Setup verification and in vivo dosimetry during intraoperative radiation therapy (IORT) for prostate cancer.

The purpose of this study was to check the setup and dose delivered to the patients during intraoperative electron beam radiation therapy (IORT) for prostate cancer. Twenty eight patients underwent IORT after radical prostatectomy for prostate cancer by means of a dedicated mobile accelerator, Novac7 (by Hitesys, SpA, Italy). A 9 MeV electron beam at high dose per pulse was used. Eighteen patients received IORT at escalating doses of 16, 18, and 20 Gy at 85% isodose, six patients for each dose level. Further, ten patients received 20 Gy at 85% isodose. The electron applicator position was checked in all cases by means of two orthogonal images obtained with brilliance intensifier. Target and organ at risk doses were measured in vivo by a MOSFETs dosimetry system. MOSFETs and microMOSFET dosimeters were inserted into sterile catheters and directly positioned into the rectal lumen, for ten patients, and into the bladder to urethra anastomosis, in the last 14 cases. Verification at 0 degree led to very few adjustments of setup while verifications at 90 degrees often suggested to bring the applicator closer to the target. In vivo dosimetry showed an absorbed dose into the rectum wall < or =1% of the total dose. The average dose value inside the anastomosis, for the 12 patients analyzed, was 23.7 Gy with a standard deviation of +/-7.6%, when the prescription was 20 Gy at 85% isodose. Using a C-arm mobile image intensifier, it is possible to assess if the positioning is correct and safe. Radio-opaque clips and liquid were necessary to obtain good visible images. In vivo MOSFETs dosimetry is feasible and reliable. A satisfactory agreement between measured and expected doses was found.