Purpose/Objective: A critical quality of life concern for prostate cancer patients is preservation of sexual function. Most correlative and mechanistic studies support a vascular mechanism for erectile dysfunction following radiotherapy. Non-radiation literature indicates that occlusive disease of the internal pudendal artery is the most common cause of erectile dysfunction. This is a study to compare forward and inverse planning strategies to preserve erectile function by sparing the internal pudendal artery and corpus cavernosum, the critical vascular structures responsible for erection. Materials/Methods: Ten patients underwent axial, sagittal, and coronal T2MRI to define the prostate for external beam treatment. Time of flight MRI angiography (<10 minutes, noncontrast) was employed to define the internal pudendal arteries. MRI datasets were registered to CT by Mutual Information. Two plans were devised for each patient: the first plan was optimized by the operator based on forward planning techniques and a second plan was designed based on inverse planning techniques using the University of Michigan treatment planning system. The forward plan was designed to minimize the dose to the internal pudendal artery (IPA) and corpus cavernosum (CC) while delivering a total dose of 75.8 Gy to the PTV. 1.8 Gy/fx to 55.8 Gy was planned to the prostate with a 1 cm expansion and a boost dose of 2 Gy/fx to 20 Gy was planned to the prostate with a 0.5 cm expansion. Three to five fields with 2 segments per field were used to deliver the forward optimized plans. Inverse plans were developed with 7 fields and 1 cm beamlets to deliver the same dose to the prostate and expansions by assigning a cost to structures outside the PTV. Dose to 50% (D50) and 5% (D5) to the vascular structures, rectum and bladder were compared. Results: Normal tissue dose was significantly lower using inverse planning techniques than forward planning. The average D5 and D50 to the left and right IPA and CC for the forward and inverse plans are shown in the table. 1. The average max dose to the right and left pudendal arteries were significantly lower (32 % (p < 0.003) and 44% (p<0.001), respectively) with inverse planned fields than with forward planned techniques. 2. The average max dose to the right and left corpus cavernosum were significantly lower (p < 0.002 and 0.001, respectively) with inverse planned fields than with forward planned techniques. 3. The average D5 to the rectum was significantly lower with inverse planned fields than with forward planned techniques. (p < 0.01) 4. The average D5 to the bladder was significantly lower with inverse planned fields than with forward planned techniques. (p < 0.01) Conclusions: The internal pudendal artery courses lateral and inferior to the prostate, directly following the curvature of the prostate. Forward planning may improve internal pudendal artery sparing over standard beam arrangements, but IMRT provides the optimal solution for a structure which curves around another. Correlation of dose to the IPA/CC and potency will allow direct testing of the vascular hypothesis.
Narayana, V,
Comparison of Forward and Inverse Planning Strategies to Preserve Erectile Function by Sparing Critical Vascular Structures Near the Prostate. Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL.
http://archive.rsna.org/2004/4417951.html
