Synchronous multiple cancers are defined as a cancer diagnosed simultaneously with another cancer or within a time frame of 6 months. The presence of two synchronous cancers in one patient is not an uncommon scenario. The risk for a second synchronous cancer at diagnosis of breast cancer is approximately 2–3%. The most common synchronous and metasynchronous malignancies after breast cancer are malignancies of the thyroid gland and the female genital tract [
4]. Synchronous cancers of the breast and the kidney occur occasionally. There are no common risk factors in carcinogenesis, except patient age. Increased detection of such synchronous and concomitant cancers may also be due to improved imaging, biochemical work-up and increased life expectancy. Sonography of the abdomen can yield incidental diagnoses of abdominal masses such as RCC. On the other hand, RCCs most often occur as a comalignancy. As many as 16–27% of patients with RCC have other synchronous malignancies [
2,
5]. In contrast to synchronous cancers, the phenomenon of cancer-to-cancer metastasis is very rare with less than 50 cases described in the literature and most of these metastases were found at autopsy [
1]. For successful metastasizing, a cancer requires different characteristics, such as the ability of invasion and dissemination and the appropriate tissue to thrive and grow. It is known that different cancers metastasize to different tissues or organs and this tissue-specific pattern of spread is called tropism [
6]. Breast cancer commonly spreads to bone, the lungs, the brain and if abdominal metastasis occur then most often in the liver and/or spleen. Breast cancer metastasizing to the kidneys is almost nonexistent, so the high blood circulation due to the rich vasculature will not be the only decisive factor for metastasis to RCC. Nevertheless, the most frequently described recipient tumor for cancer-to-cancer metastasis is RCC, which is found in up to 65% of the cases with cancer-to-cancer metastasis [
3]. The well-preserved stroma, a high content of glycogen and lipid-rich cells and the lower immunological competence of RCC may be more responsible for this predisposition [
1,
2,
5]. The metastatic features of RCC and breast cancer, especially of metastasis to the bone, are similar. Therefore, this may be another factor that favors coexistence [
7].
A PubMed literature search revealed nine cases of metastasis of a breast cancer into RCC. The six cases reported in which tumors were resected during the lifetime and the present case are presented in detail in Table
1. The present case is one of three reported cases with simultaneous metastasis from breast cancer into RCC. A high breast cancer stage (T2 or more) seems to be responsible for the metastasis. The hormone receptor status was variable among cases. The HER-2 status was negative in the previous series, if specified. In the present case the HER-2 status was positive and both breast cancer tumors had Ki67 levels of over 70%. This case is the only one in a patient with bilateral breast cancer. In contrast to the large breast cancer metastasis described by Begara Morillas et al. [
11]and the multiple foci in the other cases, only a small metastasis (0.5 cm) was found in the RCC. Mastectomy was performed in nearly all cases. Information on the outcome of the reported cases was not available in all of the cases but palliative therapy determined the outcome [
1,
8‐
12].
Table 1
Survey of case reports on breast cancer metastasis to renal cell carcinoma with diagnosis during life
Begara Morillas et al. [ 11] | 4 | 50 | No | No | No | No | No | No | No | RCC, bone | Alive 6 months after nephrectomy |
Van Wynsberge et al. [ 10] | 6 | 64 | Ductal | T3N1M0 | 2 | − | − | No | No | RCC, lung, liver, and bones | Not specified |
| 2 | 62 | Ductal | T4N3M0 | 3 | + | + | − | No | RCC, pleura and scalp | Death 10 months after nephrectomy |
| 4 | 43 | Ductal | T2N0M1 | no | + | + | − | 20% | RCC, liver, mediastinum | After 3 months disease progression—further treatment was refused |
| 0 | 60 | Ductala | T4N2M1 | 3 | + | + | − | 22% | RCC | Without recurrence for 18 months |
| 0 | 49 | Ductal | T4N1M1 | 3 | − | − | − | No | RCC, lung and bone | Not specified |
Present case | 0 | 79 | Ductal bilateral | Right: T2N3M1 | 3 | + | + | + | 80% | RCC and bone | Death 3 years after diagnosis by multimorbidity |
| | | | Left: T2N0M1 | 2 | + | + | + | 70% | | |
The literature data and this case illustrate that screening for metastases in patients presenting with cancer may lead to the detection of secondary cancers. Suspected secondary lesions should be biopsied to distinguish between metastatic and synchronous cancers. Even if finding a cancer-to-cancer metastasis is a seldom event, it should be considered especially in high stage cancer. In the present patient the finding of breast cancer metastasis in RCC was clinically important to define treatment (curative vs. palliative) goals. Finally, investigation of cancer-to-cancer metastases, even if rare, could provide clues about tumor biology and behavior. Research in this special area could sustain the development of new therapies or detection methods for metastases.