Time Feasibility of Upfront "Brachy-like" SBRT Using Adapt-to-Shape Workflow with 1.5T MR-Linac for Cervical Cancer Definitive Radiotherapy
Abstract
Purpose
To evaluate the feasibility and adapt-to-shape (ATS) workflow efficiency of a novel UPFRONT "Brachy-like" Stereotactic Body Radiation Therapy (SBRT) technique delivered via MR-LINAC. This upfront boost aims to deliver a steep-gradient high dose to a "Virtual-Tandem Core PTV" (VTC-PTV) and a moderate dose to the HR-CTV to promote immunopotentiation and tumor reduction prior to conventional chemoradiotherapy and brachytherapy.
Methods
Five treatment-naïve patients with intact-uterus cervical cancers previously treated on a 1.5T MR-LINAC (Elekta Unity) were retrospectively analyzed to maintain clinical relevance to the intended "UPFRONT" workflow. An "UPFRONT" scheme was replanned on the original planning CT. A 2-cm diameter VTC-PTV was generated along the central HR-CTV core using an automated central spline function. A single-fraction simultaneous integrated boost was prescribed: 8Gy to the VTC-PTV and 3Gy to the HR-CTV. IMRT plans (12-beam template) were optimized to meet stringent OAR constraints: Bladder D2cc <3Gy and Rectum/Sigmoid D2cc <2.2Gy. ATS workflow was implemented offline, with two medical physicists and two radiation oncologists on the treatment planning console. Workflow metrics, including registration, contouring, and optimization times, were recorded. Dosimetric consistency between the initial planning CT and post-adaptive plans was evaluated.
Results
The median (range) adaptive workflow times were: total adaptive planning time 7.8 (5.4–10.9) min; registration 1.1 (0.5–3.1) min; contouring 4.3 (2.2–6.4) min; and optimization 1.5 (1.3–3.3) min. The median estimated delivery time was 10.3 (9.6–12.6) min. No statistically significant dosimetric differences were observed between planning CT and post-adaptive plans with pre-/post- adaptive dose for VTC-PTV Dmax 11.8(10.1-12.9)/11.1(10.9-12.7) Gy, D90% 8.2(6.2-8.9)/7.8(6.3-8.8) Gy; HR-CTV D90% 3.2(3.1-3.4)/3.2(3.0-3.5) Gy; D2cc Bladder 2.7(2.5-2.7)/2.7(2.6-2.7) Gy, Rectum 1.8(1.5-1.9)/1.7(1.4-1.8) Gy, and Sigmoid 1.7 (0.7-2)/1.6(0.5-1.9) Gy.
Conclusion
Upfront MR-guided "Brachy-like" SBRT for cervical cancer is clinically feasible within a highly efficient time frame. The workflow is particularly optimized in treatment-naïve patients, supporting its role as a potential induction therapy.