Purpose/Objective: A crucial factor for a successful treatment utilizing radiation therapy is the accurate positioning of the patient and the target volume to apply a sufficient dose to the clinical target volume (CTV), while sparing surrounding organs at risk (OARs). When utilizing advanced treatment techniques to improve treatment outcome, like Intensity Modulated Radiotherapy (IMRT), which uses highly conformal dose distributions according to anatomical sites, a precise irradiation of the CTV is even more important. This work was focusing on radiation therapy of prostate cancer - the most frequent cancer in men, where proximate OARs are the rectal wall and the bladder wall.
Considering that the daily position of the prostate is depending on the filling of the rectum and the bladder and intrafraction organ and patient motion occurs, the purpose of this work was to investigate the movement of the prostate for IMRT patients during a treatment fraction.
The shift of bony structures was measured additionally in order to determine patient motion during the delivery of a fraction. Thus, a differentiation between the prostate and the whole patient movement was included in the analysis. Margins accounting for setup uncertainties and intrafraction motion were calculated.
Material/Methods: Seventeen patients with prostate cancer, each with 3 implanted fiducial gold markers (1.2 mm diameter, 3 mm length) within the prostate, were immobilized in supine position utilizing a knee support. Among them twelve patients were receiving an IMRT treatment of the prostate and the pelvic lymph nodes and five patients a 4-field box treatment of the prostate. To spare the posterior rectal wall and to immobilize the prostate an endorectal balloon (filled with 40 ccm of air) was used on a daily basis. After the first treatment setup, based on skin marks, patients were imaged using the ExacTrac stereoscopic imaging system. If the marker displacement exceeded the tolerance of 3 mm in relation to the marker positions on the planning CT, patients were shifted accordingly and verification images were taken. Additionally, all patients were imaged after treatment. Patients treated with IMRT were also imaged during the treatment (approximately at halftime).
Marker/prostate and bone drifts were evaluated as a function of treatment time for 587 treatment fractions. Margins were calculated according to the van Herk recipe .