Estimation of CTV to PTV Margins in Head and Neck Cancer Patients Using On-Board Imager [OBI]
Sangutid Thongsawad MSc1,2, Wilai Masa-nga BSc2, Sawanee Suntiwong MSc2, Kittipol Dachaworakol MSc1,2, Chirasak Khamfongkhruea MSc1,2, Sasikarn Chamchod MD1,2, Chirapha Tannanonta MS2
Affiliation : 1 Faculty of Medicine and Public Health, HRH Princess Chulabhorn College of Medical Science, Chulabhorn Royal Academy, Bangkok, Thailand 2 Department of Radiation Oncology, Chulabhorn Hospital, HRH Princess Chulabhorn College of Medical Science, Chulabhorn Royal Academy, Bangkok, Thailand
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 (Σ
total) of patient setup were determined to estimate the CTV to
PTV margins by using Van Herk equation in 1D and 3D.
total) and random errors (σ
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
