Assessment of Various Thermoplastic Masks Used for Radiation Therapy Immobilization in Terms of Shrinkage and Pressure

Abstract

The aims of this study are first, to assess and compare the amount of deformation for seven thermoplastic masks from three different manufacturers [Aquaplast RT-1889YS (Aqua variable perf), Fiberplast RT-1889KYS (Fiber variable perf), Embrace RT-8320 (Embrace), AccuPerf IMRT head mask RG461-4SHT (AccuPerf), Fiberplast RT-1889KS (Fiberplast), Aquaplast RT-1989S (Aqua thick), and Aquaplast RT-1889S( Aquaplast)] using time lapse photography, and second, to measure the amount of pressure exerted by each mask on a rigid head phantom using pressure sensitive film. For the first aim, all masks were marked with fiducial points located near forehead, nose, and chin. The masks were fabricated on a rigid head phantom following manufacturer instructions. Each mask was mounted on a plastic mounting tray, and photographed in a sagittal view over the span of one week. The fiducial coordinates were determined by a center of mass algorithm. A scale factor was determined by photographing a graph sheet mounted in place of the mask. For the second aim, the pressure exerted by each mask was evaluated by mounting the mask on the head phantom after sandwiching a strip of pressure sensitive film between the two at three anatomical locations: the forehead, nose, and chin. Measurement of translation in the posterior direction did not give reproducible results as direct caliper measurements did not agree with those from the time-lapse photography technique. In the superior/inferior direction, the Embrace mask showed the greatest deformation at forehead and nose areas (1.069±0.017mm (inferiorly), and 0.432±0.010mm (inferiorly) respectively), while the Aqua thick mask showed the greatest deformation at chin area (0.970±0.016mm (superiorly)). The averaged median pressure of the masks at the forehead location were not significantly different except for the Fiber variable perf which showed the greatest averaged median pressure ( 0.118 MPa). At the nose location, no mask showed a significantly different averaged median pressure. At the chin location, the Aqua thick mask showed a significantly higher averaged median pressure compared to the others (0.180 MPa).