Radiobiology — MCQs

Radiobiology Indian Medical PG Practice Questions and MCQs

Question 41: All of the following drugs are known for their radioprotective properties, EXCEPT?

A. Glucan
B. Etoposide (Correct Answer)
C. Amifostine
D. Sodium Selenite

Explanation: **Explanation:** The correct answer is **Etoposide**. Radioprotectors are compounds that, when administered before or during radiation exposure, reduce the damage to healthy tissues. **Etoposide** is a chemotherapy agent (a Topoisomerase II inhibitor) that acts as a **radiosensitizer**, not a radioprotector. It inhibits DNA repair and arrests the cell cycle, making cells *more* vulnerable to radiation-induced death. **Analysis of Options:** * **Amifostine (WR-2721):** This is the "Gold Standard" radioprotector. It is a sulfhydryl prodrug that scavenges free radicals produced by ionizing radiation. It is FDA-approved to reduce xerostomia in patients undergoing head and neck radiotherapy. * **Glucan:** This is an immunomodulator derived from yeast cell walls. It acts as a radioprotector by stimulating the recovery of the hematopoietic system (macrophages and neutrophils) after radiation injury. * **Sodium Selenite:** Selenium is a key component of the enzyme glutathione peroxidase. It acts as an antioxidant radioprotector by neutralizing reactive oxygen species (ROS) and has been studied for reducing side effects in pelvic and head/neck radiation. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mechanism of Radioprotectors:** Most work by scavenging free radicals (sulfhydryl groups) or by inducing hypoxia in healthy tissues. 2. **The "Oxygen Effect":** Radioprotectors are most effective against **Low LET** radiation (X-rays, Gamma rays) because these rely on indirect action via free radicals. 3. **Time of Administration:** Radioprotectors must be present in the system **at the time of or before** radiation to be effective. 4. **Amifostine Side Effects:** The most common dose-limiting side effect is **hypotension**; others include nausea and vomiting.

Question 42: Which of the following accounts for the destruction of rapidly growing cells by radiation therapy?

A. Cross-linking of DNA
B. Demethylation of DNA
C. Cleavage of DNA double strands
D. Disruption of DNA-RNA transcription complexes (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Disruption of DNA-RNA transcription complexes** **Mechanism of Action:** The primary goal of radiotherapy in oncology is to inhibit the proliferative capacity of malignant cells. While direct DNA damage occurs, the destruction of **rapidly growing cells** is specifically linked to the disruption of the **DNA-RNA transcription complex**. Rapidly dividing cells have high metabolic demands and are constantly undergoing transcription to produce proteins necessary for cell cycle progression. Radiation interferes with the RNA polymerase activity and the stability of the transcription bubble. This leads to "mitotic catastrophe" or apoptosis because the cell cannot synthesize the essential proteins required to sustain its rapid growth and division. **Analysis of Incorrect Options:** * **A. Cross-linking of DNA:** While radiation can cause intra-strand or inter-strand cross-links, this is more characteristic of **alkylating agents** (e.g., Cyclophosphamide or Cisplatin) rather than the primary mechanism of ionizing radiation. * **B. Demethylation of DNA:** This is an epigenetic modification. Radiation typically causes oxidative damage rather than targeted enzymatic demethylation. * **C. Cleavage of DNA double strands:** While double-strand breaks (DSBs) are the most **lethal** form of DNA damage caused by radiation, the question specifically asks what accounts for the destruction of *rapidly growing* cells. The immediate cessation of growth in hyper-metabolic cells is most acutely sensitive to the disruption of the transcription machinery. **NEET-PG High-Yield Pearls:** * **Law of Bergonie and Tribondeau:** Radiosensitivity of a tissue is directly proportional to the reproductive capacity (mitotic rate) and inversely proportional to the degree of differentiation. * **Most Radiosensitive Phase:** **M phase** (Mitosis), followed closely by G2. * **Most Radioresistant Phase:** **Late S phase** (due to high levels of DNA repair enzymes). * **Oxygen Enhancement Ratio (OER):** Radiation is more effective in the presence of oxygen because oxygen "fixes" the free radical damage to DNA, making it permanent.

