Radiobiology — MCQs

Radiobiology Indian Medical PG Practice Questions and MCQs

Question 101: Which compound is not considered as a radiosensitizer?

A. Hyperbaric oxygen
B. Misonidazole
C. Amifostine (Correct Answer)
D. Idoxuridine

Explanation: ***Amifostine*** - **Amifostine** is a **radioprotector**, meaning it selectively protects healthy cells from the damaging effects of radiation, rather than enhancing radiation's effect on tumor cells. - It works by being dephosphorylated by alkaline phosphatase in normal tissues to an active thiol metabolite that scavenges **free radicals** generated by radiation. *Hyperbaric oxygen* - **Hyperbaric oxygen** increases the amount of dissolved oxygen in tissues, which is a potent **radiosensitizer**. - **Oxygen** enhances the formation of **free radicals** and fixes radiation-induced DNA damage, making tumor cells more susceptible to radiation. *Misonidazole* - **Misonidazole** is a **hypoxic radiosensitizer** that mimics oxygen, becoming relatively toxic under hypoxic conditions (common in tumors). - It forms **free radicals** and binds to cellular macromolecules when reduced by nitroreductases in hypoxic cells, thereby enhancing the effects of radiation. *Idoxuridine* - **Idoxuridine** is a **halogenated pyrimidine** that acts as a radiosensitizer by being incorporated into DNA in place of thymidine. - This incorporation sensitizes the DNA to radiation by making it more susceptible to **strand breaks** and other damage.

Question 102: Ionizing radiation acts on tissue leading to

A. Ionization of electrons from orbit (Correct Answer)
B. Thermal injury
C. Linear acceleration injury
D. Formation of pyrimidine dimer

Explanation: ***Ionization of electrons from orbit*** - **Ionizing radiation** is defined by its ability to eject electrons from atoms, creating **ions** and free radicals. - This process directly damages cellular components, including **DNA**, leading to biological effects. *Thermal injury* - **Thermal injury** is caused by heat and is not the primary mechanism of damage from ionizing radiation. - While high doses of radiation can cause local heating, the characteristic damage of ionizing radiation is through **ionization**, not heat. *Linear acceleration injury* - **Linear acceleration injury** refers to trauma caused by rapid changes in speed, often associated with motor vehicle accidents. - This is a form of **mechanical trauma** and is unrelated to the effects of ionizing radiation. *Formation of pyrimidine dimer* - **Pyrimidine dimers** are formed primarily by **ultraviolet (UV) radiation**, not ionizing radiation. - UV light causes **covalent bonds** between adjacent pyrimidine bases in DNA, leading to mutations.

Question 103: Cancer patient undergoes radiotherapy, pick the true statement for radiosensitivity of tissues?

A. GI mucosa is one of the most radioresistant tissues in the body
B. Rapidly dividing cells are resistant to radiation
C. Small blood vessels are radiosensitive tissues (Correct Answer)
D. The intensity of radiation is inversely proportional to the square of distance from the source

Explanation: ***Small blood vessels are radiosensitive tissues*** - **Endothelial cells** lining small blood vessels are actively dividing and have a relatively short lifespan, making them susceptible to radiation-induced damage. - Damage to these vessels can lead to **fibrosis** and **tissue hypoxia**, contributing to late radiation effects. *GI mucosa is one of the most radioresistant tissues in the body* - The **GI mucosa** is one of the **most radiosensitive** tissues due to its rapid cell turnover and high proliferative capacity, leading to symptoms like mucositis and diarrhea after radiation. - The constant renewal of epithelial cells makes them highly vulnerable to the **cytotoxic effects** of radiation. *Rapidly dividing cells are resistant to radiation* - **Rapidly dividing cells**, such as those in the bone marrow, GI crypts, and hair follicles, are generally **most sensitive** to radiation. - This forms the basis of radiation therapy, as cancer cells often have a higher proliferative rate than many normal tissues, making them a target. *The intensity of radiation is inversely proportional to the square of distance from the source* - This statement describes the **inverse square law** for radiation intensity, which is true, but it is a principle of radiation physics, not a statement about the radiosensitivity of tissues. - The inverse square law dictates how radiation dose **attenuates with distance**, which is critical for dose calculation but not directly about biological radiosensitivity.

Question 104: Which of the following is most radioresistant?

