With the wide-scale introduction of IGRT, traditional geometrical errors such as setup error and organ motion have mostly been eliminated. As a result, other sources of error are dominating, such as delineation uncertainty, IGRT limitations including surrogate error, organ deformation and short term organ motion. Also, more and more treatments are hypo-fractionated and some assumptions used in margin calculation are not necessarily valid for their small number of fractions and unconventional ways of prescribing dose. In order to properly calculate margins from the clinical target volume (CTV) to the planning target volume (PTV) these uncertainties must be quantified, the effect of fractionation and of prescription method must be understood and finally incorporated into the margin calculation[1]. One of the most difficult uncertainties to quantify is delineation variation. Here it is important to distinguish interpretation differences from true observer variation. Interpretation differences should be dealt with through consensus and protocol development. Then the residual is taken forward into margin calculation. Interestingly margin calculation for delineation and deformation is mathematically an unsolved problem but there is some practical evidence to support use of margin recipes. However, also alternative methods such as robust treatment planning should be developed[2].

Educational aims:

- To understand the role of residual uncertainties after IGRT

- To be able to quantify these given enough data with simplified methods and calculate the appropriate safety margins

References:

1. Errors and margins in radiotherapy. van Herk M. Semin Radiat Oncol. 2004 Jan;14(1):52-64
2. Robust radiotherapy planning. Unkelbach J, Alber M, Bangert M, Bokrantz R, Chan TCY, Deasy JO, Fredriksson A, Gorissen BL, van Herk M, Liu W, Mahmoudzadeh H, Nohadani O, Siebers JV, Witte M, Xu H. Phys Med Biol. 2018 Nov 12;63(22): 22TR02

Further reading:

Is reducing irradiated margins key to improving outcomes for radiotherapy? van Herk M, Osorio EV, Troost EGC. Lancet Oncol. 2019 Sep;20(9):1208-1210.

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Duration:30 mins