In regard to “Setup error analysis in helical tomotherapy based image-guided radiation therapy treatments”

Sir, I read with interest the paper of Dr. Thondykandy et al. in the latest issue of your journal.[1] The current rapid introduction of pretreatment (tomotherapy megavoltage computed tomography (CT) and cone-beam CT on Varian and Elekta linacs) and in-treatment (CyberKnife, ViewRay) image guidance (IG) needs a careful assessment of these technologies in order to establish their optimal usage. The authors investigated the position correction shifts for 102 patients treated with helical tomotherapy using megavoltage CT for matching the patient position of the day to the planning CT study. There is no information on how planning CT studies were obtained, but for patients with a significant target motion, the usage of fast helical CT studies for planning may be suboptimal for IG purpose, and untagged average studies should be recommended. Systematic and random errors were evaluated and used for treatment margin calculation as per van Herk et al.[2] However, the authors’ concluded that the clinical margins used in their hospital were adequate enough for the brain, head and neck, and lung cancer patients while being out of clinical margins for the pelvis and cervical spine injury patients, may be confusing. The margins calculated by Thondykandy et al.[1] correspond only to a part of the total planning target volume (PTV) construction accounting for interfraction motion as explained in BIR publication.[3] These margin values can be used to account for positioning errors if no pretreatment IG is performed.