Target localization and treatment verification for intensity modulated conformal radiation therapy of the head and neck region. The AZ-VUB experience. Akademisch Ziekenhuis-Vrije Universiteit Brussel.
OBJECTIVE
Based on a previous evaluation of a conventional, non-invasive fixation technique in combination with a commercially available system for conformal radiation therapy by means of intensity modulation of the treatment beam some modifications have been proposed with respect to 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
A slice-by-slice arc-rotation approach was used to deliver a conformal dose to the target and patient fixation was performed by means of a thermoplastic cast. In a previous study a procedure for target localization and verification of patient positioning suitable for this particular treatment technique had been developed and verified dosimetrically with an anthropomorphic phantom: both absolute dose measurements (alanine and thermoluminescent detectors) and relative dose distribution measurements (film dosimetry) have been applied. Two issues needed special attention, being: a) increased accuracy in patient set-up and b) deviations between measured and predicted dose values to organs at risk (OAR) in extreme situations. In a 2nd phase, fiducial markers fixed to customized ear moulds and a bite block have been introduced for target localization and verification of patient set-up on a day-to-day basis. A dosimetric verification by means of TLD and an anthropomorphic phantom has been performed to investigate the possible influence of leakage dose.


RESULTS
The dose verification with the anthropomorphic phantom yielded a ratio between measured and predicted dose values of 1.0 for typical cases. However, large deviations (i.e. measured/calculated = 2.9) have been observed in cases with extreme dose constraints to the OAR due to a leakage dose of approximately 0.5%. Dosimetric verification showed good agreement with the dose calculation once the limitations of the system (10% of the target dose to the OAR) had been taken into consideration. Day-to-day variations in patient set-up of 0.3 cm (translations) and 2.0 degrees (rotations) were considered acceptable for this particular patient population in an initial phase, whereas variations of less than 0.1 cm can be achieved with the introduction of ear moulds and daily monitoring in a second phase. Preliminary results show a reproducibility of 0.08 cm in positioning of the fiducial landmarks with respect to anatomical landmarks.


CONCLUSIONS
The non-invasive fixation technique in combination with fiducial markers fixed to individualized ear moulds 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 degree is required. A clinical trial has been initiated recently to generate substantial data on the accuracy of the presented procedure for treatment set-up. Dose measurements confirmed the predicted dose values to the target and OAR, however, a physical limitation is represented by the 0.5% leakage through the leaves in that dose constraints to the OAR should be limited to 10% of the target dose.
Verellen
D
D
Department of Radiotherapy, Academic Hospital, Free University Brussels (AZ-VUB), Belgium. conrvnd@az.vub.ac.bc
Linthout
N
N
Storme
G
G
D000465:Algorithms; D001530:Belgium; D006258:Head and Neck Neoplasms / Q000532:radiotherapy*; D006785:Hospitals, University; D006801:Humans; D019047:Phantoms, Imaging; D011187:Posture; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000295:instrumentation / Q000379:methods* / Q000706:statistics & numerical data; D011882:Radiotherapy, High-Energy / Q000295:instrumentation / Q000379:methods* / Q000706:statistics & numerical data