Measuring radiotherapy setup errors at multiple neck levels in nasopharyngeal cancer (NPC): A case for differential PTV expansion

Abstract

Background and purposeWe aim to quantify the magnitude of the systematic and random setup errors at three different anatomical levels of the neck in Nasopharyngeal Carcinoma (NPC) when clivus matching is used, and recommend appropriate PTV margins for each level. Material and methodsThirty-six patients undergoing image-guided radiotherapy (IGRT) each with 9 scheduled CBCTs were reviewed. The magnitude of setup errors were measured at the level of the clivus, C4 and C7 vertebrae, before and after CBCT correction. The 3D displacements, systematic and random errors were calculated for each level. The appropriate PTV expansion was determined using Van Herk’s formula. ResultsMean 3D displacement was 1.88, 2.66 and 3.35mm at the clivus, C4 and C7 before correction. The differences were statistically significant (p<0.05). The PTV margin required without correction was 2.33, 4.33 and 6.52mm respectively. These were reduced to 1.20, 3.72 and 6.08mm after CBCT corrections. ConclusionsVariability is seen in setup errors at the clivus, C4 and C7 vertebral levels. A variable planning margin approach with reduced margin at the clivus is recommended. Use of daily CBCT allows the PTV expansion to be reduced to 1.2mm.