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Accurate anatomical identification is critical in gynaecological radiotherapy (RT). In our departmental clinical practice, gynaecological patients have received two CT scans at simulation; one full bladder scan and one empty bladder scan with tampon in-situ. This was complimented with a full and empty bladder sagittal and axial T2-weighted MRI scan. Diagnostic MRI exams have generated vaginal opacification using ultrasound gel to distend and delineate the vagina due to its low viscosity and high signal on T2 weighting. This is not recommended for gynaecological RT planning scans, as a distended vaginal volume is not accurately reproduced at treatment. Fusion of planning CT and MRI scans led us to ponder if tampons do not reveal the true shape and extent of the anatomy. We theorised a method to improve visualisation of the true extent of the vaginal vault, without deforming the natural anatomy using MRI simulation for RT planning.

We modified the diagnostic opacification technique for use in MRI simulation with the alternative goal of delineation and visualisation without distension. The standard diagnostic 60cc of ultrasound gel, considered the correct amount for distension, was then tested in varying lesser amounts. Aquasonic 100® ultrasound transmission gel was warmed to 36ᵒC and delivered vaginally on the MRI couch by a Radiation Oncologist using a sterile 50ml catheter tip syringe pre imaging. During testing both CT + tampon and MRI + vaginal gel simulation scans were acquired for comparative qualitative imaging.

After trialling varying volumes of ultrasound gel, 10-15cc was found to be optimal for most patients (Fig. 1).

Fig. 1 Sagittal T2-weighted MRI comparing various gel volumes showing an excessive amount of gel (A), note deformation of natural anatomy, and optimal amount of gel (B) delineating full vaginal vault without distension.

We observed with 10-15cc of gel the superior aspect of the vaginal vault and cervix is well visualised on T2 imaging, whilst tending not to unfold the natural fornices of the collapsed vagina (fig.2).

Fig. 2 A 38 year old female with cervical squamous cell carcinoma. (A) Sagittal CT reconstruction with tampon in-situ. (B) Sagittal T2-weighted image with vaginal gel in-situ. (C) Axial CT with tampon in-situ. (D) Axial T2-weighted image with vaginal gel in-situ.

The adaption of this diagnostic technique has facilitated more accurate contouring of the natural vaginal anatomy in gynaecological cancers. It is also useful in visualising disease extension into vaginal canal, which may have otherwise been obscured by the tampon. We have ceased the second ionising CT planning scan for gynaecological patients and removed the need for tampon insertion at the RT simulation session. We feel this is an example of the direct clinical benefits of increased integration of MRI simulation into routine RT clinical practice.