Measurement of shrinking immobilizing sheets for radiotherapy using a near-infrared camera

We aimed to develop a technique to precisely measure the shrinkage of immobilization sheets (ISs) using a three-dimensional (3D) tracking of multiple points on IS using a near-infrared camera. A thermoplastic sheet and an elastomer sheet were used in this study. The inter-marker distance (IMD) of neighboring marker pairs and the triangular area (TA) formed by neighboring three markers were analyzed as a function of time since molding each IS. Thermal distance ratio (TDR), IMD normalized to IMD after 48 h, and thermal area ratio (TAR), TA normalized to TA after 48 h, were analyzed using an exponential function. The 3D visualization of the initial shrinkage amplitude (ISA) was created for each IS. The mean ISA and the time constant (TC) in the exponential function (ISA, TC) for horizontal and vertical pairs were (0.34 $\pm$ 0.06, 3.9 $\pm$ 0.9) and (0.60 $\pm$ 0.05, 7.6 $\pm$ 0.9) for HFT and (0.13 $\pm$ 0.02, 25.0 $\pm$ 4.7) and (0.06 $\pm$ 0.03, 4.7 $\pm$ 3.5) for SF, respectively. The mean (ISA, TC) for HFT and SF were (0.76 $\pm$ 0.07, 7.1 $\pm$ 0.9) and (0.22 $\pm$ 0.03, 17.7 $\pm$ 4.2), respectively. Horizontal pairs showed smaller ISA and shorter TC than vertical pairs for HFT, while horizontal pairs showed larger ISA and longer TC than vertical pairs for SF, possibly due to different chemical characteristics of each material under the effect of mechanical force. The mean TDR and TAR are considered useful for evaluating the gross property of IS. The visualized distributions of ISA are considered useful to provide spatial information for investigating relationships between actual handlings and shrinkage of IS.