7.1 Anatomy, Physiology, and Pathophysiology Foundations

Pediatric hematology/oncology nursing rests on a few shared biologic facts: where blood is made at each age, which lineage a blast belongs to, how neutrophils are counted, why a toddler's immune system is naïve, why the brain is a sanctuary, why childhood tumors melt with chemotherapy and then dump potassium, and where each classic cancer goes when it leaves the primary site. CPHON Test Content Outline (TCO) II.A.1–2 items test this map. They do not ask you to recopy the ALL, neuroblastoma, or osteosarcoma chapters. They ask whether fever, pallor, limp, a belly mass, or a mediastinal crunch land in marrow, solid tumor, or airway.

Hematopoiesis moves with age

Blood production is not a single organ for the entire pediatric life span. In the fetus, the yolk sac, then the liver and spleen, carry hematopoiesis. Infants still use hepatic and splenic hematopoiesis as a backup, so hepatosplenomegaly in a neonate with leukemia is both infiltration and a still-active blood factory. After birth, marrow takes over. In young children, red marrow fills the long bones—femur, tibia, humerus—which is why a preschooler with leukemia presents with a limp or night bone pain: packed metaphyses hurt. By adolescence, red marrow recedes from the distal extremities toward the axial skeleton: vertebrae, pelvis, ribs, sternum, and skull. An adolescent with marrow disease may have back pain and pelvic discomfort more than a toddler's refusal to walk, although both can limp.

When you teach a family why a 3-year-old's marrow aspirate still comes from the posterior iliac crest even though the pain is in the shin, the answer is access and safety, not that long-bone marrow has vanished. Extramedullary hematopoiesis therefore means different things by age. Splenomegaly in an infant with leukemia is expected geography. The same spleen in a teenager still signals infiltration or sequestration, but it is no longer a primary blood factory. A 2-month-old with circulating blasts and a liver that fills the abdomen is not automatically a second solid tumor. Image to exclude a mass, but teach residual infant hematopoiesis first.

Lineage: myeloid versus lymphoid

Hematopoietic stem cells branch. The lymphoid lineage produces B cells, T cells, and natural killer cells—the cells of acute lymphoblastic leukemia (ALL) and most pediatric lymphomas. The myeloid lineage produces neutrophils, monocytes, erythrocytes, and platelets—the cells of acute myeloid leukemia (AML), and the cells you count when you calculate an ANC. Flow cytometry, not a guess from the CBC, assigns lineage. A high white-cell count is not a lineage. A mediastinal mass is a T-lymphoid clue, not a diagnosis. Nursing implication: infection risk after lymphoid-directed therapy is still neutrophil-driven, because neutrophils are myeloid cells even when the cancer is lymphoid. Filgrastim, when a protocol uses it, supports the myeloid pool; it does not treat lymphoblasts.

ANC: the number that drives fever teaching

Absolute neutrophil count (ANC) = white blood cell count × (percent segmented neutrophils + percent bands). Convert percentages to decimals. A child with WBC 2,000/µL, 15% segs, and 5% bands has ANC 2,000 × 0.20 = 400/µL. That child is severely neutropenic. A WBC of 2,000 with 80% segs is an ANC of 1,600—not the same story. Bands count because they are circulating neutrophils. Monocytes, lymphocytes, and blasts do not go in the ANC. Fever during neutropenia is an emergency because a naïve or suppressed myeloid pool cannot mount pus. Teach the family the number and a written fever threshold, not a color-coded app they cannot read at 02:00.

Work a second example at the chairside so the arithmetic sticks. A 6-year-old after delayed intensification has WBC 1,200/µL, 20% segs, and 5% bands: ANC = 1,200 × 0.25 = 300/µL. If that child is 38.3°C at midnight, the emergency department is the plan, not acetaminophen and a morning clinic call.

Developmental immune naïveté

Term infants have maternal immunoglobulin G, which wanes over the first months of life. Young children have limited antigen experience, incomplete vaccine series, and lymphoid tissue that enlarges easily. After chemotherapy, they lose both innate neutrophil defense and adaptive memory. Functional asplenia, central lines, and mucosal injury stack on that naïveté. The CPHON point is not a textbook immunology lecture: a 2-year-old on induction is not a small adult with a low WBC. Household live vaccines, daycare exposures, and a history of ordinary runny noses are different risk after myelosuppression. Pneumocystis prophylaxis with trimethoprim-sulfamethoxazole on ALL protocols exists because cellular immunity is young and then chemically stripped.

