Treating a Refractory Locally Advanced Carcinoma of the Cervix With Cone-Beam Computed Tomography-Based Adaptive External Beam Radiotherapy: A Case Report.

Adaptive radiotherapy (ART) refers to methods that allow a radiation therapy plan to be adjusted based on images obtained during the treatment. Using cutting-edge imaging methods such as computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET), ART can adjust the treatment plan in response to observed changes in anatomy and even biology while the patient is receiving treatment. The backbone of ART is intensity-modulated RT (IMRT), which permits better sparing of normal critical organs while still delivering a uniform dose to target tumor volume. Volumetric modulated arc therapy (VMAT) is a more rapid form of IMRT with more conformity, which helps in treating patients in a shorter time. Different types of ART include individualized margins using an internal target volume (ITV) and offline and online methods. ITV uses the margin to appropriately cover the clinical target volume (CTV) based on matching CT scans to different extents of the radiological anatomy of the selected area. Offline adaptive strategies include scheduled replanning throughout the external beam radiotherapy (EBRT) course, depending on intra-fraction or inter-fraction changes. The online ART (oART) strategy takes into account changes in tumor volume and the daily anatomical variations of target volumes and organs at risk structures (OARS). As such, PTV margins have the potential to be reduced. Commercially available oART systems are predominantly MRI-guided, but more recent advances have seen the creation of a cone-beam CT (CBCT)-guided oART system. In this case of FIGO (International Federation of Gynaecology and Obstetrics) stage IIB squamous cell carcinoma of the uterine cervix, we used an offline ART approach to complete the initial part of the treatment, which included concurrent chemoradiation therapy with 50 Gy/25 Fr and weekly cisplatin for five weeks. However, in the final fraction of on-couch kilovoltage CBCT (kvCBCT), it appears that the tumor only partially responded, demonstrating its refractory nature to treatment. The patient then underwent a repeat planning contrast-enhanced CT (CECT) scan, which was fused with the initial planning CECT scan. It revealed that the tumor responded poorly, with only a slight decrease in size. With the OARS toxicity limit in mind, the patient was scheduled for an adapted volumetric modulated arc therapy (VMAT) boost of 8 Gy/4 Fr as a second-phase plan for the tumor. Subsequently, the patient was taken up for intra-cavitary brachytherapy (ICBT) after a one-week gap. She received brachytherapy with 9 Gy/session for two sessions as per institutional protocol on a weekly basis. On subsequent follow-up, the patient underwent a complete response clinico-radiologically, even after two years of follow-up. This case report shows the importance of adaptive radiotherapy in treating tumors with a high therapeutic ratio and less toxicity to OARS despite employing the less frequently used EBRT boost along with ICBT brachytherapy.