Comparison of setup errors based on bony landmarks in high precision radiotherapy in head and neck cancer: Results of a prospective study

Background: Comparison of setup errors for various immobilization devices in head and neck cancer and to determine the treatment margin in high precision radiotherapy using bony landmark based matching. Materials and Methods: Total 20 patients were immobilized with BrainLAB immobilization device (BL) or thermoplastic ray cast (RC) for high precision radiotherapy. Total, systematic, and random errors in mediolateral (ML), craniocaudal (CC) and antero-posterior directions were determined and clinical target volume (CTV) to planning target volume (PTV) margin by Stroom's formula was compared. Unpaired t-test was used for comparing errors. The standard deviations (systematic and random errors) in different groups were compared by variance ratio test (Levene's test) and P < 0.05 was considered significant. Results: The total error in ML direction (BL vs. RC) was 1.00 mm versus 1.39 mm (P = 0.03), systematic error 0.09 cm versus 0.197 cm and random error 0.116 cm versus 0.258 cm (F-test, P = 0.001). CTV to PTV margin was significantly lower in BL (0.26 cm vs. 0.57 cm, P < 0.05). In CC direction, BL system had lower total error (0.075 cm vs. 0.157 cm) and a significantly less systematic error (0.116 cm vs. 0.258 cm, F = 7.149, P = 0.015). CTV to PTV margin was less in BL than RC in CC direction (0.34 cm vs. 0.92 cm, P = 0.06). Conclusion: In head and neck region, when electronic portal imaging device based verification is used, for BL margins ranged from 2.6 to 3.7 mm. For RC in the PTV margin was 5.7–9.2 mm. Therefore, a margin of 3 mm for BL and 5–10 mm for RC with online correction in head and neck is adequate.