Correct Answer: C. Neurons
Radiation sensitivity is directly proportional to the mitotic activity and inversely proportional to the degree of differentiation of cells. Neurons are post-mitotic, fully differentiated cells that have completed their division during embryogenesis and do not undergo mitosis in adult life. Therefore, they are inherently radioresistant and the least affected by ionizing radiation during prophylactic irradiation for hematopoietic stem cell transplantation. In contrast, rapidly dividing cells with high mitotic indices—such as bone marrow precursors, spermatogonia, and intestinal epithelial cells—are highly radiosensitive and suffer significant damage even at moderate radiation doses. This principle, known as the Bergonié-Tribondeau law, is fundamental to understanding radiation biology. During total body irradiation (TBI) or prophylactic CNS irradiation in ALL, the goal is to ablate hematopoietic and leukemic cells while minimizing late effects; neurons, being non-dividing, remain structurally intact despite radiation exposure, though functional CNS toxicity can occur through other mechanisms (vascular injury, glial damage) at higher doses.
Why the other options are wrong
A. Bone marrow/erythroid precursor cells — Bone marrow is one of the most radiosensitive tissues in the body due to continuous hematopoiesis and high mitotic activity of stem cells and progenitors. Erythroid precursors are particularly vulnerable, with LD50 for bone marrow being ~6–8 Gy. This is precisely why TBI is used for hematopoietic ablation before transplantation. These cells are among the first to be damaged, making this option incorrect. B. Spermatogonia — Spermatogonia are the stem cells of the testis and undergo continuous mitosis throughout adult life. They are highly radiosensitive; doses as low as 0.15 Gy can cause temporary azoospermia, and >6 Gy causes permanent sterility. In the context of prophylactic irradiation for ALL, testicular damage is a well-recognized late effect. This makes spermatogonia far more affected than neurons. D. Intestinal epithelial cells — Intestinal epithelium has one of the highest turnover rates in the body, with complete renewal every 3–5 days. Crypt cells are highly radiosensitive and are damaged at doses >2–3 Gy, leading to acute GI toxicity (mucositis, diarrhea) and late strictures. During TBI, GI toxicity is a major acute complication. These cells are far more radiosensitive than post-mitotic neurons.
High-Yield Facts
- Bergonié-Tribondeau law: Radiosensitivity is directly proportional to mitotic activity and inversely proportional to degree of differentiation.
- Neurons are post-mitotic: Adult neurons do not divide and are therefore inherently radioresistant; they are among the most radioresistant tissues in the body.
- Bone marrow LD50: ~6–8 Gy; hematopoietic stem cells and erythroid precursors are ablated during TBI for transplantation.
- Spermatogonia radiosensitivity: Temporary azoospermia at 0.15 Gy; permanent sterility at >6 Gy; major late effect in male ALL survivors.
- Intestinal crypt cells: Highly radiosensitive; acute GI toxicity occurs at >2–3 Gy; late strictures are common complications of TBI.
- CNS late effects in ALL: Despite neuronal radioresistance, prophylactic CNS irradiation causes cognitive impairment, secondary malignancies, and vascular injury through glial and endothelial damage, not neuronal death.
Mnemonics
RADIOSENSITIVE tissues (High Mitotic Index) Bone marrow, Gonadal tissue (spermatogonia), Gastrointestinal epithelium, Lens, Lymphocytes. Remember: Fast-dividing = Fast-dying from radiation. RADIORESISTANT tissues (Low/No Mitotic Index) Nervous system (neurons), Muscle (skeletal & cardiac), Bone (osteocytes), Connective tissue (fibroblasts). Remember: Mature & resting = Resistant.
NBE Trap
NBE pairs "prophylactic irradiation" with "least affected" to test whether students confuse acute radiosensitivity (bone marrow, GI) with radioresistance. Students may incorrectly assume that because CNS toxicity is a known late effect of ALL therapy, neurons themselves must be radiosensitive—missing the distinction that CNS damage occurs via vascular/glial injury, not neuronal mitotic death.
Clinical Pearl
In Indian pediatric oncology centers managing ALL, prophylactic CNS irradiation (18–24 Gy) is standard; while neurons survive the radiation, survivors develop cognitive decline and secondary CNS malignancies due to glial and vascular injury—a critical distinction when counseling families about late effects. Testicular shielding is routinely attempted in boys, but spermatogonia remain at high risk for permanent azoospermia.
_Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 9 (Environmental and Nutritional Pathology); Harrison's Principles of Internal Medicine, Ch. 384 (Oncology and Hematologic Malignancies); Guyton & Hall Textbook of Medical Physiology, Ch. 73 (Radiation and Radioactivity)_