Lymphedema is an accumulation of lymphatic fluid that causes swelling, usually, but not exclusively in the limbs. 
 
It's a side effect associated with surgical removal of lymph nodes, or after radiation therapy to lymph nodes. It may also occur as a result of tumors blocking the flow of lymphatic fluids. 

"Lymphedema is categorized as either primary or secondary.
 
Primary lymphedema is caused by congenital absence or abnormalities of lymphatic tissue and is relatively rare. 
  
Secondary lymphedema is generally caused by obstruction or interruption of the lymphatic system, which usually occurs at proximal limb segments (i.e., lymph nodes) due to infection, malignancy, or scar tissue (see table below).[1] 
 
The pelvic and inguinal groups of nodes in the lower extremities and the axillary nodes of the upper extremities are the primary sites of obstruction."  ~ cancer.gov

Risk Factor

	Radiation therapy to the axilla following axillary node dissection increases the incidence of lymphedema.  Approximately 41% (range, 21%-51%) of patients who underwent axillary radiation and surgery,  compared with 17% (range, 6%-39%) of those receiving axillary surgery without radiation.[6]  The extent of axillary dissection also increases the risk of lymphedema.  cancer.gov
	Nodal dissection of axillary, inguinal, or pelvic regions.   "Arm edema occurs in 20 to 30 per cent of patients who undergo axillary lymph node dissection ... "  ncbi.nlm.nih.gov
	Radiation therapy of axillary, inguinal, or pelvic regions, or supraclavicular (mantle field) radiation.
	Scarring of the left or right subclavian lymphatic ducts and veins by either surgical or radiation procedures.  cancer.gov
	Advanced cancer causing bulky lymphadenopathy of the anterior cervical, thoracic, axillary, pelvic, or  abdominal nodes.  cancer.gov
	Intrapelvic or intra-abdominal tumors that involve or directly compress lymphatic vessels and/or the  cisterna chyli and thoracic duct.  cancer.gov
	Poor nutritional status and obesity, that may lead to delayed wound healing, which in turn is an  important risk factor for the development of lymphedema.[9-11]  cancer.gov

Recommended resources 

	Lymphedema PDQ  cancer.gov
	The Lymphedema Program  Cancer Supportive Care  (John P. Cooke, MD, PhD and Gary Abrams, MD) Comprehensive site
	Treatment for Lymphedema  Cancer Supportive Care