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Deformable Image Registration used to Localize Tumor Bed in Breast Cancer Radiotherapy

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Most recently, the method of deformable image registration (DIR) has been developed and tested in the hopes of providing more accurate image-guided radiotherapy and adoptive radiotherapy. DIR use in the localization of tumor bed may be possible employing preoperative imaging work-up without the need of additional study for the sole purpose of radiotherapy planning.

Therefore, the study aimed to assess the feasibility of initial PET-CT in localization of the tumor bed through the method of deformable image registration with planning CT.Whole breast irradiation after breast conserving surgery is one of the basic treatments for the patients with early-stage breast ct breast cancer

Following breast irradiation, it has been noted that significant improvement in local tumor bed control has sharply risen; yet attaining the desired level of control of localization in the tumor bed is vital.

CT-based simulation, in which lumpectomy cavity and surgical clips provide localizing information, has been used to determine whether the tumor bed had been improved. However, due to the poor visualization of lumpectomy cavity and absence of surgical clips, it is difficult to determine the tumor bed in some instances.

Radiation oncologists determine tumor bed by looking at surgical scar markings and preoperative images that include mammography, ultrasonography, and breast MR. However, there exists flaws in these methods as well, since surgical scars are not always representational of the exact location of the tumor bed, and the use of preoperative images differs from CT simulation, as there may be a geographic discrepancy in the estimation of tumor bed. Some studies suggest that breast MR coupled with CT offers more accurate results on tumor bed localization, however the option is not always made available.

Twenty-five patients who had received an initial diagnostic PET-CT and underwent breast-conserving surgery with surgical clips in tumor bed were selected for the study. In every single patient, two target volumes were separately outlined on planning CT; 1) target volume based on surgical clips with a margin of 1 cm (TVclip) and 2) tumor volume based on 90% of maximum SUV on PET-CT registered by DIR (TVPET). The percent of TVPET in TVclip (Vin) was determined and the distance between the two center points of the two volumes (Dcenter) was also taken into account.

Automatic rigid image registration (RIR) was the first step of DIR between diagnostic PET-CT and planning CT. After RIR, DIR was lead to the next step. The spatial discrepancy of nipple between the two images was measured in each step and compared to assess the performance of the DIR process of breast tissue.

For RIR, the distance between the nipples between PET-CT and planning CT ranged from 0.4 to 3.9 cm (average, 2.3) and following the DIR process it ranged from 0.0 to 3.4 cm (average, 0.8).Dcenter between two volumes was 1.4 cm (range, 0.33 – 2.53). Average Vin was 94.8 percent (range, 60.9-100) and 100 percent in 18 out of 25 patients.

When compared to the center of TVclip the center of TVPE tended to be located posteriorly (average 0.3 cm, standard deviation 0.6), laterally (average 0.3cm, standard deviation 0.8) and inferiorly (average 0.4 cm, standard deviation, 0.9).

Based on the results, researchers were able to conclude that with the use of DIR, initial diagnostic PET-CT can be a feasible indicator to localize the tumor bed in breast cancer radiotherapy. 

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