Random and systematic set-up errors in three-dimensional conformal radiotherapy – impact on planning target volume margins: the experience of the Radiation Oncology Centre of Sassari

Purpose: Geometric uncertainties limit the accuracy of three-dimensional conformal radiotherapy
treatments. This study aims to evaluate typical random and systematic set-up errors and analyse the
impact of no action level (NAL) correction protocol on systematic set-up errors and clinical target volume
(CTV)–planning target volume (PTV) margins.
Materials and methods: A total 668 pairs of orthogonal electronic portal images were compared with digitally
reconstructed radiographs from computed tomography planning scans for 100 patients consecutively treated
during 2011. Patients were divided into groups depending on the treated anatomical region. Patient-specific
and population random and systematic set-up errors were calculated. Impact of application of NAL correction
protocol on systematic set-up errors and CTV–PTV expansions were evaluated.
Results: Population set-up errors resulted from about 1 mm in head and neck to 2–3 mm in prostate,
rectum, lung, breast and gynaecological districts. Patient-specific systematic set-up errors were higher for
breast and gynaecological districts and application of NAL correction protocol gave significant reductions,
even higher than 30%. Calculated CTV–PTV margins ranged from 10 mm on left–right direction for prostate
to 20 mm on superior–inferior direction for lung.
Conclusions: Set-up errors resulted reasonably controlled and application of NAL correction protocol could
further improve the level of accuracy. However, the NAL application alone did not seem to add any
substantial benefit on CTV–PTV total margins without the adoption of corrective strategies to reduce other
important uncertainties limiting accuracy of three-dimensional conformal radiotherapy.