Can initial diagnostic PET-CT aid to localize tumor bed in breast cancer radiotherapy: feasibility study using deformable image registration

BACKGROUND Localization of the tumor bed of breast cancer is crucial for accurate planning of boost irradiation. Lumpectomy cavity and surgical clips provide localizing information about tumor bed. However, defining the tumor bed is often difficult because of presence of unclear lumpectomy cavity and lack of certain information such as absence of surgical clips. In the present study, we evaluated the feasibility of initial diagnostic PET-CT in localization of the tumor bed using deformable image registration (DIR). METHODS We selected twenty-five patients who had an initial diagnostic PET-CT performed and underwent breast-conserving surgery with surgical clips in tumor bed. In every individual patient, two target volumes were separately delineated on planning CT; 1) target volume based on surgical clips with a margin of 1 cm (TV(clip)) and 2) tumor volume based on 90% of maximum SUV on PET-CT registered by DIR (TV(PET)). The percent of TV(PET) in TV(clip) (V(in)) was calculated and distance between center points of two volumes (D(center)) was also measured. RESULTS Mean D(center) between two volumes was 1.4 cm (range, 0.33-2.53). Mean V(in) was 94.8% (range, 60.9-100) and 100% in 18 out of 25 patients. When compared to the center of TV(clip), the center of TV(PET) tended to be located posteriorly (mean 0.3 cm, standard deviation 0.6), laterally (mean 0.3 cm, standard deviation 0.8) and inferiorly (mean 0.4 cm, standard deviation 0.9). CONCLUSION Initial diagnostic PET-CT can be one of the possible references to localize the tumor bed in breast cancer radiotherapy.