Question 43: About Linear energy transfer, all statements are false except?

A. Low LET radiations are more lethal.
B. It is a property of the tissue.
C. Same as relative biological effectiveness.
D. High LET radiations have a low oxygen enhancement ratio. (Correct Answer)

Explanation: ### Explanation **Linear Energy Transfer (LET)** is the rate at which energy is deposited as an ionizing particle travels through matter (expressed in keV/μm). It is a crucial concept in radiobiology that determines the biological impact of radiation. **Why Option D is Correct:** The **Oxygen Enhancement Ratio (OER)** describes the phenomenon where cells are more sensitive to radiation in the presence of oxygen. * **Low LET radiation** (e.g., X-rays, Gamma rays) causes damage primarily through **indirect action** (free radical formation), which requires oxygen to "fix" the damage. Thus, they have a **high OER**. * **High LET radiation** (e.g., Alpha particles, Neutrons) causes **direct ionization** of DNA. Because the damage is so dense and direct, the presence or absence of oxygen makes little difference. Therefore, **High LET radiations have a low OER (approaching 1).** **Analysis of Incorrect Options:** * **A. Low LET radiations are more lethal:** Incorrect. High LET radiations are more lethal because they produce dense ionizations along a short track, leading to complex, irreparable double-strand DNA breaks. * **B. It is a property of the tissue:** Incorrect. LET is a **property of the radiation** type and its energy, not the medium it passes through. * **C. Same as relative biological effectiveness (RBE):** Incorrect. While RBE generally increases with LET (up to 100 keV/μm), they are different concepts. RBE is a ratio comparing the dose of a test radiation to a standard dose (250 kVp X-rays) required to produce the same biological effect. **High-Yield Clinical Pearls for NEET-PG:** * **LET of X-rays/Gamma rays:** ~3 keV/μm (Low LET). * **The "Overkill" Effect:** RBE increases with LET until it peaks at **100 keV/μm**. Beyond this, RBE decreases because energy is wasted (more than enough to kill the cell is deposited). * **Direct vs. Indirect Action:** High LET = Direct action; Low LET = Indirect action (dominant in clinical radiotherapy).

Question 44: Which of the following organs is highly susceptible to radiation-induced cancer?

A. Bone marrow (Correct Answer)
B. Thyroid
C. Bone
D. Brain

Explanation: ### Explanation The susceptibility of an organ to radiation-induced cancer depends on its **radiosensitivity**, which is governed by the **Law of Bergonié and Tribondeau**. This law states that tissues with high mitotic activity, a long dividing future, and a low degree of differentiation are the most sensitive to radiation. **1. Why Bone Marrow is Correct:** The **Bone Marrow** (specifically the hematopoietic stem cells) is one of the most radiosensitive tissues in the body. Because these cells are rapidly dividing and undifferentiated, they are highly prone to DNA damage and subsequent malignant transformation. Radiation exposure to the bone marrow is a primary risk factor for developing **Leukemia** (excluding Chronic Lymphocytic Leukemia). **2. Analysis of Incorrect Options:** * **B. Thyroid:** While the thyroid is sensitive to radiation (especially in children, leading to papillary carcinoma), it is considered "moderately" sensitive compared to the high-turnover cells of the bone marrow. * **C. Bone:** Mature bone consists of highly differentiated cells (osteocytes) with very low mitotic activity, making it **radioresistant**. Radiation-induced osteosarcomas are rare and usually require very high localized doses. * **D. Brain:** The adult brain consists of permanent cells (neurons) that do not divide. It is one of the most **radioresistant** organs in the adult body. **3. Clinical Pearls for NEET-PG:** * **Most Radiosensitive Cell:** Lymphocyte (Exception to the law: it is sensitive despite being a non-dividing cell). * **Most Radiosensitive Phase of Cell Cycle:** **M phase** (Mitosis), followed by G2. * **Most Radioresistant Phase:** **S phase** (DNA synthesis). * **Order of Sensitivity (High to Low):** Bone marrow > Gastrointestinal tract > Skin > Lungs > Liver > Muscle/Nerve. * **100% Fatal Radiation Dose (LD100):** Approximately 10 Gray (without medical intervention).

