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Effect of translational couch shifts in volumetric modulated arc therapy (VMAT) plans and predicting its impact on daily dose delivery

Published online by Cambridge University Press:  10 November 2017

Noufal M. Padannayil ,
Kallikuzhiyil K. Abdullah ,
Pallimanhayil A. R. Subha  and
Sanudev Sadanadan
Noufal M. Padannayil*
Affiliation:
Department of Medical Physics and Radiotherapy, Baby Memorial Hospital, Calicut, India Department of Physics, Farook College, Calicut, India University of Calicut, Malapuram, Kerala, India
Kallikuzhiyil K. Abdullah
Affiliation:
Department of Physics, Farook College, Calicut, India University of Calicut, Malapuram, Kerala, India
Pallimanhayil A. R. Subha
Affiliation:
Department of Physics, Farook College, Calicut, India University of Calicut, Malapuram, Kerala, India
Sanudev Sadanadan
Affiliation:
Department of Medical Physics and Radiotherapy, Baby Memorial Hospital, Calicut, India
*
Correspondence to: Noufal M. Padannayil, Baby Memorial Hospital, Calicut, Kerala, India. Tel: +917358083838. E-mail: noufalsh@gmail.com
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Abstract

Aim

To evaluate the impact of couch translational shifts on dose–volume histogram (DVH) and radiobiological parameters [tumour control probability (TCP), equivalent uniform dose (EUD) and normal tissue complication probability (NTCP)] of volumetric modulated arc therapy (VMAT) plans and to develop a simple and swift method to predict the same online, on a daily basis.

Methods

In total, ten prostate patients treated with VMAT technology were selected for this study. The plans were generated using Eclipse TPS and delivered using Clinac ix LINAC equipped with a Millennium 120 multileaf collimator. In order to find the effect of systematic translational couch shifts on the DVH and radiobiological parameters, errors were introduced in the clinically accepted base plan with an increment of 1 mm and up to 5 mm from the iso-centre in both positive and negative directions of each of the three axis, x [right–left (R-L)], y [superior–inferior (S-I)] and z [anterior–posterior (A-P)]. The percentages of difference in these parameters (∆D, ∆TCP, ∆EUD and ∆NTCP) were analyzed between the base plan and the error introduced plans. DVHs of the base plan and the error plans were imported into the MATLAB software (R2013a) and an in-house MATLAB code was generated to find the best curve fitted polynomial functions for each point on the DVH, there by generating predicted DVH for planning target volume (PTV), clinical target volume (CTV) and organs at risks (OARs). Functions f(x, vj), f(y, vj) and f(z, vj) were found to represent the variation in the dose when there are couch translation shifts in R-L, S-I and A-P directions, respectively. The validation of this method was done by introducing daily couch shifts and comparing the treatment planning system (TPS) generated DVHs and radiobiological parameters with MATLAB code predicted parameters.

Results

It was noted that the variations in the dose to the CTV, due to both systematic and random shifts, were very small. For CTV and PTV, the maximum variations in both DVH and radiobiological parameters were observed in the S-I direction than in the A-P or R-L directions. ∆V70 Gy and ∆V60 Gy of the bladder varied more due to S-I shift whereas, ∆V40 Gy, ∆EUD and ∆NTCP varied due to A-P shifts. All the parameters in rectum were most affected by the A-P shifts than the shifts in other two directions. The maximum percentage of deviation between the TPS calculated and MATLAB predicted DVHs of plans were calculated for targets and OARs and were found to be less than 0·5%.

Conclusion

The variations in the parameters depend upon the direction and magnitude of the shift. The DVH curves generated by the TPS and predicted by the MATLAB showed good correlation.

Keywords

dose volume histogramequivalent uniform dosenormal tissue complication probabilitysetup errortumour control probability
Type
Original Article
Information
Journal of Radiotherapy in Practice , Volume 17 , Issue 2 , June 2018 , pp. 230 - 243
Copyright
© Cambridge University Press 2017 

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