PMC8576335_01
Chest CT images. (A) At baseline (July 2020), CT indicated a 2.7 cmx1.7 cm nodule in the left interpectoral region, and a 3.5 cmx3.0 cm mass in the left internal mammary area. (B) After two cycles of chemotherapy (September 2020), CT indicated stable disease (SD) with a decreased nodule size (2.4x1.5 cm) and a slight increase in the mass size (4.5x3.3 cm). (C) After four cycles of chemotherapy (November 2020), CT indicated SD with a decreased nodule size (2.2x1.2 cm) and mass (4.4x3.1 cm). (D) After six cycles of chemotherapy (February 2021), CT indicated SD with a decreased nodule size (1.5x0.7 cm) and mass (3.6x3.1 cm). (E) After surgery, E1 shows the left interpectoral node shrinkage to 1.2x0.8 cm in size, and E2, E3, E4 show appearance of multiple left pleural metastatic nodules.
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In July 2020, 2.7 years after the PMRT, chest computed tomography (CT) for routine examination revealed a 2.7 cmx1.7 cm nodule in the left interpectoral region, and a 3.5 cmx3.0 cm mass in the left internal mammary area ( Figure 1 ). CT evaluation showed stable disease (SD) after 2, 4, and 6 cycles ( Figure 1 ). In May 2021, her chest CT showed that the left interpectoral node shrank to 1.2x0.8 cm in size; however, multiple left pleural metastatic nodules appeared ( Figure 1 ).
Representative histologic images of rhabdomyosarcoma. (A) Malignant mesenchymal tumor, mainly spindle cells, clear cytoplasm or acidophilic tumor cells, severe atypia, large and hyperchromatic nuclei, irregular karyotype, and visible mitotic image. (B) MyoD1 (+) by immunohistochemistry. (C) Myogenin (focal +) by immunohistochemistry.
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Microscopically, the lesion consisted of spindle malignant cells, and IHC revealed that the tumor was positive for desmin (focal +), myoD1 (+), myogenin (focal +), Ki-67 (40%), and CK (focal +), and negative for CK7, SMA, ER, and PR, indicating rhabdomyosarcoma ( Figure 2 ).
Timeline of historical and current treatments.
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The timeline of her historical and current treatments is illustrated in  Figure 3 .
The dose distribution of PMRT the patient received for breast cancer. Upper image, horizontal view; left image, sagittal view; and right image, coronal view.
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Her PMRT treatment plan was retrieved, and the unintentional mean radiation dose to the left internal mammary region, where the rhabdomyosarcoma originated, was estimated as 32.5 Gy in 15 fractions ( Figure 4 ).