Question 45: Radiosensitizing substances include:

A. Oxygen
B. SR 250 8
C. Metronidazole
D. All of the above (Correct Answer)

Explanation: **Explanation:** Radiosensitizers are chemical agents that enhance the lethal effects of ionizing radiation on tumor cells. The correct answer is **All of the above** because each option represents a different class of radiosensitizing agents. 1. **Oxygen (Option A):** Oxygen is the most potent and universal radiosensitizer. It acts via the **"Oxygen Fixation Hypothesis,"** where oxygen reacts with free radicals produced by radiation to "fix" the damage into an irreparable form. Hypoxic tumors are significantly more resistant to radiation. 2. **Metronidazole (Option C):** This belongs to the **Nitroimidazole** group. These are "Oxygen Mimics" that simulate the effect of oxygen in hypoxic cells. They have a high electron affinity, allowing them to stabilize radiation-induced damage in the absence of actual oxygen. 3. **SR 2508 (Option B):** Also known as **Etanidazole**, this is a second-generation nitroimidazole. It was developed to be more effective and less neurotoxic than metronidazole or misonidazole, specifically targeting hypoxic tumor cells. **High-Yield NEET-PG Pearls:** * **Oxygen Enhancement Ratio (OER):** The ratio of the dose required to produce a biological effect in hypoxia vs. air. For X-rays, the OER is typically **2.5 to 3.0**. * **Other Radiosensitizers:** 5-Fluorouracil (5-FU), Cisplatin, Hydroxyurea, and Halogenated pyrimidines (e.g., BUdR, IUdR). * **Radioprotectors:** Conversely, substances like **Amifostine** (a sulfhydryl compound) protect normal tissues from radiation damage by scavenging free radicals. * **The 4 R’s of Radiobiology:** Repair, Reassortment, Repopulation, and Reoxygenation.

Question 46: What is considered a stochastic effect of radiation?

A. Genetic mutation (Correct Answer)
B. Radiation-induced myelitis
C. Mucosal enteritis
D. Alopecia

Explanation: ### Explanation Biological effects of radiation are classified into two categories: **Stochastic** and **Deterministic (Non-stochastic)** effects. #### 1. Why Genetic Mutation is Correct **Stochastic effects** are "probabilistic" in nature. They occur by chance, and their **probability** of occurrence increases with the radiation dose, but their **severity** is independent of the dose. There is **no threshold dose**; theoretically, even a single photon can cause a mutation. * **Genetic mutations** and **Carcinogenesis** (Cancer induction) are the hallmark examples of stochastic effects. If a germ cell DNA is damaged but not killed, it may result in hereditary disorders in offspring. #### 2. Why Other Options are Incorrect Options B, C, and D are **Deterministic effects**. These occur only after a specific **threshold dose** is exceeded. Once the threshold is crossed, the **severity** of the effect increases as the dose increases. * **Radiation-induced myelitis (B):** Damage to the spinal cord occurs only after high-dose radiotherapy (usually >45-50 Gy). * **Mucosal enteritis (C):** Inflammation of the gut lining occurs due to the killing of rapidly dividing stem cells after a specific dose. * **Alopecia (D):** Hair loss is a threshold-dependent event; temporary hair loss occurs at ~3 Gy, while permanent loss occurs at ~7 Gy. #### 3. Clinical Pearls for NEET-PG * **Mnemonic for Stochastic:** **S**tochastic = **S**ecrecy (you don't know if it will happen) and **S**everity is constant. * **Thresholds:** Stochastic effects have **Zero threshold** (Linear No-Threshold model). Deterministic effects have a **Clear threshold**. * **Teratogenic effects:** While most are deterministic (e.g., microcephaly, organ malformation), the induction of childhood leukemia in utero is considered **stochastic**. * **High-Yield Example:** Cataract was previously considered deterministic, but recent evidence suggests it may have stochastic components (threshold is now considered much lower at 0.5 Gy).

