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Robustness of the Organ Doses Related to Setup Error in Helical Tomotherapy Planning of Restricted Incident Beam Angles

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H Shimizu

H Shimizu1*, K Sasaki2 , M Ito1 , T Aoyama1 , H Tachibana1 , N Tomita1 , C Makita1 , Y Koide1 , T Iwata1 , T Kodaira1 , (1) Aichi Cancer Center Hospital, Nagoya, Aichi, (2) Gunma Prefectural College of Health Sciences, Maebashi, Gunma


SU-I-GPD-T-440 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Helical tomotherapy can restrict the incident beam to specified regions in the block function of dose optimization computing. The application of the function is useful to reduce lung dose of cervical esophageal carcinoma (CEC) cases; however, the robustness of doses to the target and organ at risk (OAR) related to setup error has not been fully evaluated. The aim of this study was to determine the robustness of the dose depending on setup error in the blocked plan.

Methods: The treatment plan was designed using CT images of RANDO phantom. To simulate a CEC case, a virtual target volume (VTV), a planning target volume (PTV) with 5-mm margin to VTV, bilateral lungs, heart, thyroid, and spinal cord were contoured. The virtual block was applied for semi-circle region along a lung shape to the 8-cm outside from tracheal bifurcation. The shift of 1 to 3 pixel (2.2 mm/pixel) for each direction (LR, AP, and SI) were added to the original CT images, and dose distribution were recalculated.

Results: If patient movement was within the PTV margin for any direction, the difference of dosimetric parameters (D98%, D95%, D50%, and D2%) in the VTV between the blocked plan and non-blocked plan were within 1%. And the dose difference of heart, thyroid, and spinal cord was within several percent. On the other hands, the difference for V20Gy and V5Gy of right or left lung between two types of plans exceeded 10% by 2-pixel shift of LR direction, nevertheless the differences for AP and SI directions were several percent.

Conclusion: In the blocked plan for the CEC case, the setup error within PTV margin would not significantly affect the doses of target region, heart, thyroid, and spinal cord. However, pulmonary dose would vary greatly for the setup error of LR direction.

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