Evaluation of intra-fraction patient movement for the head & neck IMRT

Abstract

Purpose/Objective: Patients often have more chances to move during IMRT delivery compared to conventional radiation therapy primarily due to longer treatment time. It is impossible to totally eliminate patient movement. Therefore, it is important to quantify patient movement for each specific clinical situation and appropriately account for it in planning margin. The goal of this study is to quantify the intra-fraction movement of patients during the head and neck IMRT. We have developed a novel concept of cumulative time versus patient positioning uncertainty histogram (TUH) that has allowed us to establish adequate margins for intra-fraction patient movement. Materials/Methods: The movement of head & neck IMRT patient was monitored using a commercial camera monitoring system (ExacTrac; BrainLab). The infrared markers seen by this camera system were rigidly placed on a custom bite-frame that was fabricated for each patient. All patients were immobilized using a Vac Fix mold and strap system. Patient movement was monitored and recorded every 2 seconds during the IMRT treatment. Prior to the initiation of this study, we also evaluated the effectiveness of immobilization of Vac Fix mold with straps with non-patient subjects. We found that the maximum/mean displacements for non-patient subjects were 1.29/0.75 mm. Based on this result; we conservatively set 2 mm tolerance for intra-fraction patient movement. Results: Intra-fraction patient movement data for 4 patients acquired for 20 fractions showed that all patients have a general tendency to move as the treatment progresses in time. We plotted the patient movement uncertainty as a function of cumulated treatment time (TUH) from 1, 2, 3, 4, 5, 10, 15, and 20 treatment fractions. Figure 1 shows that the TUH for an IMRT patient treatment has a distinct sigmoid shape. We define a quantity, U(x,y) as uncertainty that will assure target coverage for × % of treatment time through Y treatment fractions. For example, U(95,20) is 0.88 mm in Fig 1. TUH indicates that U(95,y) is more meaningful as uncertainty because it is close to where the high uncertainty shoulder on the curve begins. We obtained U(95,20) values of 1.24, 0.88, 1.22, and 1.75 mm in 20 fractions for 4 patients. All were within 2 mm of tolerance. Data shows that U(95,5) can also be used for patient specific uncertainty in re-planning without a significant loss of accuracy. Differences between U(95,5) and U(95,20) were less than .15 mm for 4 patients. Conclusions: New quantities, TUH and U(x,y) are useful in the interpretation of intra-faction uncertainty and are ideally suited for adaptive radiation therapy.