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

Abstract

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.

Optimal fractionation and timing of weekly cone-beam CT in daily surface-guided radiotherapy for breast cancer

“…Supplementary Table 1 shows that the CTV–PTV margins largely depended on the CBCT frequency for all modes with the ipsilateral ROI and decreased with increasing CBCT frequency; however, the CBCT time interval had almost no effect on them. In line with this, some studies reported that the CTV–PTV margins depended on the immobilisation devices, radiation method, and frequency of verification imaging, and a 5–10 mm CTV–PTV margin is required in breast cancer radiotherapy [ 29 , 30 ]. In this study, we found that the margin in the SI direction was the largest, and only when the CBCT frequency was ≥ 4 could the margins in all directions be approximately 5 mm for patients who underwent supine mastectomy with cushions stuffed with foam pieces.…”

Factors Affecting Isocenter Displacement and Planning Target Volume Margin for Patients With Rectal Cancer Receiving Radiation Therapy

“…Tamponi et al 24 showed systematic and random errors of 2 to 3 mm in the patients with prostate, rectum, and gynecologic cancers receiving RT. The calculated CTV-PTV margins were 10 mm in the RL direction and 20 mm in the SI direction.…”

Unravelling the landscape of image-guided radiotherapy: a comprehensive overview

“…To minimize geometric uncertainties, IGRT helps to adjust the patient's position or modifies the treatment plan based on anatomical changes. The image of the patient acquired immediately before a treatment offers opportunities for a more precise set-up [29].…”