Improving CTVboost delineation after preoperative systemic therapy in breast cancer using deformable PET/CT registration

<h2>Abstract</h2><h3>Background</h3> Delineating the CTV_boost in postoperative breast radiation therapy (RT) may be challenging after preoperative systemic therapy (PST), due to treatment-induced anatomical changes and the potential discordance between the postoperative surgical cavity, as identified by surgical clips, and the spatial extent of the pretreatment tumor. The objective of this study was to assess whether [¹⁸F]-FDG PET/CT (PET/CT)-guided delineation using deformable image registration (DIR) improves inter-observer reproducibility compared to conventional strategies relying primarily on surgical clips and anatomical landmarks. <h3>Materials and Methods</h3> Fifty-eight patients treated with PST, surgery, and postoperative RT were retrospectively included. Three radiation oncologists performed four delineation strategies per patient: (1) CTV_CLI, based on clips and anatomical landmarks; (2) CTV_PET, a 10 mm isotropic expansion around the PET-defined biological tumor volume (BTV); (3) CTV_COM, generated by a 15–20 mm isotropic expansion around the BTV centroid and adapted to pathological features; and (4) CTV_INT, manually refined volumes integrating clips, PET signal, and registration accuracy. Inter-observer variability (IOV) was assessed using the Dice similarity coefficient (DSC) and Hausdorff distance (HD). <h3>Results</h3> All patients underwent lumpectomy after PST, with a mean age of 53.9 years, and 44.2% had triple-negative tumors. The mean volume of the CTV_CLI was 34.4 cm³, compared to 36.5 cm³ for CTV_PET, 21.2 cm³ for CTV_COM, and 43.6 cm³ for CTV_INT. Mean DSCs were 0.86 for both CTV_PET and CTV_COM, 0.77 for CTV_INT, and 0.62 for CTV_CLI (<i>p</i> < 10^-8). <h3>Conclusion</h3> PET/CT-guided delineation after DIR significantly improves inter-observer reproducibility of CTV_boost definition after PST compared with conventional methods.