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Magnetic resonance imaging (MRI)-compatible combined intracavitary/interstitial (IC/IS) applicators are commonly used in the image-guided adaptive brachytherapy (BT) treatment of locally advanced cervical cancer (LACC) and primary/recurrences vaginal tumours. But all the potential coverage allowed by IC/IS applicators is scarce if there is: medial or distal parametrial extension, unresponsive bulky disease, cervical tumors with vaginal extension of the middle or lower third, stage IV, unfavorable topography, very narrow vaginas or total/subtotal hysterectomy. In these cases, it is recommended to add a larger interstitial component using transperineal templates.

Our institution designed in 2013 an MRI-compatible applicator combining an IC component (intrauterine tandem) with a perineal template and Titanium needles. It is an attempt to combine the technical advantages of the MUPIT (Elekta AB, Stockholm, Sweden) and the imaging advantages of MRI-based while preserving the stability, geometry, and robustness of the implant and including an intrauterine component to provide a central BT dose. Pre-planning techniques are not frequently employed in BT. Potential reasons of this are the possible anatomic changes that are produced after the applicator insertion (uterus straighten).

The purpose of this work is to present the noticeable benefice of the implementation of a preplanning technique using a multi-interstitial perineal and endocavitary MRI gynecological brachytherapy in-house developed applicator.

A pre-BT MRI T2 acquisition is performed with the template (without intrauterine tandem and interstitial needles but with the vaginal cylinder) in place 3-5 days before the BT implant. The required needles and their depths are selected to encompass the CTV (as conformal as possible). To facilitate this task, a Java based application linked to the TPS (Oncentra Prostate v 4.3 Elekta) has been developed. An applicator library has been specifically developed for this applicator, using free available software. The anchor points are three inserted A-vitamin pellets. The main inconvenient of preplan is the lack of an IC/IS component doing that the position of the uterus varies in the majority of the patients being straight after the insertion of the intrauterine tube. To evaluate the benefit of the preplan, implant coverage for a group of patients in which it was used was compared with another group in which it was not.

Forty three implants with preplan have been compared with 132 without it, in patients treated of LACC and primary/vaginal recurrences (Table 1). The D90 in CTV obtained were 80.4 Gy and 75.93 Gy respectively.

A virtual MRI preplanning has been presented. Although a statistical study has not been carried out, the data show an improvement in the D90 obtained in the patients in whom the preplan has been carried out, due to the better distribution and depth of the needles used.