The evolution of quality assurance for intensity- modulated radiation therapy (IMRT): sequential tomotherapy

To identify the pertinent issues to be addressed in successfully implementing IMRT using sequential tomotherapy into clinical reality and presenting the maturation of quality assurance (QA) programs for both the delivery system and patient treatments that allow routine clinical use of the system.

Initially, a cubic phantom containing silver halide film was exposed to the entire treatment before patient treatment. The processed films were digitized with a laser densitometer and the dose distributions were compared with that generated by the planning system. Later, software that calculates the dose delivered to any phantom employing the intensity patterns developed in the inverse planning system for an individual patient was implemented for point checks of dose. A measurement phantom for use with this software was developed and evaluated on a large number of patients. Invasive fixation was used for all cranial patients initially. To use sequential tomotherapy for other sites and larger targets, noninvasive immobilization systems using two types of thermoplastic masks for cranial targets and reusable, evacuated body cradles were evaluated for positional accuracy and suitability for use with port films for patient QA.

The program for equipment validation is divided into daily, weekly, and monthly programs that add only small amounts of time to routine QA programs. For the first 15 patients treated with this modality, the maximum dose measured on the film was within 5% of that predicted by the planning computer. The prescription isodose line was measured in the anteroposterior and lateral dimensions and the average discrepancy between measured and predicted was less than 2 mm. For an isodose line between 50% and 70% of the prescribed dose, the agreement was better than 3 mm. Success with the volume QA program was followed by a point check QA program that reduced the time required for individual patient QA from days to hours. Phantom measurements compared with computer predictions for 588 data points resulted in only 8% being outside a +/-5% criterion. These cases were identified and allow a further reduction in the frequency of tests. Thermoplastic mask materials have adequate restraint characteristics for use with the system and port films on 21 patients resulted in one standard deviation = 1.3 mm. Body cradles are less accurate and require more frequent port films. A QA system that reduces the frequency of port films was developed.

The evolution of sequential tomotherapy in our department has been from a maximum of 3 cranial patients per day with invasive fixation to 60 patients per day for treatment of cranial, head-and-neck, and prostate tumors using different immobilization techniques. With proper preparation and refinement of tools used in commissioning and validation, sequential tomotherapy IMRT can become a routine clinical treatment modality.