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

BACKGROUND

Delineating the CTVboost 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.

MATERIALS AND

METHODS

Fifty-eight patients treated with PST, surgery, and postoperative RT were retrospectively included. Three radiation oncologists performed four delineation strategies per patient: (1) CTVCLI, based on clips and anatomical landmarks; (2) CTVPET, a 10 mm isotropic expansion around the PET-defined biological tumor volume (BTV); (3) CTVCOM, generated by a 15-20 mm isotropic expansion around the BTV centroid and adapted to pathological features; and (4) CTVINT, 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).

RESULTS

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 CTVCLI was 34.4 cm3, compared to 36.5 cm3 for CTVPET, 21.2 cm3 for CTVCOM, and 43.6 cm3 for CTVINT. Mean DSCs were 0.86 for both CTVPET and CTVCOM, 0.77 for CTVINT, and 0.62 for CTVCLI (p < 10-8).

CONCLUSION

PET/CT-guided delineation after DIR significantly improves inter-observer reproducibility of CTVboost definition after PST compared with conventional methods.