A. Cartilage (Correct Answer)
B. Ewing's sarcoma
C. GIT epithelium
D. Gonadal tumours

Explanation: ***Cartilage*** - **Cartilage** is a connective tissue with a relatively **low metabolic rate** and **avascular nature**, making its cells (chondrocytes) less susceptible to rapid turnover and DNA damage from radiation. - Its **dense extracellular matrix** and limited cellular division contribute to its inherent resistance to ionizing radiation, requiring higher doses to induce significant damage. *Ewing's sarcoma* - **Ewing's sarcoma** is a highly **malignant bone tumor** that is generally considered **radiosensitive** and often treated with radiation therapy. - Its cells are rapidly dividing, making them more vulnerable to the DNA-damaging effects of radiation. *GIT epithelium* - The **gastrointestinal tract (GIT) epithelium** is characterized by **rapid cell turnover** and high mitotic activity to constantly replace damaged cells and absorb nutrients. - This high proliferative rate makes the GIT epithelium highly **radiosensitive**, leading to common side effects like mucositis and diarrhea during radiation therapy. *Gonadal tumours* - Tumors of the **gonads** (e.g., testicular seminoma, ovarian dysgerminoma) are often highly **radiosensitive** and respond well to radiation therapy due to the germ cell origin and rapid proliferation of tumor cells. - The germ cells themselves are very sensitive to radiation, leading to concerns about **fertility preservation** in patients undergoing treatment.

Question 105: Which of the following cells are most radiosensitive?

A. Cells of hematopoietic series (Correct Answer)
B. Fibroblasts
C. Erythrocytes
D. Cells of erythroblastic series

Explanation: ***Cells of hematopoietic series*** - Among the **hematopoietic cells**, **lymphocytes** are the **most radiosensitive cells** in the human body, susceptible to damage at doses as low as 2 Gy. - The hematopoietic series includes highly radiosensitive cells like lymphocytes, bone marrow stem cells, and various precursor cells that are actively dividing. - According to the **Bergonié-Tribondeau law**, cells that are rapidly dividing, undifferentiated, and have a long mitotic future are most radiosensitive. *Cells of erythroblastic series* - While **erythroblasts** are radiosensitive due to their high proliferation rate, they are **not the most radiosensitive** cells. - **Lymphocytes** (part of the broader hematopoietic series) are significantly more radiosensitive than erythroblastic cells. - Erythroblastic cells require higher radiation doses for damage compared to lymphocytes. *Fibroblasts* - **Fibroblasts** are relatively radioresistant because they are generally quiescent with low mitotic activity. - Their primary function in producing **collagen** and structural components means they are less affected by radiation unless the dose is very high. *Erythrocytes* - **Mature erythrocytes (red blood cells)** are highly radioresistant because they are terminally differentiated, anucleated, and no longer divide. - They lack the cellular machinery for division and repair that would make them susceptible to radiation damage.

Question 106: The least radiosensitive tissue is:

A. Thyroid
B. Kidney
C. Nervous tissue (Correct Answer)
D. Bone

Explanation: ***Nervous tissue*** - **Nervous tissue** is considered the least radiosensitive due to its **highly differentiated state**, low mitotic activity, and limited capacity for cell division. - Mature neurons are post-mitotic and therefore much less susceptible to radiation-induced cell death compared to rapidly dividing cells. *Thyroid* - The **thyroid gland** is relatively radiosensitive, particularly in children, and exposure to radiation can increase the risk of developing **thyroid cancer** or hypothyroidism. - It contains cells with moderate mitotic activity and is an **endocrine gland** with active metabolic processes sensitive to radiation damage. *Kidney* - The **kidney** is moderately radiosensitive, with radiation damage potentially leading to **renal fibrosis**, impaired kidney function, and hypertension. - While cells do not divide rapidly, they are susceptible to damage from radiation, which can affect the **glomeruli** and tubules. *Bone* - **Bone tissue**, especially red bone marrow within the bone, is considered radiosensitive due to the presence of **hematopoietic stem cells** with high mitotic activity. - Radiotherapy to bone can lead to **osteoradionecrosis** and damage to blood-forming elements within the marrow.

Question 107: Which of the following is/are most radioresistant?

A. Neurons
B. Muscle cells
C. Erythrocytes (Correct Answer)
D. All of the options

Explanation: ***Erythrocytes*** - Erythrocytes are **anucleated** and terminally differentiated cells, meaning they do not divide. Cells that do not divide are generally **radioresistant**. - Their primary function is oxygen transport, and they have a limited metabolic capacity, making them less susceptible to the genetic damage that typically leads to radiation-induced cell death. *Neurons* - While neurons are **post-mitotic** and generally radioresistant compared to rapidly dividing cells, they are still more susceptible than mature erythrocytes. - High doses of radiation can lead to neuronal damage and death through mechanisms like **apoptosis** and indirect effects from damage to surrounding glial cells and vasculature. *Muscle cells* - Muscle cells (myocytes) are **terminally differentiated** and have a low mitotic rate, making them relatively radioresistant. - However, they are still more sensitive to radiation than erythrocytes, and high doses can cause muscle degeneration and fibrosis. *All of the options* - This option is incorrect because while neurons and muscle cells are relatively radioresistant, **erythrocytes are demonstrably the most radioresistant** among the choices due to their complete lack of a nucleus and inability to divide.

