Rachid Errifai*, Abdelouahed Chetaine and Siham Bouaouad
Introduction: The procedures for treatment with external radiotherapy are very precise. However, there are many sources of error which operate during the planning and delivery of treatment; that limit accuracy. The difficulty of reproducing the positioning of the patient from day to day is a major source of error which must be given special attention. This geometric precision is an essential parameter in the radiotherapy of cancers nasopharyngeal cancer, and the determination of the margin between the CTV and the PTV is the result of the calculation of these uncertainties.
Objective: The aim of this work is to assess the positioning accuracy of patients on the treatment table on a daily basis and to describe the method of calculating systematic and random errors, in order to establish appropriate margins from the CTV to the PTV for the patients.
Materials and methods: Deviations positioning were analyzed in 16 patients irradiated for nasopharyngeal cancer by radiotherapy with VMAT, analyzing their images portals acquired by the CBCT 3D Elekta XVI ®, for 33 sessions establishment, is calculated the overall mean Displacement (M), the Systematic Errors (Σ) and the Random Errors (σ), the PTV margins were calculated according to the Van Herk formula (2.5 Σ+0.7 σ).
Results: A total of 528 portal images were analyzed for nasopharyngeal cancer cases, the M value was 2.7 mm in all directions, the Σ and σ values were 1.3 mm and 5.1 mm respectively, so the calculated PTV margin was 6.82 mm.
Conclusion: In the daily clinical routine for nasopharyngeal cancer patients, the PTV-CTV margins of 7 mm are safe and adoptable, however one could afford in some special situations such as proximity to organs at risk and high dose areas, to put lower margins.
