It can look like a simple bump or bruise, a complaint of a bellyache or pain in the bones. But it persists.
The symptoms of childhood cancer can seem like ordinary ailments, but when a young boy or girl isn't getting better and there's no reason why, it's time to see a doctor, according to Dr. Teresa York, the University of Maryland Children's Hospital interim division head of pediatric hematology and oncology.
It's estimated that about 11,630 children under the age of 15 will be diagnosed with cancer in the United States this year, according to the American Cancer Society. It's rare, comprising less than 1 percent of cancers diagnosed annually.
"It comes out of nowhere for parents and families," said York, who is also a University of Maryland School of Medicine assistant professor of pediatrics. "It's awful. It's so shocking, and a lot of times that's the hard part of it. "
And because young children can't always express what hurts -- and because kids bleed and bruise just from being kids -- it can be hard to know when it's time to take symptoms seriously.
"In this day and age, we're all working moms," York said, "and you take care of your kids, and you don't run them to the doctor for every little thing they have. If they're not getting better, I would take them in."
That's the best rule of thumb: If symptoms persist without an explanation for their cause, it's important to get the child checked out. However, preventative care appointments play an important role in ensuring a child is healthy because there isn't a catch-all screening for childhood cancer.
"For melanoma, you're checking your moles," York said. "For breast cancer, you're doing your mammogram. There really isn't any screening process for children except their wellness visits."
Due to medical advances, more than 80 percent of children diagnosed with cancer survive five years or more. This is substantially higher than in the mid-1970s when the five-year survival rate was less than 60 percent, according to the American Cancer Society.
And the causes of childhood cancers aren't really known. A small portion of cases can be attributed to causes such as Down Syndrome, and chromosomal and genetic abnormalities.
There are several main types of childhood cancers.

About 1/3 of childhood cancers are leukemia, which are blood cell cancers that start in blood-forming tissue like bone marrow. Acute lymphoblastic leukemia is the most common in children, according to the National Cancer Institute, which is when too many immature white blood cells are found in the blood and bone marrow.
Symptoms: Fatigue, bone and joint pain, bleeding or bruising, pale skin, fever, weight loss, weakness and more, according to the American Cancer Society.


Brain and central nervous system tumors
Brain and central nervous system tumors are the second-most-common cancers, comprising about 25 percent of childhood cancers, according to the American Cancer Society.
Symptoms: Headaches, nausea, vomiting, blurred or double vision, dizziness and trouble walking and/or handling objects.

Neuroblastoma arises in immature nerve cells that are found in an embryo or fetus, according to the American Cancer Society. They make up about 6 percent of childhood cancers, typically occurring in infants and young children.
Symptoms: These vary widely and are dependent on the size and location of the tumor. The most common is an unusual lump or mass, generally in the abdomen that causes it to swell, and a child may complain of a bellyache. The pressure from the tumor can cause problems with urination, diarrhea or constipation. If the disease has spread to the bones, a child may complain of bone pain and refuse to walk, according to the American Cancer Society.

Wilms tumor
Starting in one or, rarely, both kidneys, Wilms tumor comprises about 5 percent of childhood cancers and is generally found in children 3 to 4 years of age, according to the American Cancer Society.
Symptoms: Fever, pain, nausea and/or poor appetite.

Lymphoma is cancer in the lymphocytes, which are certain cells of the immune system. The disease typically grows in the lymph nodes or lymph tissue, such as in the tonsils, but can affect bone marrow and other organs.
There are two main types: Hodgkin lymphoma and non-Hodgkin lymphoma, according to the American Cancer Society.
Hodgkin lymphoma is a cancer of the immune system that is marked by the presence of certain cells called the Reed-Sternberg cell. It makes up about 4 percent of childhood cancers and is more common in those 15 to 40 years of age (especially those in their 20s) and those older than 55.
Symptoms: Lumps under the skin in the neck, under arm or groin; fever; drenching night sweats; weight loss and more.
Non-Hodgkin lymphoma is identified by the presence of lymphocyte-predominant cells, which are different from the ones found in Hodgkin disease, according to the National Cancer Institute.
It comprises about 4 percent of childhood cancers, generally occurring in younger children.
Symptoms: Dependent on the location of the cancer but include weight loss, fever, sweats and fatigue. Lumps that are actually swollen lymph nodes can be found under the skin, in the neck, armpit or groin, according to the American Cancer Society.

Occurring in the soft tissue of certain muscles, rhabdomyosarcoma makes up about 3 percent of childhood cancers. It can be found in the head and neck, groin, abdomen, pelvis or in the arms and legs, the American Cancer Society's website states.
Symptoms: Dependent on where the cancer forms. Swelling that looks like a lump that may be painful; bleeding; blood in the urine; and more.

Cancer of the eye, called retinoblastoma, accounts for about 2 percent of childhood cancers and is generally found in those about 2 years old.
Symptoms: The eye looks unusual. When a light is shined in the eye, the pupil typically looks red, but it looks white or pink for those who have retinoblastoma.

Bone cancer
Bone cancers typically occur in older children, and there are two main types: osteosarcoma and ewing sarcoma, according to the American Cancer Society.
The former is more common, comprising about 3 percent of childhood cancers and forms in spots where the bone grows quickly, such as in the legs or the arms.
Symptoms: Bone pain at night or during physical activity and swelling around the bone.

Ewing sarcoma
Ewing sarcoma makes up about 1 percent of childhood cancers and commonly occurs in the pelvis, ribs or shoulder blades or in the middle of the long leg bones. It can form in the bone or in soft tissue, according to the National Cancer Institute.
Symptoms: Bone pain; lump in the arms, legs chest or pelvis; and more.