The usual breast cancers are non-special type carcinomas and invasive lobular carcinomas or mixed subtypes and represent the majority of malignant breast tumors. The most common benign breast tumors are fibroadenoma and intraductal papilloma. We will focus on rarer breast tumors and will look in detail at some of them: angiosarcoma of the breast, usually affecting the dermis but occasionally developing in the breast parenchyma., desmoid tumor, phyllodes tumors, granular cell tumors, malignant adenomyoepithelioma, and finally hemangioma. For each of these tumors, we will look in detail at clinical cases with illustrations in mammography/ultrasound, breast MRI +/- PET scan, their histopathological features as well as the key points of their treatment options. 

Phyllodes tumors

• These are fibroepithelial tumors^1-3 with a rare prevalence (0.3 to 1% of breast tumors and < 3% of fibroepithelial tumors). The incidence is about 2.1 cases per million/year, with a peak of incidence between 45 – 49 yo (35 – 55 yo).
• Clinically, this type of tumor is often a painless mass rapidly increasing in size (Size between 4 and 7 cm).
• There are 3 different prognostic grades : Benign = grade 1 : 60-70%; Intermediate = grade 2 : 7-25% and Malignant = grade 3 = phyllodes sarcoma : 8-20%
• They are present in Li Fraumeni Syndrome (TP 53 mutation).  MED12 gene mutations are described in fibroadenoma and phyllodes tumors (grades 1 and 2), whereas TERT alterations are found in phyllodes sarcoma +++ (never in fibroadenomas)
• The differential diagnoses of a well-circumscribed mass are described in Figure 1; and the different features distinguishing phyllodes tumors from fibroadenomas^4,5 are summarized in Figure 2. 
• The following figures 3-8 show imaging in different modalities and histopathological correlation of the different grades of phyllodes tumors.
• With regard to prognosis, local recurrence occurs mostly in the first 2 years, in 15% of cases of all types of phyllodes tumors. It increases according to grade (Figure 9).
• Treatment^6-8 is summarized in Figure 10.

Desmoid tumors

• These are intra or extra-abdominal monoclonal fibroblastic proliferations, very rare in breast (incidence of 5-6 cases per million per year for all sites). The prevalence is < 0,03% of breast tumors and they occur in young women between 20 – 40 yo.
• There is no specific presentation. The evolution of the tumor is unpredictable : stability, decrease or increase in size. There is no metastatic risk but local recurrence.
• 85% of desmoid tumors are sporadic : a beta-catenin mutation in the tumor is found in 80% of cases (p.T41A mutation in the CTNNB1 gene). 15% are hereditary : abnormality is present in all cells and in the tumor : APC (adenomatous polyposis coli) gene mutation, FAP
• The etiologies⁹ and differential diagnoses are presented in Figure 11.
• The following Figures 12-13 show imaging in different modalities^10,11 and histopathological correlation.
• Treatment^12-15 is summarized in the Figure 14 and an example of cryotherapy is shown in Figure 15.

Angiosarcoma

Angiosarcoma^16-17 is a rare malignant mesenchymal tumor originating from breast vascular tissue. It represents < 5% of all sarcoma and 0,004 to 1% of malignant breast tumors. Most often, it is a superficial form :  purplish or erythematous skin patches or nodules, skin thickening or bruising. Rarely, it is located in deep-tissue with frequent involvement of the pectoral muscle.

• 80% are secondary to locoregional radiotherapy or chronic lymphedema; 8 - 10 years after initial cancer (>5 years) : cumulative dose > 40Gy / latency period from 3 to 12 years
• 20% are primary in young women aged between 20 – 40 yo, with no previous medical history, appearing as a large, rapidly-appearing palpable mass.
• A breast MRI is performed for locoregional extension and PET CT for distant metastasis extension.
• The following Figures 16-21 show imaging of superficial and deep angiosarcoma in different modalities and histopathological correlation
• The differential diagnoses are presented in Figure 22.
• Prognosis and treatment are summarized in Figure 23.

Granular cell tumor

• A Granular cell tumor^18,19 is a benign neuro-ectodermal tumor originating from perineural Schwann cells, also known as Abrikossoff’s tumor. It is a very rare tumor, benign in 99% of cases, and typically occurs in pre-menopausal women aged between 40 – 60 yo. It mimics classic invasive carcinoma so a microbiopsy is always needed.
• The following Figures 24-25 show imaging of granular cell tumors in different modalities and histopathological correlation.
• The treatment is a conservative surgery without axillary lymph node dissection, then standard follow-up. The prognosis is excellent.

Adenomyoepithelioma

• Adenomyoepithelioma²⁰ is a mixed tumor with bicellular ductal and myoepithelial proliferation, with no lymphatic tropism. It can occur from 20 to 80 yo and there is a past medical history of breast cancer in 20% of cases. Differential diagnoses are Intracanal papilloma and Tubular adenoma. It is mostly benign but can be malignant. There is a risk of local recurrence and metastases in lung, thyroid, bone, brain.
• The Figure 26 shows imaging of malignant adenomyoepithelioma in different modalities²¹.
• Histology and treatment are summarized in Figure 27.

Hemangioma

• Hemangioma^22;23 is a rare (0,4% of breast tumors) benign vascular breast tumor (≠ angiosarcoma). It is often discovered incidentally. There are several risk factors: hormone-dependent : pregnancy, oral contraception, HRT; family history of hemangioma; personal history of vascular pathology.
• The following Figures 28-30 show imaging of hemangioma in different modalities²⁴ and histopathological correlation.
• These tumors do not need any treatment, only standard monitoring. If the lesion is growing, it can be reviewed in tumor board and surgery is the reference treatment.