Question 47: Which of the following is NOT considered one of the 'R's of radiobiology?

A. Repair
B. Redistribution
C. Repopulation
D. Reperfusion (Correct Answer)

Explanation: The **"5 Rs of Radiobiology"** are the fundamental principles that govern the response of tissues (both normal and cancerous) to fractionated radiotherapy. Understanding these is crucial for NEET-PG as they explain why radiation is delivered in multiple small doses rather than a single large dose. ### **Explanation of the Correct Answer** **D. Reperfusion:** This is **not** one of the 5 Rs. Reperfusion refers to the restoration of blood flow to an organ or tissue, typically discussed in the context of myocardial infarction or stroke. While "Reoxygenation" is an R of radiobiology, "Reperfusion" is a distractor. ### **Explanation of the Incorrect Options (The actual Rs)** * **A. Repair:** Refers to the ability of cells to repair sublethal radiation damage. Normal cells generally repair more efficiently than tumor cells between fractions. * **B. Redistribution (or Reassortment):** Radiation is most effective in the **M and G2 phases** of the cell cycle. Fractionation allows surviving cells to move from resistant phases (like S-phase) into more sensitive phases before the next dose. * **C. Repopulation:** This describes the division of surviving cells between fractions. While it helps normal tissue recover, accelerated repopulation of tumor cells can lead to treatment failure if the course is too long. ### **High-Yield Clinical Pearls for NEET-PG** * **The 5 Rs include:** Repair, Redistribution, Repopulation, Reoxygenation, and Radiosensitivity (intrinsic). * **Reoxygenation:** As a tumor shrinks, previously hypoxic (radioresistant) central cells get better access to oxygen, making them more sensitive to subsequent radiation. * **Most Radiosensitive Phase:** M phase (Mitosis). * **Most Radioresistant Phase:** Late S phase. * **Law of Bergonie and Tribondeau:** Stem cells and rapidly dividing cells (high mitotic index) are the most radiosensitive.

Question 48: Which of the following cell types is least radiosensitive?

A. RBC, nerve, muscle, epithelial cells (Correct Answer)
B. Endothelial cells, bone cells
C. Nerve, muscle, bone
D. All of the above

Explanation: ### Explanation The radiosensitivity of a cell is governed by the **Law of Bergonie and Tribondeau**, which states that cells are most sensitive to radiation when they are **undifferentiated**, have a **high mitotic rate**, and a **long dividing future**. Conversely, cells that are highly specialized (differentiated) and do not divide are the most radioresistant. **1. Why Option A is Correct:** Mature **RBCs, nerve cells, and muscle cells** are highly differentiated and do not undergo mitosis. According to the law, these are the **least radiosensitive** (most radioresistant) cells in the human body. While some epithelial cells divide, mature specialized epithelial cells are significantly less sensitive than primitive stem cells. **2. Analysis of Other Options:** * **Option B:** While bone cells (osteocytes) have low sensitivity, **endothelial cells** (lining blood vessels) are considered to have **intermediate radiosensitivity**. They divide more frequently than nerve or muscle cells, making this option less accurate than A. * **Option C:** Although nerve and muscle are highly resistant, **bone** (specifically growing bone with active osteoblasts) is more sensitive than a mature erythrocyte or a neuron. * **Option D:** Incorrect because the degree of sensitivity varies significantly between these groups. **Clinical Pearls for NEET-PG:** * **Most Radiosensitive Cell:** Lymphocyte (Exception to the law: it is highly sensitive despite being differentiated and non-dividing). * **Most Radiosensitive Phase of Cell Cycle:** **M phase** (Mitosis), followed by G2. * **Most Radioresistant Phase:** **S phase** (DNA synthesis). * **Order of Sensitivity (High to Low):** Lymphocytes → Spermatogonia → Erythroblasts → Intestinal crypt cells → Endothelial cells → Osteoblasts → Muscle/Nerve cells.

