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Magnetic Resonance Imaging-Guided High-Dose Rate Brachytherapy for Cervical Cancer.

Introduction Cervical cancer represents 0.8% of all new cancer cases and in 2012 there was an estimated 249,512 women living with cervical cancer in the United States. Concurrent chemoradiation followed by brachytherapy represents the standard of care in patients with International Federation of Gynecology and Obstetrics stage IB2 to IVA tumors (tumors larger than 4 cm or with parametrial involvement). During the first phase of treatment, patients receive external beam radiotherapy (EBRT) to the pelvis specifically targeting the cervix, uterus, and regional lymphatics. Patients typically receive 45-50.4 Gy with concurrentweekly cisplatin over 5weeks. This is followed by brachytherapy where applicators are placed in close proximity to the tumor typically via an intracavitary approach for temporary loading of a radioactive source. Brachytherapy plays a crucial role in the management of invasive cervical cancer. It has a rapid dose fall off allowing the tumor to receive a high dose while relatively sparing nearby structures such as the bladder, sigmoid, and rectum. Brachytherapy delivery options have evolved over time and can be delivered through multiple (typically 5) outpatient procedures using a high–dose rate source or during 2 inpatient stays using a low–dose rate source. By taking advantage of the geometric andphysical properties of brachytherapy, the central principle is to escalate the tumor dose to a curative rangewhileminimizing the dose and toxicity to the surrounding organs at risk (rectum, bladder, and sigmoid). Over time with advances in imaging, brachytherapy treatment planning has evolved tomeet these goals. In the early era, brachytherapy was planned using 2-dimensional images (Fig. 1). Defined points in space were used to evaluate the quality of brachytherapy implants and to estimate the tumor dose and dose to organs at risk. With the widespread use of computed tomography (CT) within radiation oncology departments, CT-based planning (Fig. 2) is now feasible. The