Purpose: To investigate dosimetric uncertainties of MRI-based cervix cancer brachytherapy, when applying two HDR fractions for each applicator insertion and their clinical relevance.
Methods: 21 patients with 84 MRI-examinations and fractions were investigated. After insertion of the MRI compatible tandem-ring applicator, an MRI-set was recorded and the treatment plan optimised for the first fraction. Prior to the second fraction 16-20 h later a second MRI-set was recorded, and the dose distribution from the plan of the previous day superimposed and analysed. The same procedure was repeated for fractions 3 and 4. Dose from EBRT and brachytherapy was normalised to 2 Gy-fractionation (EQD2), added up to a total dose, and compared to a calculated total dose if only 1 MRI-examination per insertion is available.
Results: The total D(90) for High risk (HR) CTV was 1.2±2.7 Gy(αβ10) (1±3%) (mean±1SD) lower by individual MRI-evaluation of each fraction compared to 1 MRI per insertion. The D(2cm(3)) increased by 0.7±4.7 Gy(αβ3) (1±6%) for bladder, 1.1±2.4 Gy(αβ3) (2±4%) for rectum and decreased by 0.8±3.4 Gy(αβ3) (1±5%) for sigmoid. For HR CTV the individual approach did not identify any case with a decrease of D(90) >5 Gy(αβ10). For the bladder 3 cases, for the rectum no case and for the sigmoid 1 case was identified with an increase of D(2cm(3)) >5 Gy(αβ3). For the bladder all dose variations of more than 5 Gy(αβ3) could have been avoided by ensuring a constant bladder filling. Individual MRI-evaluation did not determine any case where dose constraints were not fulfilled.
Conclusions: For the treatment schedule as applied in this study, geometric differences between applicator, target and OAR result in overall dosimetric changes, which seem to be of minor relevance in regard to clinical dose volume constraints applied at present.
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