Question 108: Which of the following is a stochastic effect of radiation?

A. Alopecia in the irradiated portal
B. Local desquamation in the irradiated field
C. Genetic mutation (Correct Answer)
D. All of the options

Explanation: ***Genetic mutation*** - **Stochastic effects** are those for which the probability of occurrence, rather than the severity, is a function of radiation dose without a threshold. **Genetic mutations** are a classic example, as any dose carries some probability of inducing a change in DNA that can be passed to offspring. - The severity of a genetic mutation, if it occurs, is independent of the dose. It's the chance of it happening that increases with exposure. *Alopecia in the irradiated portal* - **Alopecia** (hair loss) due to radiation is a **deterministic effect**, meaning there is a threshold dose below which it does not occur, and above that threshold, the severity increases with dose. - It occurs locally in the **irradiated field** because it is a direct tissue reaction to cellular damage. *Local desquamation in the irradiated field* - **Desquamation** (skin peeling) is a **deterministic effect** that results from direct cell death and damage in the skin, a tissue reaction with a dose threshold. - Its occurrence and severity are directly related to the **radiation dose received** in the specific area. *All of the options* - This option is incorrect because **alopecia** and **desquamation** are deterministic effects, not stochastic effects. - Only **genetic mutation** falls under the category of stochastic effects among the choices provided.

Question 109: Most sensitive structure in the cell for radiotherapy is

A. Mitochondrial membrane
B. Enzymes
C. Cell membrane
D. DNA (Correct Answer)

Explanation: ***DNA*** - **DNA** is the most sensitive structure to radiotherapy because radiation primarily induces damage through **direct ionization and free radical formation**, which critically affects **DNA integrity**. - Damage to **DNA** can lead to **strand breaks, base modifications, and cross-links**, ultimately impairing cell division and triggering **apoptosis** or **reproductive cell death**. *Mitochondrial membrane* - While radiation can damage mitochondrial membranes, leading to **oxidative stress** and release of pro-apoptotic factors, it is less critical for immediate cell survival compared to **DNA**. - **Mitochondrial damage** often contributes to the overall cell death pathway but is not the primary target for the cytotoxic effects of radiation. *Enzymes* - **Enzymes** can be damaged by radiation, leading to a loss of catalytic activity, but the cell has mechanisms to repair or replace damaged enzymes. - While enzyme damage can disrupt cellular processes, it is usually not the direct cause of cell death unless essential enzymes involved in **DNA repair** or *cell cycle regulation* are severely compromised. *Cell membrane* - The **cell membrane** can be damaged by radiation, affecting its permeability and signaling, but this damage is generally less detrimental and more repairable than **DNA damage**. - Significant cell membrane damage usually requires higher doses of radiation and is often secondary to more fundamental damage within the cell.

Question 110: Osteoradionecrosis occurs at a dose of:

A. > 10 Gy
B. > 70 Gy
C. > 30 Gy
D. > 60 Gy (Correct Answer)

Explanation: ***> 60 Gy*** - **Osteoradionecrosis (ORN)** is a severe complication of radiation therapy characterized by devitalized bone tissue that fails to heal over a period of 3-6 months in the absence of tumor recurrence. - The critical threshold for ORN development is **> 60 Gy**, as this dose causes significant damage to bone vascularity and osteocytes, leading to impaired healing capacity and potential bone necrosis. - Risk factors include mandibular location, poor oral hygiene, dental extractions post-radiation, and cumulative doses exceeding this threshold. *> 10 Gy* - A radiation dose of **10 Gy** is too low to cause osteoradionecrosis as it does not produce sufficient vascular and cellular damage to bone tissue. - Such doses are typically used in palliative radiation therapy where the goal is symptom relief rather than definitive treatment. *> 30 Gy* - While **30 Gy** represents a moderate radiation dose, it remains below the established threshold for significant ORN risk. - Though some soft tissue changes may occur at this dose level, the vascular compromise and osteocyte death required for ORN typically require higher cumulative doses. *> 70 Gy* - While ORN certainly occurs at doses **> 70 Gy**, this represents the higher end of the risk spectrum rather than the threshold dose. - The established threshold in radiobiology literature is **60 Gy**, with risk increasing progressively beyond this point.

Practice by Chapter

Cellular Effects of Radiation

Practice Questions

Radiation-Induced DNA Damage

Practice Questions

Cell Survival Curves

Practice Questions

Radiation Effects on Normal Tissues

Practice Questions

Acute Radiation Syndrome

Practice Questions

Late Effects of Radiation

Practice Questions

Radiotherapeutic Ratio

Practice Questions

Fractionation in Radiotherapy

Practice Questions

Oxygen Effect and Radiosensitizers

Practice Questions

Radiation Carcinogenesis

Practice Questions

Radiation in Pregnancy

Practice Questions

Biological Dosimetry

Practice Questions

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