ELSEVIER Medical Dosimetry. Vol. 22. No. 4. pp. 293-297, 1997 Copyright 0 1997 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 09%3947/97 $17.00 + .oo PI1 SO958-3947(97)00102-7 DESIGN AND ANALYSIS OF AN IMMOBILIZATION AND REPOSITIONING SYSTEM FOR TREATMENT OF NECK MALIGNANCIES ROBIN MARSH, C.M.D., JAMES BALTER, PH.D., VICKI L. EVANS, R.T.T., and AVRAHAM EISBRUCH, M.D. University of Michigan Hospitals, Department of Radiation Oncology, 1500 E. Medical Center Drive, Ann Arbor, MI 48109 Abstract-Recent applications of three-dimensional treatment planning using non-coplanar treatment angles have demonstrated the potential for improved target dose homogeneity as well as normal tissue sparing for tissues in the neck, notably the parotid gland. Implementation of these highly targeted treatments requires a reliable method for accurate daily reproduction of the treatment position, as patient setup error could lead to significant decreases in target dose and normal tissue sparing. Additionally, the constrained geometry of this anatomic site requires the freedom to deliver treatments from any arbitrary angle in order to maximize the expected benefit. In order to permit accurate setup while maintaining access from most angles, a hybrid immobilization system has been developed, consisting of a custom thermoplastic mask with attachment to a foam cradle shaped to the back of the patient. To evaluate the accuracy of this system, setup errors were measured in 20 patients treated while immobilized with the positioning aide. Two orthogonal film sets consisting of anterior and lateral projections, one set taken at the beginning of treatment and a second 4-5 weeks into therapy, were compared to baseline simulator films or digitally reconstructed radiographs. The average setup deviation in any direction ranged from 0.8 to 1.4 mm and the largest single setup error observed was 4.5 mm. For the film sets taken late in treatment only 3 of the 20 patients required setup adjustment followed by repeat filming to obtain an acceptable tilm pair. This system has been implemented for routine clinical use since March 1995. 0 1997 American Association of Medical Dosimetrists. Key Words: Head and neck, Immobilization, 3D treatment planning. INTRODUCTION Proper clinical implementation of three-dimensional (3D) treatment planning (TP) techniques requires in- creased precision in patient setup compared to more conventional treatment approaches.‘m3 As a result, the use of reliable immobilization and repositioning aides becomes more critical. This is of particular concern in managementof malignancies of the head and neck due to the close proximity of tumor and nodal tissues to normal structures. Historically, the radiation therapist depended on immobilization methodsthat ranged from tape and bite blocks to therm0 plastic masks secured by a rigid frame.l.5 When used with standard beam arrangements which consist primarily of opposed lat- eral portals, these devices have performed adequate- 1~;~ however, with the implementation of 3D tech- niques, these same devices may no longer afford the therapist the accuracy necessary for precise treatment delivery.7 Many of these devices, while providing excellent cranial immobilization, do little or nothing to immobilize or reproduce the position of the neck and shoulders. Reprint requests to: Robin Marsh, University of Michigan Hos- pitals. Department of Radiation Oncology, 1500 E. Medical Center Drive. Ann Arbor. MI 48 109. The use of 3D TP may produce some number of beams directed posteriorly (under the treatment ta- ble).* In this case the dense and varied materials that form the rigid frame used to secure a face mask can cause significant attenuation of the beam. This atten- uation can be complicated to model and difficult to compensate for. The materials of these devices may cause significant artifacts in the TP Computerized Tomography (CT) scans.5 The need for a head and neck immobilization device (HNIM) capable of providing the necessary reliable immobilization and repositioning, and allow- ing flexibility in beam arrangement becomesapparent. Additionally, due to the current trend of cost reduction in health care, this must be done in a cost effective manner. In an effort to achieve this repositioning accuracy and relaxed constraints on beam direction, an immobilization device specifically for 3D treatment of the head and neck has been constructed and imple- mented on a routine clinical basis. This device incor- porates the best qualities of two common positioning aides. The first is a foam cradle which provides im- mobilization of the neck and shoulders. The second is a thermoplastic face mask which secures the head position.“.x ‘93