Institution-specific Validation of CTV-PTV Margins in Head and Neck Radiotherapy using Daily Cone Beam Computed Tomography on Halcyon Elite: A Population - Based Study

Abstract

Aims: To quantitatively evaluate setup errors in head and neck cancer radiotherapy using Cone Beam Computed Tomography (CBCT) and determine whether the institutional Clinical Target Volume (CTV)–Planning Target Volume (PTV) margin adequately compensates for these uncertainties. Study Design: Retrospective observational study. Place and Duration of Study: Sample: Department of Radiation Oncology, Father Muller Medical College Hospital, Mangalore, India, conducted over a period of six months. Methodology: Twenty patients with head and neck malignancies treated using Intensity‑Modulated Radiotherapy (IMRT) or Volumetric Modulated Arc Therapy (VMAT) were retrospectively analyzed. All patients were immobilized with thermoplastic head and neck masks. Daily setup verification was performed using kV cone‑beam CT on the Halcyon Elite linear accelerator. Translational setup deviations in vertical, longitudinal and lateral directions were recorded. Population systematic (Σ) and random (σ) errors were calculated using individual patient mean and standard deviation values. The CTV–PTV margins were derived using the Van Herk margin formula. Results: Population systematic errors (Σ) were 0.041 cm, 0.072 cm and 0.076 cm in vertical, longitudinal and lateral directions respectively. Random errors (σ) were 0.148 cm, 0.154 cm and 0.213 cm respectively. The calculated CTV–PTV margins were 0.21 cm (vertical), 0.29 cm (longitudinal) and 0.34 cm (lateral), all smaller than the institutional margin of 3 mm. Conclusion: Daily CBCT‑based image guidance combined with thermoplastic immobilization provides high setup reproducibility in head and neck radiotherapy. The institutional 3 mm margin remains adequate as it accounts for additional clinical uncertainties beyond setup errors.