Dynamic assessment of the dose perturbation by sliding rails in intensity-modulated radiotherapy for spinal metastases / 中华放射医学与防护杂志

ABSTRACT

Objective:To investigate the impact of carbon based non-conductive sliding rails on intensity-modulated radiation therapy (IMRT) planning for vertebral tumors.Methods:A reconstruction imaging of the couch plate and its rails removed from the EDGE linear accelerator was acquired by helical computed tomography (CT), and pushed into the treatment planning system (TPS). Based on CT images of homogeneous phantom and patients in our database, 6 and 10 MV photon IMRT plans were designed with five fields (180°, 200°, 220°, 160°, 140°), setting a prescribed dose of 3 Gy/F ×10 F to the planning target volume (PTV), and the dose was calculated by AcruosXB. In addition, optimization plans (OP) without rails and with rails symmetrically moving (4-19 cm, step size 1 cm) were created and verification plans(VP) were created by inserting and removing slide rail′s structure from the corresponding OP in VP. The differences in mean dose ( Dmean), homogeneity index (HI), and conformability index (CI) of the PTV, and maximum dose ( Dmax) of the spinal cord were compared and analyzed between the VP and OP. Results:HI of PTV with and without the rails were worse in the 6/10 MV verification plans, with a maximum difference of 2%. In 6 MV plans, with rails Dmean of PTV decreased by (2.07±0.99)%, CI increased by (4.91±3.12)%, and Dmax of spinal cord decreased by (1.83±1.37)%. Without rails, the Dmean of PTV increased by (2.02±0.96)%, but CI decreased by (3.07±1.31)% and Dmax of the spinal cord increased by (2.03±1.44)% in the patient respectively. There were significant statistical differences between groupswith and without rails( F=27.55, 361.32, 13.05, P<0.05). The target volume Dmean and spinal cord Dmax decreased with a sloping " W" with the change of the rail position. The difference appeared to be noticeable in the range of less than 10 cm, but it gradually decreased as the rails slid to the outside until it reached zero, and the Dmean of PTV and Dmax of the spinal cordchanges were less in 10 MV plans, comparing to 6 MV plans. Conclusions:The dose attenuation by the sliding rails of the Qfix kVue should not ignored in the IMRT of spinal metastases and the slide position should be consistent with the planned position.