ASSESSING DOSIMETRIC IMPACT OF GYNECOLOGICAL BRACHYTHERAPY APPLICATOR RECONSTRUCTION ON T1- WEIGHTED AND T2-WEIGHTED MR IMAGES VERSUS CT IMAGES

Abstract

Purpose:

Gynecological brachytherapy (BT) treatment planning traditionally uses CT images for applicator and catheter reconstruction, and MRI imaging for contouring. Some centres now employ MRI-only BT planning. Constructing on MRI images poses challenges; inherent MRI distortions can lead to dosimetric effects of 2-7% per mm of displacement (Tanderup, Hellebust and Lang 2008) (Richart, et al. 2018) (Schindel, et al. 2013) (Oonsiri, et al. 2017). This research retrospectively quantified dosimetric differences for gynecological brachytherapy treatment plans using applicators reconstructed on CT versus MRI images acquired with a T₂-PROPELLER or a T₁-3D-LAVA-FLEX sequence.

Materials and Methods:

MRI and CT images were obtained for 10 cervical cancer patients undergoing brachytherapy with a Venezia (Elekta) applicator. The patients were scanned on a 1.5 Tesla Avanto fit GE scanner and a Philips Big Bore Radiation Therapy CT scanner. The MRI images acquired included a T₂-weighted PROPELLER sequence and a T₁-weighted 3D-LAVA-FLEX sequence. Images were oriented in the plane of the tandem to minimize distortions. MRI images were imported into MIM (Medical Image Merge, version 7.2.8) for reorientation. Applicator reconstruction was completed for each CT or MRI image using the Oncentra Treatment Planning System (Elekta, version 4.6.2). The clinical treatment dwell positions and dwell times were copied to each image with its corresponding reconstruction. A three-dimensional dose grid was calculated in Oncentra for each independent MRI or CT reconstruction. The dose calculated using each MRI reconstruction was then compared to that calculated using the CT reconstruction via a gamma index analysis (Das, et al. 2022) in MIM. A passing rate of 90% using a 3%/2mm dose difference/ distance to agreement, with 10 % maximum dose thresholding, was considered acceptable (Miften, et al. 2018). The gamma results were averaged for the T₂-weighted images and T₁-weighted images. A student t-test was completed (p<0.05) to determine if there were significant differences in gamma index results obtained with the T₂ reconstruction versus the T₁ reconstructions.

Results:

Each of the 10 patients underwent 3 fractions of brachytherapy. In some cases, one or both of the MRI images were not acquired due to hospital resource constraints. Thus, a total of 22 T₁ images and 23 T₂ images were assessed. The average and minimum gamma result over all T₁-weighted image reconstructions were 96.6% and 90.5%, respectively. For T₂-weighted images, average gamma results were 96.0 % and the minimum result was 90.2%. The student t-test yielded a p-value of 0.52, indicating there is no significant difference between reconstructions performed on the T₂-weighed versus the T₁-weighted images.

Conclusions:

The results illustrate that applicator reconstructions on T₂-PROPELLER and T₁-3D-LAVA-FLEX MRI images produce dose maps that align with CT-based reconstructions within established criteria.