Daily CT planning during boost irradiation of prostate cancer - Feasibility and time requirements

Background: In the irradiation of prostate cancer internal organ movement l eads to uncertainties in the daily localization of the clinical target volu me. Therefore more or less large safety margins are added when designing th e treatment portals. With daily CT planning internal organ movement can be compensated to some extent, safety margins can be reduced and irradiated no rmal tissue can be spared. The feasibility of daily CT-based 3D treatment p lanning is studied in a patient with localized prostate carcinoma using a n ew patient positioning system. Methods: Daily CT planning was applied during boost irradiation of a patien t with prostate cancer: After patient immobilization the pelvis was scanned in 3 mm CT slices. Planning was done with the BrainSCAN planning system fo r stereotactic body irradiation. The prostate was contoured in all slices a nd the safety margins of the micromultileafs were automatically set to the distance chosen by the physician (0.8 cm). Patient positioning was done wit h the BrainLAB ExacTrac positioning system on the basis of skin attached st ereotactic body markers. Before each treatment verification images of the i socenter were taken. Results: The total time requirement for planning and irradiation was about 1 hour 15 minutes. Patient positioning on the treatment couch took about 10 minutes. The accuracy of the positioning system was good (75% of the devia tions were smaller than 3 mm). The shift of the single markers from CT scan to CT scan was more extensive than those of the center of all 7 markers co mbined (47% of the deviations were smaller than 3 mm). The location of the markers stems to influence the magnitude of their dislocation. Conclusion: Daily CT planning is feasible but time consuming. The new patie nt positioning system ExacTrac is an interesting tool especially for daily CT planning since conventional simulation can be omitted.