Question 49: Which of the following cells in the body are most radiosensitive?

A. Endothelial cells
B. Epithelial cells
C. Red blood cells
D. White blood cells (Correct Answer)

Explanation: **Explanation:** The radiosensitivity of cells is primarily governed by the **Law of Bergonié and Tribondeau**, which states that cells are most sensitive to radiation when they have a high mitotic rate, a long mitotic future, and are least differentiated. **1. Why White Blood Cells (WBCs) are the Correct Answer:** Among all cells in the human body, **Lymphocytes** (a type of WBC) are the **most radiosensitive**. Interestingly, lymphocytes are an exception to the Law of Bergonié and Tribondeau; although they are mature, non-dividing cells, they are highly sensitive and undergo rapid interphase death (apoptosis) following even low doses of radiation. In the event of acute radiation syndrome, the absolute lymphocyte count is the first to drop, making it a critical prognostic indicator. **2. Analysis of Incorrect Options:** * **Endothelial cells (A) and Epithelial cells (B):** These possess intermediate radiosensitivity. While they divide more frequently than muscle or nerve cells, they are significantly more resistant than hematopoietic stem cells or lymphocytes. * **Red blood cells (C):** Mature RBCs are highly radioresistant because they lack a nucleus and do not undergo cell division. However, their precursors in the bone marrow (erythroblasts) are highly sensitive. **3. High-Yield Clinical Pearls for NEET-PG:** * **Most Radiosensitive Cell:** Lymphocyte (specifically Small Lymphocytes). * **Most Radioresistant Cell:** Nerve cells (followed by mature Muscle cells). * **Most Radiosensitive Phase of Cell Cycle:** **M phase** (Mitosis), followed by G2. * **Most Radioresistant Phase of Cell Cycle:** **Late S phase** (due to DNA repair mechanisms). * **Order of sensitivity in blood:** Lymphocytes > Neutrophils > Platelets > RBCs.

Question 50: Which of the following is a radioprotector?

A. Amifostine (Correct Answer)
B. Mesna
C. Paraffin wax
D. Leucovorin

Explanation: **Explanation:** **Amifostine (Correct Answer):** Amifostine is a prodrug that is converted by alkaline phosphatase into an active thiol compound (**WR-2721**). It acts as a potent **free radical scavenger**. Since radiation causes cellular damage primarily through the radiolysis of water and the subsequent generation of reactive oxygen species (ROS), Amifostine protects normal tissues by neutralizing these radicals. It is the only FDA-approved radioprotector used clinically to reduce xerostomia in patients undergoing radiotherapy for head and neck cancers. **Analysis of Incorrect Options:** * **Mesna:** It is a cytoprotective agent used to prevent **hemorrhagic cystitis** caused by chemotherapy drugs like Cyclophosphamide and Ifosfamide. It is not a radioprotector. * **Paraffin wax:** In radiotherapy, paraffin wax is used as a **tissue-equivalent bolus** material to increase the surface dose to the skin; it does not provide biological protection to cells. * **Leucovorin (Folinic acid):** It is used as a "rescue" agent following high-dose **Methotrexate** therapy or to enhance the activity of 5-Fluorouracil. It has no role in radiobiology. **High-Yield Clinical Pearls for NEET-PG:** * **Differential Protection:** Amifostine protects normal cells more than tumor cells because normal tissues have higher alkaline phosphatase activity and better vascularity. * **Radiosensitizers:** Contrastingly, drugs like **Nimorazole** and **Metronidazole** (hypoxic cell sensitizers) or **Halogenated Pyrimidines** (IUDR, BUDR) make tumor cells more sensitive to radiation. * **Dose Reduction Factor (DRF):** The effectiveness of a radioprotector is measured by the DRF (Dose with protector / Dose without protector). Amifostine has one of the highest DRFs.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free