How to determine margins for planning target volume (PTV): from 2D to 3D planning in radiotherapy for head and neck cancer? Portal imaging assessment for set-up errors

OBJECTIVES: To evaluate set-up errors using Electronic Portal Imaging (EPI) for Three Dimensional Conformal Radiotherapy (3D CRT) protocol for head and neck carcinoma in Dokuz Eylül University Department of Radiation Oncology (DEUDRO). METHODS: Ten patients between July 2004 - September 2005 were included. Seven EPIs/5 weeks per patient were planned to evaluate by two independent observers. Differences between Digitally Reconstructed Radiography (DRR) images and EPIs measured in cranio-caudal (CC), anterior-posterior (AP) and medio-lateral (ML) directions. Random (σ) and the systematic (∑) errors were calculated and used in van Herk margin formula (2.5 x Σ + 0.7 x σ) for PTV margin. RESULTS: Ninety three EPIs with 186 [93 (50%) CC, 78 (42%) AP and 15 (8%) ML] measurements were evaluated. The σ were AP: 2.6 mm, CC: 2.9 mm, ML: 1 mm. The ∑ AP: 3.9 mm, CC: 1.8 mm, ML: 1.9 mm. PTV margins calculated were AP: 11.7 mm, CC: 6.6 mm, ML: 5.8 mm. CONCLUSION: 3DCRT protocol for head and neck cancer was amended using these PTV margins. These maybe wide especially for IMRT, and can reduce with better immobilization systems.