A non-invasive immobilization system and related quality assurance for dynamic intensity modulated radiation therapy of intracranial and head and neck disease

Purpose: To develop and implement a non-invasive immobilization system guid ed by a dedicated quality assurance (QA) program for dynamic intensity-modu lated radiotherapy (IMRT) of intracranial and head and neck disease, with I MRT delivered using the NOMOS Corporation's Peacock System and MIMiC collim ator. Methods and Materials: Thermoplastic face masks are combined with cradle-sh aped polyurethane foaming agents and a dedicated quality assurance program to create a customized headholder system (CHS). Plastic shrinkage was studi ed to understand its effect on immobilization. Fiducial points for computer ized tomography (CT) are obtained by placing multiple dabs of barium paste on mask surfaces at intersections of laser projections used for patient pos itioning. Fiducial lines are drawn on the cradle along laser projections al igned with nasal surfaces. Lateral CT topograms are annotated with a crossh air indicating the origin of the treatment planning and delivery coordinate system, and with lines delineating the projections of superior-inferior fi eld borders of the linear accelerator's secondary collimators, or with thos e of the fully open MIMiC. Port films exposed with and without the MIMIC ar e compared to annotated topograms to measure positional variance (PV) in su perior-inferior (SI), right-left (RL), and anterior posterior (AP) directio ns. MIMiC vane patterns superposed on port films are applied to verify plan ned patterns. A 12-patient study of PV was performed by analyzing positions of 10 anatomic points on repeat CT topograms, plotting histograms of PV, a nd determining average PV. Results and Discussion: A 1.5 +/- 0.3 mm SD shrinkage per 70 cm of thermopl astic was observed over 24 h. Average PV of 1.0 +/- 0.8, 1.2 +/- 1.1, and 1 .3 +/- 0.8 mm were measured in SI, AP, and RL directions, respectively. Lat eral port films exposed with and without the MIMiC showed PV of 0.2 +/- 1.3 and 0.8 +/- 2.2 mm in AP and SI directions. Vane patterns superimposed on port films consistently verified the planned patterns. Conclusion: The CHS provided adequately reproducible immobilization for dyn amic IMRT, and may be applicable to decrease PV for other cranial and head and neck external beam radiation therapy. (C) 1999 Elsevier Science Inc.