(Monday, 7/15/2019) 9:30 AM - 10:00 AM
Room: Exhibit Hall | Forum 6
Purpose: In our institution, chest wall patients routinely have expanders. For left-sided Deep Inspiration Breath Hold (DIBH) patients with bi-lateral expanders, we use VMAT instead of conventional tangential beams to achieve better normal tissues sparing. We herein analyze the motion during the breath hold and estimate how much that motion affected the delivered dose distribution.
Methods: Varian Optical Surface Monitoring System (OSMS) was used. Breath hold motion was recorded in three translational and three rotational displacements of the treatment surface. These are called Real Time Deltas (RTD). The beam on and off status was also recorded. 3 patients’ RTDs were exported. The mean and standard deviation of the beam on portion of the RTDs were calculated. The translational RTDs were used to calculate the delivered dose in Eclipse. The delivered DVH and planned DVH were compared.
Results: The average over all patients (mean ± standard deviation) of the beam on portions of the RTDs in the order of vertical, longitudinal, lateral, rotation, roll, and pitch, were (-0.3±0.5) mm, (-0.1±0.6) mm, (-0.7±0.5) mm, (-0.15±0.23)°, (-0.2±0.16)°, (-0.04±0.18)°. The planned and delivered CTV, PTV, left lung, heart, right breast DVHs were compared, and the differences for most structures were less than 1%. However, due to the small volume and the location by the edge of PTV, when IMN was involved, even a small motion could lead to a big dose coverage difference.
Conclusion: The averaged mean motion during deep inspiration breath hold was less than 1 mm and 1°. This indicates a good relative reproducibility of the patient breath hold. On average, the PTV coverage, the heart and lung doses have barely noticeable differences from what have been planned. However, the IMN coverage might be modestly different for certain cases.
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