Estimation of CTV to PTV Margins in Head and Neck Cancer Patients Using On-Board Imager [OBI]

Background: At Chulabhorn Hospital, uniform clinical target volume [CTV] to planning target volume [PTV] margin of 0.5 cm is used for head and neck cancer radiation therapy treatment as Radiation Therapy Oncology Group [RTOG] protocol suggested. However, PTV margins relate to types of thermoplastic mask, image verification modality, and image guidance protocol. Objective: To estimate the CTV to PTV margins in head and neck cancer patients treated with intensity modulated radiation therapy [IMRT] at Chulabhorn Hospital in order to verify whether our current addition of 0.5 cm was appropriate. Materials and Methods: Twenty-three head and neck cancer patients treated with IMRT technique were included in this study. The population systematic (Σtotal) and random errors (σtotal) of patient setup were determined to estimate the CTV to PTV margins by using Van Herk equation in 1D and 3D. Results: The CTV to PTV margins calculated with 1D margin were 0.12, 0.16, and 0.10 cm for Vert, Lng, and Lat couch directions, respectively. The CTV to PTV margin calculated with anisotropic 3D margin expansion was 0.32 cm. Conclusion: This study indicated that the uniform CTV to PTV margin of 0.5 cm used for head and neck cancer patients at our hospital is adequate to cover set up errors. Keywords: PTV margins estimation; Setup errors; On-board imager; Head and neck cancer