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Archived Comments for: Rotational IMRT techniques compared to fixed gantry IMRT and Tomotherapy: multi-institutional planning study for head-and-neck cases

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  1. trade-off between PTV coverage and OAR doses

    Simon Thomas, Cambridge University Hospitals

    12 June 2011

    Sir,

    The recent planning study by Wiezorek et al (2011) is a commendable attempt to benchmark IMRT planning software and techniques in standardised cases of head and neck cancer. In order to minimise variation between the groups the planning objectives of the study are clearly defined with a requirement for at least 93% of the prescribed dose to a minimal 99% of the PTV and the authors state that ┬┐fulfilling the objectives of the dose criteria for the PTV is given highest priority for treatment planning, except the criteria for the spinal cord could not be met┬┐. Although one technology / treatment planning system achieved this objective (RapidArc), in figure 2 of the paper it is evident that the other groups had accepted much lower doses to PTV2 with most only achieving 90% or lower. This was presumably a compromise to allow them to attempt to keep parotid and mandible doses low. We have plotted the parotid doses from table 3 as a function of the PTV2 D99% values from figure 2. With the exception of one outlier, they are clustered around a trendline that shows the generally accepted trade-off between PTV coverage and OAR doses.

    Unfortunately the results and the conclusion presented in this study have highlighted a problem of variable application of the dose constraints in different groups with different emphasis given to the PTV and OAR objectives. If all the centres had followed the objectives set out in the method, and prioritised achieving 93% or more to the D99%, it would be possible to compare the performance of the planning systems. The planning system that gave 14.11 Gy to the parotids (whilst failing the PTV objective) could probably, if parotid doses were allowed to rise to the 26Gy permitted in the objectives, have met the PTV objective for all patients. However this study fails to discover if this is so.

    Regards

    Simon Thomas and Richard Benson

    Competing interests

    none

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