Setup uncertainties and PTV margins at different anatomical levels in intensity modulated radiotherapy for nasopharyngeal cancer

AimTo determine the systematic error (∑), random error (σ) and derive PTV margin at different levels of the target volumes in Nasopharyngeal Cancer (NPC).Materials and methodsA retrospective offline review was done for patients who underwent IMRT for NPC from June 2015 to May 2016 at our institution.Alternate day kV images were matched with digitally reconstructed radiographs to know the setup errors. All radiographs were matched at three levels – the clivus, third cervical (C3) and sixth cervical (C6) vertebra. The shifts in positions along the vertical, longitudinal and lateral axes were noted and the ∑ and σ at three levels were calculated. PTV margins were derived using van Herk's formula.ResultsTwenty patients and 300 pairs of orthogonal portal films were reviewed. The ∑ for the clivus, C3 and C6 along vertical, longitudinal and lateral directions were 1.6 vs. 1.8 vs. 2[[ce:hsp sp="0.25"/]]mm; 1.2 vs. 1.4 vs. 1.4[[ce:hsp sp="0.25"/]]mm and 0.9 vs. 1.6 and 2.3[[ce:hsp sp="0.25"/]]mm, respectively. Similarly, the random errors were 1.1 vs. 1.4 vs. 1.8[[ce:hsp sp="0.25"/]]mm; 1.1 vs. 1.2 vs. 1.2[[ce:hsp sp="0.25"/]]mm and 1.2 vs. 1.3 vs. 1.6[[ce:hsp sp="0.25"/]]mm. The PTV margin at the clivus was 4.4[[ce:hsp sp="0.25"/]]mm along the vertical, 4[[ce:hsp sp="0.25"/]]mm along the longitudinal direction and 3.2[[ce:hsp sp="0.25"/]]m in the lateral direction. At the C3 level, it was 5.5[[ce:hsp sp="0.25"/]]mm in the vertical, 5[[ce:hsp sp="0.25"/]]mm in the lateral direction and 4.4[[ce:hsp sp="0.25"/]]mm in the longitudinal direction. At the C6 level, it was 6.4[[ce:hsp sp="0.25"/]]mm in the vertical, 6.9[[ce:hsp sp="0.25"/]]mm in the lateral direction and 4.4[[ce:hsp sp="0.25"/]]mm in the longitudinal direction.ConclusionA differential margin along different levels of target may be necessary to adequately cover the target