TARGET LOCALIZATION AND TREATMENT VERIFICATION FOR INTENSITY-MODULATED CONFORMAL RADIATION-THERAPY OF THE HEAD AND NECK REGION - THE AZ-VUBEXPERIENCE

Purpose: Based on a previous evaluation of a conventional, non-invasiv e fixation technique in combination with a commercially available syst em for conformal radiation therapy by means of intensity modulation of the treatment beam some modifications have been proposed with respect Co target localization and treatment verification. This paper reports on an on-going study of which some preliminary results are presented and discussed with respect to reproducibility of the treatment set-up and verification of the dose. Methods and Materials: A slice-by-slice are-rotation approach was used to deliver a conformal dose to the targ et and patient fixation was performed by means of a thermoplastic cast . In a previous study a procedure for target localization and verifica tion of patient positioning suitable for this particular treatment tec hnique had been developed and verified dosimetrically with an anthropo morphic phantom: both. absolute dose measurements (alanine and thermol uminescent detectors) and relative dose distribution measurements (fil m dosimetry) have been applied. Two issues needed special attention, b eing: a) increased accuracy in patient set-up and b) deviations betwee n measured and predicted dose values to organs at risk (OAR) in extrem e situations. In a 2nd phase, fiducial markers fixed to customized ear moulds and a bite block have been introduced for target localization a nd verification of patient set-up on a day-to-day basis. A dosimetric verification by means of TLD and an anthropomorphic phantom has been p erformed to investigate the possible influence of leakage dose. Result s: The dose verification with the anthropomorphic phantom yielded a ra tio between measured and predicted dose values of 1.0 for typical case s. However, large deviations (i. e. measured/calculated = 2.9) have be en observed in cases with extreme dose constraints to the OAR due to a leakage dose of approximately 0.5%. Dosimetric verification showed go od agreement with the dose calculation once the limitations of the sys tem (10% of the target dose to the OAR) had been taken into considerat ion. Day-to-day variations in patient set-up of 0.3 cm (translations) and 2.0 degrees (rotations) were considered acceptable for this partic ular patient population in an initial phase, whereas variations of les s than 0.1 cm can be achieved with the introduction of earmoulds and d aily monitoring in a second phase. Preliminary results show a reproduc ibility of 0.08 cm in positioning of the fiducial landmarks with respe ct to anatomical landmarks. Conclusions: The non-invasive fixation tec hnique in combination with fiducial markers fixed to individualized ea rmoulds and a bite block proved to be acceptable for IMRT of the head and neck region and allow for daily monitoring of patient set-up. The latter becomes mandatory if an accuracy of 0.1 cm and 1.0 degrees is r equired. A clinical trial has been initiated recently to generate subs tantial data on the accuracy of tie presented procedure for treatment set-up. Dose measurements confirmed the predicted dose values to the t arget and OAR, however, a physical limitation is represented by the 0. 5% leakage through the leaves in that dose constraints to the OAR shou ld be limited to 10% of the target dose.