You will be presented with the full text of a biomedical research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
tumor - to - tumor metastasis , although uncommon , is not as rare as once thought . since berent 's first documentation of a case in 1902 , over 150 cases
the most commonly reported recipient tumor is a renal cell carcinoma   and there are several theories to support this phenomenon . pulmonary carcinoma is noted to be the most frequent donor tumor   but other tumors are also known to metastasize , including breast , gastrointestinal , prostate and thyroid cancers .
we report an unusual case of tumor - to - tumor metastasis that involves a benign recipient tumor .
a 67-year - old caucasian female presented for evaluation of a right chest wall mass 19 years after a modified radical mastectomy for a stage iib hormone - receptor positive invasive ductal carcinoma .
a pet - ct was performed , which demonstrated a round enhancing 1.8  1.5 cm right subpectoral lymph node with fdg uptake and a maximum suv of 4.0 suspicious for metastatic disease ( figs  1 and 2 ) . additionally , an incidental 4.1  3.4 cm mass within the midpole of the left kidney was heterogeneously enhancing with increased fdg uptake and a maximum suv of 8.6 suspicious for a renal cell carcinoma or less likely a transitional cell carcinoma ( figs  3 and 4 ) . 

an ultrasound guided core biopsy of the subpectoral lymph node verified metastatic carcinoma of breast origin , confirmed by positive immunostaining for mammaglobulin , e - cadherin and estrogen receptors ( fig . 
figure  5:microscopic findings of the subpectoral lymph node : h&e stain ( a , b ) ; e - cadherin and p120 dural stain to confirm the ductal nature of the cancer ( c ) ; er staining ( d ) .
microscopic findings of the subpectoral lymph node : h&e stain ( a , b ) ; e - cadherin and p120 dural stain to confirm the ductal nature of the cancer ( c ) ; er staining ( d ) . with the relatively small burden of metastatic breast disease , surgical management for the renal mass was deemed appropriate and she underwent a left total nephrectomy without complications .
the kidney measured 10.5  7.5  5.5 cm and was distorted by a 6.0  5.5  4.0 cm tan - brown mass with a pseudocapsule at the inferior pole ( fig . 
the final pathology demonstrated a 2 cm metastatic mammary carcinoma within a 6 cm angiomyolipoma , confirmed by staining with melan a. all the margins were clear and the renal parenchyma was uninvolved .
the metastatic focus was composed of cytologically atypical glands with intraluminal blue mucin , morphologically similar to the subpectoral lymph node biopsy .
the metastatic cells also stained positive for the estrogen receptor , mammaglobulin and gcdfp-15 , confirming the tumor to be of breast origin ( fig . 
figure  8:microscopic findings of left nephrectomy : h&e stain ( a , b ) , with insert in b demonstrating melan a stain to confirm the aml ; mammaglobin stain ( c ) ; estrogen receptor stain ( d ) , with insert demonstrating gcdfp15 stain .
microscopic findings of left nephrectomy : h&e stain ( a , b ) , with insert in b demonstrating melan a stain to confirm the aml ; mammaglobin stain ( c ) ; estrogen receptor stain ( d ) , with insert demonstrating gcdfp15 stain .
the patient did well post - operatively and was discharged home in a timely fashion after her nephrectomy . at her
7 month follow - up , she continues to take anti - estrogen hormonal therapy and has no demonstrable progression of disease .
she has been offered surgical excision of the subpectoral lymph node , but declines at this time .
with the improvement in cancer care and increased life expectancy in the us , tumor - to - tumor metastasis may become a more frequently encountered clinical scenario .
first defined tumor - to - tumor mestastasis in 1968 with the following criteria : ( i ) documentation of more than one primary tumor ; ( ii ) the recipient tumor is a true neoplasm ; ( iii ) the second neoplasm is a true metastasis with established growth or invasion in the host and not a result of contiguous growth or embolization of tumor cells ; and ( iv ) tumors that have metastasized to the lymphatic system , where lymphoreticular malignant disease already exists , are excluded .
renal cell carcinomas are overwhelmingly the most common recipient tumor involved in tumor - to - tumor metastasis .
two theories supporting this phenomenon include : ( i ) the rich vascularity of the kidney and renal cell carcinomas and ( ii ) the high lipid and glycogen content of renal cell carcinomas , which make them a favorable , nutrient - rich environment for metastasis .
although tumor - to - tumor metastasis has been reported with benign recipient tumors , the occurrence is less common .
recipient tumors that have been reported include endocrine adenomas , angiomyolipomas , meningiomas [ 8 , 9 ] and uterine leiomyomas .
in such cases , lung and breast cancers proved to be the most common donor tumor .
our current case , however , involves metastasis of a breast carcinoma to a benign renal angiomyolipoma , a scenario that was not diagnosed or expected pre - operatively , but one that significantly affects her prognosis .
only one other such case has been reported in a patient with tuberous sclerosis complex and multiple renal angiomyolipomas with a distant history of breast cancer .
the occurrence of tumor - to - tumor metastasis remains a rare phenomenon that may become a more common clinical entity with improvements in health care in developed countries and the increasing life expectancy , proving the point that a distant history of cancer should always raise suspicion of a recurrence .
this case also reminds us of the enigmatic nature of breast cancer and its behavior : although metastatic , it can be indolent and slow - growing to allow detection before local symptoms arise .
tumor - to - tumor metastasis is a clinical entity that has been documented in medical literature as early as the 1900s . 
 it is rare and published primarily as case reports , although its incidence seems to be on the rise as a result of improving medical care in developed countries . 
 we report an unusual case of metatstatic breast tumor to a benign renal tumor , in a patient with a remote history of breast cancer . 
 this case reinforces in our minds the enigmatic nature of breast cancer and its wide spectrum of clinical behavior .