Sir,

The aim of our study was to assess the patient setup errors for various tumor sites based on clinical data treated with helical tomotherapy using pretreatment image guidance (IG) based on the institutional practice. In addition, we also calculated and compared the planning target volume (PTV) margins for all disease sites based on the analysis of systematic and random errors. The motivation of this study was to analyze our institution\'s specific setup errors considering the fact that setup errors vary according to each immobilization system and patient which in turn influence the PTV margins. We have divided the tumor sites into various categories as brain, head and neck, thorax - abdomen, pelvis, and craniospinal irradiation. Planning computed tomography (CT) images were obtained on lightSpeed Xtra (GE Health care Pvt Limited, India.) with a slice thickness of 3 mm for all the disease sites as per the protocol. For thorax and abdominal tumors, four-dimensional CT scans were obtained, from which internal target volume was generated for planning purposes.

In our investigation, we have analyzed our data using fast CT scans and based on these results, we cannot recommend that untagged average studies would be advantageous, which is not supported by our results. However, we agree with the author of the letter that in principle, the usage of fast helical CT studies for planning may be suboptimal for IG purposes, and untagged average studies may be used.

Although image-guided radiation therapy (IGRT) addresses reducing geometric uncertainties, there are various other uncertainties related to target delineation, imaging, etc., and hence reducing the margins with the introduction of IGRT, has to be done with caution, otherwise, might lead to poorer outcome, especially in higher stage disease.\[[@ref1]\] There are various other literature supporting this with mixed opinions and hence, it is debatable and requires further research.\[[@ref2]\]

With respect to establishing comprehensive recommendations for the PTV margins for all the disease sites, we agree with the authors that many parameters have to be taken into account, which include planning CT parameters, immobilization devices, patient preparation protocols, imaging schedules (non-IG, IG for limited number of fractions, and daily IG), and patient-specific features. We would like to however emphasize here that the purpose of the present investigation is to analyze institution-specific setup errors, and we consider that based on these preliminary findings, it will be too early to recommend comprehensive PTV margins for all the disease sites.

We agree with the final comment of the authors that compiling clearly defined instructions for IG procedures and corresponding PTV margins needs a multi-institutional collaboration followed by a careful analysis of clinical outcomes for robust evaluation.

 {#sec2-1}

Financial support and sponsorship {#sec3-1}
---------------------------------

Nil.

Conflicts of interest {#sec3-2}
---------------------

There are no conflicts of interest.
