Set-up variations/set-up margin

 During a fractionated course of radiotherapy, variations in patient position and in alignment of beams will occur both intra- and inter-fractionally, and a margin for set-up error must be incorporated into the CTV-PTV margin. Errors may be systematic or random.

Systematic errors may result from incorrect data transfer from planning to dose delivery, or inaccurate placing of devices such as compensators, shields, etc. Such systematic errors can be corrected.

Random errors in set-up may be operator dependent, or result from changes in patient anatomy from day to day which are impossible to correct. Accuracy of set-up may be improved with better immobilisation, attention to staff training and/or implanted opaque fiducial markers, such as gold seeds, whose position can be determined in three dimensions at planning, and checked during treatment using portal imaging or IGRT. Translational errors can thereby be reduced to 1 mm and rotational errors to 1°.

Each department should measure its own systematic and random errors for each treatment technique by comparing portal imaging and digitally reconstructed radiographs (DRRs). These measurements are then incorporated into the CTVPTV margin using the formula devised by Van Herk, where the CTV is covered for 90 per cent of the patients with the 95 per cent isodoses:

 PTV margin =  2.5Σ + 0.74 σ 

where Σ total standard deviation (SD) computed as the square root of the sum of the squared individual SD values of all systematic errors for organ motion and set-up; and σ total SD of all random errors combined quadratically in a similar way.

This provides a population-derived standard CTV-PTV margin for a particular technique in a given centre and can be non-isotropic in cranio-caudal, transverse and anteroposterior (AP) directions. Accurate treatment delivery depends on reducing or eliminating systematic errors and requires a high level of awareness of all staff throughout the many different work areas from localisation through to treatment.

Other theories about how to incorporate organ motion and the uncertainty of the ‘mean’ position of the CTV on a snapshot CT scan used for localisation have been proposed. Van Herk suggests a volume large enough to contain the mean position of the CTV in 90 per cent of cases, called the systematic target volume (STV) (British Institute of Radiology 2003). Collection of data on the precise CT location of tumour recurrences in relation to the original target volume is important to improve margin definition.

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