QUALITY ASSURANCE OF SERIAL TOMOTHERAPY FOR HEAD AND NECK PATIENT TREATMENTS

Purpose: A commercial serial tomotherapy intensity-modulated radiation therapy (IMRT) treatment planning (Peacock, NOMOS Corp., Sewickley, P A) 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. Acce lerator and patient-specific quality assurance (QA) procedures have be en developed that address the localization, normalization, and deliver y of the IMRT dose distributions. Methods and Materials: The dose dist ribution delivered by serial tomotherapy is highly sensitive to the ac curacy of the longitudinal couch motion. There is also an unknown sens itivity of the dose distribution on the dynamic mutlileaf collimator a lignment. QA procedures were implemented that assess these geometric p arameters. Evaluations of patient positioning accuracy and stability w ere conducted by exposing portal films before (single exposure) and af ter (single or double exposure) treatments. The films were acquired wi th sequential exposures using the largest available fixed multileaf po rtal (3.36 x 20 cm(2)). Comparison was made against digitally reconstr ucted radiographs generated using independent software and appropriate beam geometries. The delivered dose was verified using homogeneous cu bic phantoms. Radiographic film was used to determine the localization accuracy of the delivered isodose distributions, and ionization chamb ers and thermoluminescent dosimetry (TLD) chips were used to verify ab solute dose at selected points. Ionization chamber measurements were c onfined 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. Results: The accelerator QA techniques provided a dequate monitoring of the geometric patient movement and dynamic multi leaf collimator alignment and positional stability. The absolute deliv ered dose as measured with the ionization chamber varied from 0.94 to 0.98. Based on these measurements, the delivered monitor units for bot h subsequent QA measurements and patient treatments were adjusted by t he ratio of measured to calculated dose. TLD measurements showed agree ment, on average, with the ionization chamber measurements. The distri bution of TLD measurements in the high-dose regions indicated that mea sured doses agreed within 4.2% standard deviation of the calculated do ses. In the low dose regions, the measured doses were on average 5% gr eater than the calculated doses, due to a lack of leakage dose in the dose calculation algorithm. Conclusions The QA system provided adequat e determination of the geometric and dosimetric quantities involved in the use of IMRT for the head and neck. Ionization chamber and TLD mea surements provided accurate determination of the absolute delivered do se throughout target volumes and critical structures, and radiographic film yielded precise dose distribution localization verification. Por tal film acquisition and subsequent portal film analysis using 3.36 x 20 cm(2) portals proved useful in the evaluation of patient immobiliza tion quality. Adequate bony landmarks were imaged when carefully selec ted portals were used. (C) 1998 Elsevier Science Inc.