Assessment of Anatomic Surface Contour Variations During Radiation Therapy in Oral Cavity Cancer Patients with Flap Reconstruction by Co-Registering Planning CT and Cone Beam CT Scan

Background: Oral cavity cancer (OCC) patients undergoing flap reconstruction during surgery present unique anatomical challenges during postoperative radiation therapy (PORT). Variations in body contour and internal anatomy can affect dose distribution and necessitate adaptive radiotherapy (ART). However, limited data is available regarding the frequency, timing, and causes of re-planning. Objective: To evaluate anatomical surface contour variations and the need for adaptive re-planning in OCC patients with flap reconstruction (OCCwFR) receiving PORT using co-registration of planning CT and cone-beam CT (CBCT). Methods: This prospective observational study included adult OCCwFR patients treated with curative-intent PORT. Daily portal imaging and weekly CBCTs were analyzed. Variations greater than 1 cm across three or more consecutive CBCTs were considered significant. Changes were categorized as progression, regression, internal target motion, or setup error. Factors associated with adaptive re-planning were recorded. Results: Among 111 patients, 39 (35%) required re-planning. Regression was the predominant change (92%), mainly due to weight loss (80%), followed by edema resolution and tumor response. Re-planning occurred mostly around the 20^th fraction. Progression-related changes were rare (5%) and occurred earlier (3^rd week). No association was found between flap type and re-planning (p= 0.21). Weight loss strongly correlated with re-planning (χ²= 45.5, p< 0.0001) and mean volume reduction of 50 cc. Conclusion: Anatomical changes during PORT in OCCwFR patients are frequent, predominantly due to weight loss. Mid-treatment ART around the 5th week and proactive nutritional support are essential to maintain dosimetric accuracy and treatment efficacy.