Blood-brain barrier and why IT therapy exists

The blood-brain barrier (BBB) limits many systemic drugs, including several leukemia agents at ordinary doses. The CNS and, in boys, the testes are sanctuary sites: places where blasts survive while the blood looks better. That is the rationale for intrathecal (IT) chemotherapy—typically methotrexate, with cytarabine and a corticosteroid as protocols specify—delivered into cerebrospinal fluid at lumbar puncture. High-dose methotrexate and dexamethasone also help because they penetrate better, but they do not replace scheduled IT doses. You do not skip IT because the child looked well or because the peripheral blast count fell. You also do not lumbar-puncture a child with a posterior fossa mass and obstructive hydrocephalus; that is a herniation rule. The shared science is sanctuary anatomy, not nurse preference.

Tumor kinetics: why pediatric cancers melt and then lyse

Many pediatric malignancies are embryonal or hematopoietic: neuroblastoma, Wilms tumor, hepatoblastoma, medulloblastoma, rhabdomyosarcoma, ALL, Burkitt lymphoma. They have a high growth fraction—a large share of cells are cycling. High growth fraction means two nursing facts at once. First, chemotherapy sensitivity: cycle-active drugs kill a larger fraction per dose than they do in many indolent adult carcinomas. Second, tumor lysis syndrome (TLS) risk: rapid kill dumps potassium, phosphate, and uric acid, and phosphate binds calcium. Start hydration and uric-acid control (allopurinol or rasburicase per protocol) before the first cytoreductive doses when disease is bulky or hematopoietic. Lactate dehydrogenase (LDH) elevation flags turnover; it is not a diagnosis. Osteosarcoma is a relative exception in the kinetics story—still treated with intensive chemotherapy, but TLS is not its signature emergency the way new Burkitt lymphoma or hyperleukocytic ALL is.

A 7-year-old starting ALL induction with a high uric acid and bulky nodes needs TLS precautions on admission, not after day-8 counts. Looking better on day 3 does not cancel electrolyte checks.

Metastasis patterns you must not mix

Relapse and staging geography is disease-specific. Teach the pattern, then stop before restating entire protocols.

• ALL: marrow, CNS, and testis (boys). Isolated CNS or testicular relapse can occur with marrow still in remission.
• Neuroblastoma: bone, marrow, liver, and skin (blueberry-muffin cutaneous nodules in infants); orbital metastases make raccoon eyes. Lungs are less classic than in Wilms tumor or osteosarcoma.
• Osteosarcoma: lung first; skip metastases in the same bone.
• Wilms tumor: lung is the dominant distant site.

Mixing these is a scored error. A new pulmonary nodule in osteosarcoma is expected geography. Raccoon-eye bruising with an abdominal mass is neuroblastoma, not Wilms lung disease. A painless testicular enlargement during ALL maintenance is sanctuary-site relapse until proven otherwise, not a sports hernia.

Presentation: map the first sentence to a compartment

Fever, pallor, bruising, petechiae, and limp map to marrow failure or marrow occupation—leukemia, marrow metastases, aplastic anemia. The limp is metaphyseal pressure, not a sprain. Lymphadenopathy plus fever can still be leukemia or lymphoma; if nodes are bulky and the mediastinum is wide, think T-ALL or lymphoblastic lymphoma and protect the airway. Superior vena cava signs, orthopnea, and a child who will not lie flat are a mediastinum emergency, not a good moment for unplanned sedation. A painless abdominal mass maps to a solid tumor (Wilms tumor, neuroblastoma, hepatoblastoma) until imaging assigns the organ. Do not deeply palpate a suspected Wilms tumor. Opsoclonus or raccoon eyes with a belly mass map to neuroblastoma biology, not a second disease chapter here.

Walk a 4-year-old with fever, pallor, and refusal to walk toward a CBC and marrow pathway. Walk a 3-year-old with a bath-time flank mass toward imaging and a hands-off belly. Walk a 14-year-old who cannot lie flat toward mediastinal precautions. That triage is the shared pathophysiology exam.