01). Bladder and rectum V(75) were maintained at <1 cm(3) and not significantly different between plans. Prostate coverage was maintained per protocol at V(100)>90%, with mean for the SRP V(100)=93% versus UDS plan V(100)=90%. Prostate D(90) for SRP was 104% versus UDS plan D(90)=101%. For all patients, the UDS achieved a dose tunnel surrounding the length of the intraprostatic urethra. The class solution for generating UDS is presented.

CONCLUSIONS: A urethral sparing-focused planning solution using IPSA reduces mean urethral dose by 34%, as compared with IPSA-generated plans based on the RTOG 0321 protocol. This is done while maintaining prostate coverage and critical structure dose. This technique can be applied to all patients in whom urethra toxicity is of particular concern.