Biologically adaptive radiation oncology in oligometastatic and oligoprogressive disease: MR- and CBCT-guided...

Oligometastatic disease (OMD) and oligoprogression are clinically actionable states in which local treatment of limited tumour burden can prolong disease control for selected patients. Randomised phase II trials support metastasis-directed therapy (MDT), usually stereotactic body radiotherapy (SBRT/SABR), in several settings, but the effect is not uniform across tumour types. SABR-COMET and two non-small-cell lung cancer (NSCLC) trials showed progression-free or overall-survival gains, whereas NRG-BR002 did not improve progression-free or overall survival in newly oligometastatic breast cancer. In oligoprogression, CURB improved progression-free survival in NSCLC but not breast cancer. These divergent results make biological and treatment-context selection more important than lesion count alone. Biologically adaptive radiation oncology extends adaptation beyond setup correction by linking anatomy-of-the-day, molecular imaging, tumour phenotype, and response to target definition, dose, fractionation, and timing. This narrative review critically compares conventional image-guided SBRT, cone-beam CT (CBCT)-guided online adaptive radiotherapy, MR-guided online adaptive radiotherapy, PET/PSMA-guided planning, biology-guided radiotherapy, dose painting, and radiomics/AI. CBCT-guided adaptation is positioned as an increasingly available bridge between conventional image guidance and MR-guided adaptation. The practical value of MR guidance is greatest when soft-tissue visibility, motion, or daily organ-at-risk proximity would otherwise prevent safe ablative dosing; biology-guided delivery, biomarker-triggered adaptation, predictive AI, and radioligand-MDT combinations remain investigational. A clinical decision framework is proposed to separate approaches ready for routine use from selectively useful and research-only strategies.