Original paper Clinical significance of treatment delivery errors for helical TomoTherapy nasopharyngeal plans – A dosimetric simulation study Shrikant Deshpande a,b,⇑ , Mark Geurts c , Philip Vial a,d , Peter Metcalfe a,b , Mark Lee a , Lois Holloway a,b,d,e a Liverpool and Macarthur Cancer Therapy Centres and Ingham Institute, Sydney, NSW, Australia b Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, Australia c Department of Human Oncology, School of Medicine and Public Health, University of Wisconsin-Madison, USA d Institute of Medical Physics, School of Physics, University of Sydney, Sydney, NSW, Australia e South West Clinical School, University of New South Wales, Sydney, NSW, Australia article info Article history: Received 10 September 2016 Received in Revised form 6 December 2016 Accepted 7 January 2017 Available online 16 January 2017 Keywords: Helical tomotherapy Dosimetric simulation QA tolerance abstract Purpose: Develop a framework to characterize helical TomoTherapy (HT) machine delivery errors and their clinical significance. Method and materials: Ten nasopharynx HT plans were edited to introduce errors in Jaw width (JW), couch speed (CS), gantry period (GP), gantry start position (GSP), multi leaf collimator leaf open times (MLC LOT). In case of MLC LOT only, both systematic and random delivery errors were investigated. Each error type was simulated independently for a range of magnitudes. Dose distributions for the clin- ical reference plans and the error simulated plans were compared to establish the magnitude for each error type which resulted in a change in clinical tolerance, defined as 5% variation in D95 of PTV70, D0.1 cc of spinal cord, D0.1 cc of brainstem and the smallest value of either a 10% or 3.6 Gy dose variation in mean parotid dose. Results: Dose variation from systematic delivery errors in JW ±0.5 mm, CS ranges between À1% to 1.5%, GP ±1 s, GSP ranges between À2 0 to 2.5 0 and MLC LOT random error up to 2% from the planned value rel- ative to the clinical reference plan was within the set tolerance values for all the patient cohorts. GSP errors and the random MLC LOT errors with up to 10% standard deviation were found to be relatively insensitive compared to other delivery errors. Conclusion: This work has established a framework to characterize HT machine delivery errors. This framework could be applied to any patient dataset to determine clinically relevant HT QA tolerances. Ó 2017 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved. 1. Introduction High radiation dose delivery to tumours with improved sparing of normal tissues is an ongoing goal of radiotherapy practice. Advanced radiation therapy techniques such as fixed beam intensity-modulated radiation therapy (IMRT), volumetric modu- lated arc therapy (VMAT) and helical tomotherapy (HT) have improved our ability to achieve this goal [1]. However, these tech- niques are also more complex to deliver. As such, an understand- ing of delivery uncertainties within these advanced planning and delivery techniques is required to ensure the potential for improved effectiveness from these techniques is actually being achieved. Margalit et al. [2] outlined the different errors associ- ated with either IMRT or 3D/conventional treatment techniques and highlighted how the occurrence of error types differs between treatment techniques. The dosimetric impact of multileaf collima- tor (MLC) leaf position errors have been thoroughly studied and reported [3–8] or IMRT and VMAT delivery. Further, Tatsumi et al. [9] demonstrated that the sensitivity of MLC errors depends upon the treatment planning system (TPS) type, implementation of inverse optimization, and MLC segmentation algorithm within the TPS. The dosimetric impact from simulated delivery errors from other mechanical parameters such as gantry angle, collima- tor angle and table position in IMRT and VMAT delivery has also been studied [4,10,11]. Betzel et al. [12] reported that RapidArc TM (Varian Medical Systems, Palo Alto, CA) deliveries were found to be more tolerant to variations in dose rate, gantry position, and MLC leaf position compared to IMRT delivery. All of the above reported studies on the sensitivity of treatment delivery error http://dx.doi.org/10.1016/j.ejmp.2017.01.006 1120-1797/Ó 2017 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Liverpool Cancer Therapy Centre, Liverpool Hospital, Cnr Goulburn and Campbell St, Liverpool 2170, NSW, Australia. E-mail addresses: Shrikant.deshpande@sswahs.nsw.gov.au (S. Deshpande), mwgeurts@humonc.wisc.edu (M. Geurts), Philip.Vial@sswahs.nsw.gov.au (P. Vial), metcalfe@uow.edu.au (P. Metcalfe), Mark.Lee@sswahs.nsw.gov.au (M. Lee), Lois. Holloway@sswahs.nsw.gov.au (L. Holloway). Physica Medica 33 (2017) 159–169 Contents lists available at ScienceDirect Physica Medica journal homepage: http://www.physicamedica.com