| Background:Intracavitary brachytherapy(ICBT) is an essential component of the curative radiotherapy of locally advanced cervical cancer. Compared with conventional2D orthogonal X ray films based ICBT, CT/MRI-guided brachytherapy has shown superiorities on dose escalation of target volumme and sparing the critical organs at risk with improved both local control and complications. CT is a common modality of IGBT which is widely adopted by the institutions worldspread. HR CTV and OARs can be contoured based on the CT images followed by the process of planning and dose optimization, which usually takes a longer time of hours compared with2D planning. HR CTV and OARs movements in the interval betweent scanning and dose delivery will result in a dosimetric variation. To explore whether the variation causes a clinical concerned consequence provides meaningful information for clinical practice.Materials and methods:9patients with cervical cancer were recruited to the research from December2013to June2014for CT-guided intracavitary brachytherapy with five fractions and the prescription dose is6Gy per fraction. After applicator placement, the CT images with applicator in place were used for contouring and planning wich usually took about an hour. An extra CBCT(Cone beam computed tomography) scan was performed just before dose delivery for each fraction of every patient. A CBCT-based contouring and planning data was comapred with that of CT. The HR CTV and OARs which include bladder, rectum, sigmoid and small intestine were delineated on both image sets. The dwell time and postion of source from CT-plans were just manually adopted to the structure of CBCT by modifying the source activity meanwhile. Seventy-six scans with two different image sets were produced. The two different data sets of DVH parameters were analysed to interpret the intrafractional variations of dosimetry of HR CTV and OARs. Results:The variations (mean±SD) of the volume, D90, D50and D100for HR CTV are-2.00±3.26%,-1.18±4.47%,-0.02±4.51%,-3.83±8.23%and-9.0±2.77%individually. Most of them are statistically non-significant except the variations of the volume and D100(p values are0.000and0.006individually). The variations of D2cc for OARs which include bladder, rectum, sigmoid and small intestine are-0.6±17.05%,9.34±14.59%,7.15%±20.49%and1.52±12.57%individually. Most of them are statistically non-significant except the D2cc for rectum, which showed a significant increase(p=0.001).Conclusion:In a short time interval with an average less than1h, the variations of HR CTV trend to be minimal. The uncertainty(1SD) of the physical dose of D90for HR CTV is within5%, over70%of the EQD2of D90for the intrafractional variations are less than±0.5Gy with a clinical acceptable level. However for individual fraction the change may reach to1.5Gy and for all of the five fractions of individual patient the sum of EQD2may decrease with a clinical concerned level of3Gy. The variations of D2cc for OARS range from15%to20%, the majority of the whole sample changed by at least10%for at least one OAR, which would result in an overdose in a single fraction or the constraintof individual organ at risk for the whole treatment(includes EBRT). Methological improvements by controlling the uncertainties may reduce the risks of morbidities. |
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