**2. Applicators for IGABT**

**1. Introduction**

90 Radiotherapy

of 33 months.

cols of MRI sequences [5].

that in institutions in other countries.

institutions.

Image-guided adaptive brachytherapy (IGABT) using magnetic resonance imaging (MRI) has been widely accepted as a novel treatment technique for gynecologic malignancies, especially for cervical cancer. In 2005, the primary concept of IGABT, which is high-risk clinical target volume (HR-CTV), was described by the Gynaeacologic (GYN) GEO-ESTRO working group [1]. Around the same time, the first clinical report on the effectiveness of MRI‐based IGABT was published by Pötter et al. [2]. In that study, 44 of 48 patients had stage IIB to IVA disease. Overall survival at 3 years was 61%, progression-free survival was 51%, and continuous complete remission for true pelvis was 85% with a median follow up

By using MRI or computed tomography (CT), three-dimensional (3D) description of the HR-CTV and the organs at risk (OAR), such as the bladder, rectum, sigmoid colon, and small bowel, can be achieved. Therefore, dosimetric evaluation for both the HR-CTV and OAR using dose volume histograms (DVH) can be performed with greater accuracy than that with traditional point A treatment planning using the definition of the ICRU report 38 for the OAR doses [3]. The GYN GEO-EATRO working group also published recommendations regarding the 3D dosimetric parameters [4]. Although various dosimetric parameters were described by GYN GEO-EATRO, HR-CTV D90 and OAR D2cc have been emphasized in clinical brachytherapy (BT). The working group also published recommendations for the acquisition proto-

Implementation of IGABT made it possible to use interstitial needles more safely. This is one of the most important points in performing IGABT, because the use of interstitial needles can change the dose distribution dramatically, especially in large tumors. In European institutions, MRI-based IGABT is a common technique and interstitial needles are frequently used. However, CT-based IGABT is most common in Japanese institutions and interstitial needles are less frequently used than in European institutions. In addition, 3D-conformal radiotherapy (3D-CRT) with a central shielding (CS) technique for the protection of OAR from higher doses has been applied as the standard external beam radiotherapy (EBRT) for many years at most Japanese institutions, even after the introduction of intensity-modulated radiotherapy (IMRT), which is performed without a CS technique at many European institutions. Therefore, radiotherapy (RT) for cervical cancer in Japan is somewhat different from

MRI-based IGABT for cervical cancer was initiated at Kobe University Hospital in September 2014 and 50 patients were treated with definitive IGABT through March 2016. Similar to other institutions in Japan, for EBRT, a 3D-CRT with a CS technique is still performed. Interstitial needles are applied for some patients with large tumors. The purpose of this chapter is to provide an overview of MRI-based IGABT and the introduction of the experience at Kobe University Hospital along with a comparison with European representative Applicators compatible with MRI must be used for MRI-based IGABT. Even if CT-based IGABT is performed, traditional stainless steel applicators should not be used because severe metal artifacts may occur when CT images are acquired. Tandem and ovoid applicators are used most frequently in Japanese institutions.

The Medical University of Vienna, the most representative institution in Europe, and a lot of other institutions in other countries have used Vienna applicators compatible with MRI that were developed for combined intracavitary (IC) and interstitial (IS) BT [6]. This applicator has a tandem and a ring part. The ring part includes the source pathway and it also has a templated function for titanium interstitial needle implantation. By setting the dwell point in the ring, a dose distribution the same as when using a tandem and ovoid applicator can be developed. Therefore, both MRIbased IC-BT alone and combined with IC- and IS-BT can be performed by using the Vienna applicator. Unfortunately, this applicator is not allowed to be used in Japan. In addition, only plastic or stainless steel needles are available for IGABT. When using stainless steel needles, MRI cannot be performed after needle implantation. CT-based IGABT, or acquisition of MR images before needle implantation and fusion of the MRI images to the CT images acquired after needle implantation, is one of the ways to deal with this problem. Plastic needles are also compatible with MRI; however, these are not suitable for hard tumors because of the dull edge. Metal stainless needles may help to create a pathway for plastic needles. After pathway preparation with metal stainless needles, implantation may be easier and acquisition of MR images can be achieved.

Jürgenliemk‐Schulz et al. investigated the potential benefit of newly designed tandem and ovoid applicators compatible with IC- and IS-BT using plastic needles [7]. They performed MRI-based IGABT using the applicators in six patients and reported that additional improvement was achieved with a combined IC/IS approach. The results are encouraging because the newly developed applicators for both IC/IS approaches may be used in many countries including Japan, resulting in significant progress in IGABT.
