The impact of daily setup variations on head-and-neck intensity-modulated radiation therapy

Intensity-modulated radiation therapy (IMRT) in the treatment of head-and-neck (H&N) cancer provides the opportunity to diminish normal tissue toxicity profiles and thereby enhance patient quality of life. However, highly conformal treatment techniques commonly establish steep dose gradients between tumor and avoidance structures. Daily setup variations can therefore significantly compromise the ultimate precision of idealized H&N IMRT delivery. This study provides a detailed analysis regarding the potential impact of daily setup variations on the overall integrity of H&N IMRT.

A series of 10 patients with advanced H&N cancer were prospectively enrolled in a clinical trial to examine daily H&N radiation setup accuracy. These patients were treated with conventional shrinking field design using three-dimensional treatment planning techniques (not IMRT). Immobilization and alignment were performed using modern H&N practice techniques including conventional thermoplastic masking, baseplate fixation to the treatment couch, three-point laser alignment, and weekly portal film evaluation. After traditional laser alignment, setup accuracy was assessed daily for each patient by measuring 3 Cartesian and 3 angular deviations from the specified isocenter using a high-precision, optically guided patient localization system, which affords submillimeter setup accuracy. These positional errors were then applied to a distinct series of 10 H&N IMRT plans for detailed analysis regarding the impact of daily setup variation (without optical guidance) on the ultimate integrity of IMRT plans over a 30-day treatment course. Dose-volume histogram (DVH), equivalent uniform dose (EUD), mean total dose (mTd), and maximal total dose (MTD) for normal structures were analyzed for IMRT plans with and without incorporation of daily setup variation.

Using conventional H&N masking and laser alignment for daily positioning, the mean setup error in any single dimension averaged 3.33 mm. However, when all six degrees of freedom were accounted for, using the optically guided patient localization system, the mean composite vector offset was 6.97 mm with a standard deviation of 3.63 mm. Superimposition of mean offset vectors on idealized H&N IMRT treatment plans enabled evaluation of resultant shifts in DVH, EUD, mTd, and MTD calculations. Partial geographic tumor miss (GTV underdosing) and normal tissue overdosing was common when these mean positional offsets were incorporated. The decrease in EUD for defined tumor volumes ranged up to 21% when the largest offset histories were applied, and 3-14% for plans when the least and median offset histories were applied.

The successful implementation of H&N IMRT requires accurate and reproducible treatment delivery over a 6- to 7-week treatment course. The adverse impact of daily setup variation, which occurs routinely with conventional H&N masking techniques, may be considerably greater than recognized. Isocenter verification checks on two-dimensional orthogonal films may not sufficiently alert the clinician to the magnitude of three-dimensional offset vectors and the resultant impact on the quality of overall IMRT delivery. Unrecognized geographic miss and resultant target underdose may occur. Similarly, selected normal structures such as parotid glands may receive higher doses than intended. The results of this study suggest that more rigorous immobilization techniques than conventional masking and routine patient setup tracking methodologies are important for the accurate monitoring and successful delivery of high-quality IMRT for H&N cancer.