Quality assurance of serial tomotherapy for head and neck patient treatments

A commercial serial tomotherapy intensity-modulated radiation therapy (IMRT) treatment planning (Peacock, NOMOS Corp., Sewickley, PA) and delivery system is in clinical use. The dose distributions are highly conformal, with large dose gradients often surrounding critical structures, and require accurate localization and dose delivery. Accelerator and patient-specific quality assurance (QA) procedures have been developed that address the localization, normalization, and delivery of the IMRT dose distributions.

The dose distribution delivered by serial tomotherapy is highly sensitive to the accuracy of the longitudinal couch motion. There is also an unknown sensitivity of the dose distribution on the dynamic mutlileaf collimator alignment. QA procedures were implemented that assess these geometric parameters. Evaluations of patient positioning accuracy and stability were conducted by exposing portal films before (single exposure) and after (single or double exposure) treatments. The films were acquired with sequential exposures using the largest available fixed multileaf portal (3.36 x 20 cm2). Comparison was made against digitally reconstructed radiographs generated using independent software and appropriate beam geometries. The delivered dose was verified using homogeneous cubic phantoms. Radiographic film was used to determine the localization accuracy of the delivered isodose distributions, and ionization chambers and thermoluminescent dosimetry (TLD) chips were used to verify absolute dose at selected points. Ionization chamber measurements were confined to the target dose regions and TLD measurements were obtained throughout the irradiated volumes. Because many more TLD measurements were made, a statistical evaluation of the measured-to-calculated dose ratio was possible.

The accelerator QA techniques provided adequate monitoring of the geometric patient movement and dynamic multileaf collimator alignment and positional stability. The absolute delivered dose as measured with the ionization chamber varied from 0.94 to 0.98. Based on these measurements, the delivered monitor units for both subsequent QA measurements and patient treatments were adjusted by the ratio of measured to calculated dose. TLD measurements showed agreement, on average, with the ionization chamber measurements. The distribution of TLD measurements in the high-dose regions indicated that measured doses agreed within 4.2% standard deviation of the calculated doses. In the low-dose regions, the measured doses were on average 5% greater than the calculated doses, due to a lack of leakage dose in the dose calculation algorithm.

The QA system provided adequate determination of the geometric and dosimetric quantities involved in the use of IMRT for the head and neck. Ionization chamber and TLD measurements provided accurate determination of the absolute delivered dose throughout target volumes and critical structures, and radiographic film yielded precise dose distribution localization verification. Portal film acquisition and subsequent portal film analysis using 3.36 x 20 cm2 portals proved useful in the evaluation of patient immobilization quality. Adequate bony landmarks were imaged when carefully selected portals were used.