The application of PET-CT to post-mastectomy regional radiation therapy using a deformable image registration

BACKGROUND To evaluate the utility of the preoperative PET-CT using deformable image registration (DIR) in the treatment of patients with locally advanced breast cancer and to find appropriate radiotherapy technique for further adequate treatment of axillary nodal area. METHODS Sixty-five breast cancer patients who had level II, III axillary or supraclavicular lymph node metastasis on ¹⁸F-FDG PET-CT and received postoperative radiotherapy after modified radical mastectomy were enrolled. One radiation oncologist contoured normal organs (axillary vessels, clavicular head, coracoids process and humeral head) and involved lymph nodes on PET-CT and simulation CT slices. After contouring, deformable image registration of PET-CT on simulation CT was carried out. To evaluate the performance of the DIR, Dice similarity coefficient (DSC) and Center of mass (COM) were used. We created two plans, one was the historically designed three field plan and the other was the modified plan based on the location of axillary lymph node, and we compared the doses that irradiated the axillary lymph nodes. RESULTS The DSCs for axillary artery, axillary vein, clavicular head, coracoids process and humeral head were 0.43 ± 0.15, 0.39 ± 0.20, 0.85 ± 0.10, 0.72 ± 0.20 and 0.77 ± 0.20, respectively. The distances between the COMs of axillary artery, axillary vein, clavicular head, coracoids process and humeral head in simulation CT and from PET-CT were 13.0 ±7.1, 20.2 ± 11.2, 4.4 ± 6.3, 3.7 ± 6.7, and 9.5 ± 25.0 mm, respectively. In the historically designed plan, only 57.7% of level II lymph nodes received more than 95% of prescribed dose and the coverage was improved to 70.0% with the modified plan (p < 0.01). For level III lymph nodes, the volumes received more than 95% of prescribed dose were similar in both plans (96.8 % vs 97.9%, p = 0.35). CONCLUSION Deformable image registration of PET-CT on simulation CT was helpful in the identification of the location of the preoperatively involved axillary lymph node. Historically designed three-field plan was not adequate to treat the axillary level II lymph node area. Novel treatment technique based on the location of axillary lymph node from PET-CT using DIR can result in more adequate coverage of nodal area.