Adaptive-Driven CT Simulation-Free Soft Tissue Stereotactic Body Radiation Therapy: A Single-Patient Case Report

Online adaptive radiotherapy (ART) enables accommodation for variations in patient setup and anatomical changes, allowing for fractional replanning for target coverage, organ at risk (OAR) sparing, and application of CT simulation-free (SF) workflows. SF workflows bypass the conventional simulation CT scan at the potential trade-off in dosimetric uncertainty. ART can alleviate many of these uncertainties, and this work extends previous experience with an Ethos adaptive cone-beam computed tomography (CBCT)-based SF process to treating a unique bony and soft tissue case with stereotactic body radiation therapy (SBRT). The patient is an 83-year-old male with metastatic prostate cancer, presenting with metastases near the right posterior ischium and a right perirectal lymph node. The patient's history includes multiple radiation treatments and androgen deprivation therapy (ADT). Rising prostate-specific antigen(PSA) levels and new metastases identified via positron emission tomography (PET)/CT prostate-specific membrane antigen (PSMA) led to SBRT re-irradiation, considered safe due to the time lapse since previous treatments. Using a HyperSight-equipped Ethos ART system, an SF SBRT workflow utilized the patient's recent PET/CT images for target and OAR delineation. A nine-field adaptive intensity-modulated radiotherapy(IMRT) treatment plan was generated to deliver 3600 Gy in three fractions with a primary focus to limit the dose to proximal OARs and the previously treated region. At the adaptive treatment, the patient is positioned based on anatomical marks, and axial images from HyperSight CBCT are used to contour the OARs and targets. These modified contours accommodate daily variations and are used to recalculate the reference plan and generate a new adapted plan. The adapted plan is selected if coverage improvement and OAR sparing are achieved. For each newly adapted plan, Ethos-generated synthetic CT is reviewed prior to treatment to verify no errors occurred in the deformable propagation between the reference image and the fractional CBCT. For this patient, the adapted plan was selected for all fractions due to improved target coverage, particularly of the soft tissue target, and OAR sparing. The patient tolerated the treatment well and demonstrated a good response on three-month follow-up PSMA PET/CT imaging. This case highlights the efficacy of CBCT-driven SF ART in complex re-irradiation scenario. Future enhancements in the Ethos treatment planning system, including direct dose computation on HyperSight CBCT images, will streamline SF workflows and expand their applicability. Careful consideration of potential on-unit OAR changes and target motion remains crucial for successful SF ART applications.