Stereotactic Body Radiotherapy Boost with the CyberKnife for Locally Advanced Cervical Cancer: Dosimetric Analysis and Potential Clinical Benefits

SIMPLE SUMMARY: The recommended treatment for locally advanced cervical cancer (LACC) consists of chemoradiotherapy (CRT) followed by brachytherapy (BT). Although BT is considered a minimally invasive procedure, patients still suffer severe discomfort from it and risk uterine perforation. Dosimetric uncertainties are often inevitable due to anatomical variations and inconsistencies in applicator loadings. These issues prompted us to explore the use of stereotactic body radiotherapy (SBRT) as a viable alternative. It has been well described that the CyberKnife (CK), a robotic image-guided SBRT delivery system, is capable of producing rapidly fall-off dose gradients with submillimeter accuracy. The aim of this study was to compare the dose distributions and radiobiological effects of a CK-based SBRT boost and a BT boost. We found a tumor volume threshold target, below which the CK-based SBRT plan could result in significantly better target coverage, OAR sparing and radiobiological effects compared to the BT plan. With improved precision of target localization, a reduced PTV margin might increase the eligibility of patients to receive a CK-based SBRT boost after CRT, rather than BT. CK-based SBRT could be an alternative option for patients who are not candidate for BT. ABSTRACT: (1) Aim: To compare the treatment plans of stereotactic body radiotherapy (SBRT) with CyberKnife (CK) and high-dose-rate (HDR) intracavitary/interstitial brachytherapy (IC/ISBT) and examine the feasibility of CK-SBRT as a viable alternative to BT in patients with locally advanced cervical cancer (LACC). (2) Methods: A BT plan of 28 Gy in four fractions delivered previously to 20 patients with LACC was compared with a CK plan based on the same CT images with structures delineation for BT. The SBRT treatment plan was further divided according to two different approaches, with the high-risk planning target volume (HR-PTV) defined by the high-risk clinical target volume (HR-CTV) without and with a 5 mm margin, which were named CK-CTV plan and CK-PTV plan, respectively. The dose distributions and dosimetric parameters of the target volumes and organs at risk (OARs) were recorded and compared for the three boost plans. Radiobiological metrics were calculated based on the EUD for the hybrid plans. Additionally, the relationship between tumor volume and tolerance doses for the OARs in the BT plan and CK-PTV plan was investigated. (3) Results: Target coverage was better with the CK plan than with the BT plan, as the D95%, D98%, HI and CI of the CK-CTV plan and CK-PTV plan were higher than those of the BT plan; an exception was the D50%. Similarly, the TCP of the target was also significantly in favor of the CK hybrid plans (p < 0.01). For the OARs, the CK-CTV plan was superior to the BT plan as regards the rectum D2cc, bladder D2cc and bladder Dmax. The CK-PTV plan could achieve dosimetric parameters comparable to those of the BT plan for OARs concerning the small residual tumor volume. The NTCP of the rectum for the WPI+CK-CTV plans was significantly lower than that of the WPI+BT plans (p < 0.01). (4) Conclusions: CK-based SBRT can achieve better target coverage, dose sparing for the OARs and radiobiological effects compared with the BT plan for tumors that are not excessively large. CK-based SBRT could be an alternative option to administer a radiation boost for patients with LACC.