A quantitative analysis of tumor site specific setup based on 1 485 daily MVCT scans from helical tomotherapy

Abstract

Objective To assess the variation in patient setup corrections for three different anatomic treatment sites using daily pretreatment megavoltage CT(MVCT) in helical tomotherapy, and to analyze alternative reference margins for specific tumor site. Methods Sixty patients treated for three anatomical sites on helical tomotherapy were analyzed.Daily MVCT was carried out for registration and setup corrections before each treatment fraction.Setup errors and rotational setup corrections from 587 head and neck, 500 thoracic, 371 abdomen and gynecology MCVT imaging were input to statistical analysis.Incidences of three dimensional vector error lengths were calculated for each anatomic site.Overall distribution histograms of the three-dimensionally error was presented using GraphPad Prism 5.The distributions of systematic and random setup errors were quantitative analyzed and the additional margins required were also taken into account. Results 1 458 MVCT scans were implemented for sixty patients.Head and neck had lower frequencies of translational setup errors than others.Frequency of at least 3mm three-dimensional setup errors for head and neck, thoracic, abdomen and gynecology was 55.3%, 70.8%, 79.8%, respectively.This frequency decreased to 17.5%, 40.6%, 47.2% if 3D vector distance ≥5mm was scored.Overall systematic errors ranged from-1.4mm to 2.7mm, abdomen and gynecology had the largest setup errors in the vertical direction which was statistically significant(χ2=19.3, P<0.05). The suggested margins should be increased by 4-7 mm in three-dimensional direction for head and neck, 9-14mm for thoracic, 10-17mm for abdomen and gynecology. Conclusion Differences in setup corrections are perceived between head and neck, thoracic, abdomen and gynecology.The accuracy of patient positioning can be improved if pre-treatment daily MVCT scans are put into use.Results from setup correction can provide evidence for tumor treatment margin and improve the accuracy of regular radiotherapy. Key words: Tomography, spiral computed; Radiotherapy; Image processing, computer-assisted