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Radiation treatment planning (RTP) is based on the clinical stage at diagnosis and the pathological stage after surgery in postoperative radiotherapy for patients with breast cancer who undergo neoadjuvant chemotherapy (NAC). However, adopting initial PET-CT images for RTP is challenging because of image registration inconsistency due to different acquisition conditions. We compared relevant clinical parameters in between patients who underwent PET-CT under the same conditions as those at the RTP (group A) and those who did not (group B).

Dose-volume histogram parameters of targets and organs at risk were evaluated for thirty consecutive cases from May 2019 to November 2020, in which PET-CT imaging was performed under the same postural conditions as for radiotherapy (flat panel on the patient back and both upper arms raised, group A), and thirty consecutive cases before then (group B). Regions of interests were generally set according to the RTOG contouring guideline, with appropriate individual modifications. All primary tumors and lymph node metastases considered to be positive at initial PET-CT before NAC were included in the target with ANACONDA deformable image registration technique. Prescribed doses were 50 Gy in 25 fractions or 42.56 Gy in 16 fractions.

Both groups were well-balanced in age, BMI, laterality, clinical/pathological T stages, and pathological N stage without significant difference in clinical N stage. Among the targets, favorable coverages in CTVs and PTVs were observed in the group A, and statistically significant improvement was seen especially in CTV and PTV of axillary lymph nodes. Such advantage was not observed for supraclavicular nodes or internal mammary nodes. We did not find any meaningful difference in sparing the risk organs such as lung, heart, LAD, oesophagus, trachea, thyroid gland, or spinal cord within two groups.

RTP by using PET-CT under the same conditions as treatment planning CT can improve the coverage of targets, affected axillary lymph nodes at diagnosis, in particular. Proposed “direct fusion” approach can contribute to avoid insufficient axillary management due to ostensible downstaging after NAC.