Radiobiology — MCQs

Radiobiology Indian Medical PG Practice Questions and MCQs

Question 31: Which of the following is the most radiosensitive stage of the cell cycle?

A. G0
B. G1
C. G2 (Correct Answer)
D. S

Explanation: **Explanation:** The radiosensitivity of a cell varies significantly throughout its cycle, a concept known as the **Law of Bergonié and Tribondeau**. **Why G2/M is the correct answer:** The **M (Mitosis)** phase is the most radiosensitive phase of the cell cycle, followed closely by late **G2**. During these stages, the DNA is condensed, and the cell’s natural repair mechanisms are less active or have insufficient time to fix radiation-induced double-strand breaks before division occurs. Damage during G2/M is lethal because it is directly propagated during mitosis, leading to mitotic death. **Analysis of Incorrect Options:** * **G0 (Quiescent phase):** These cells are non-cycling and generally more radioresistant because they are not actively dividing. * **G1 (Gap 1):** Sensitivity varies, but it is generally less sensitive than G2/M. In cells with a long G1, there is a period of relative resistance in early G1. * **S (Synthesis phase):** This is the **most radioresistant** phase of the cell cycle. During the S phase, DNA replication occurs, and homologous recombination (a high-fidelity repair mechanism) is most active, allowing the cell to efficiently repair radiation damage. **NEET-PG High-Yield Pearls:** * **Most Radiosensitive Phase:** M phase (Mitosis) > G2. * **Most Radioresistant Phase:** Late S phase. * **Law of Bergonié and Tribondeau:** Radiosensitivity is directly proportional to the reproductive/mitotic rate and inversely proportional to the degree of differentiation. * **Oxygen Enhancement Ratio (OER):** Radiation is most effective in the presence of oxygen (hyperoxic cells are more sensitive than hypoxic cells).

Question 32: Months or years following radiotherapy, the irradiated oral mucosa:

A. Undergoes necrosis
B. Develops candidiasis
C. Becomes atrophic (Correct Answer)
D. Develops granulomatosis

Explanation: **Explanation:** The correct answer is **C. Becomes atrophic**. **Why it is correct:** Radiation-induced changes in tissues are categorized into acute and late effects. **Atrophy** is a classic **late effect** of radiotherapy (occurring months to years post-exposure). The underlying mechanism involves progressive **obliterative endarteritis** (narrowing of small blood vessels) and interstitial fibrosis. This leads to a chronic reduction in blood supply and nutrient delivery to the oral mucosa, causing the epithelium to become thin, friable, and "atrophic." This thin mucosa is highly susceptible to trauma and ulceration. **Why the other options are incorrect:** * **A. Necrosis:** While osteoradionecrosis (bone death) is a serious late complication, the mucosa itself typically undergoes atrophy first. Spontaneous mucosal necrosis is less common than chronic thinning unless triggered by secondary trauma or extreme doses. * **B. Candidiasis:** This is typically an **acute or subacute** complication. It occurs during or shortly after treatment due to radiation-induced xerostomia (dry mouth) and changes in oral flora, rather than being a primary structural change of the mucosa years later. * **D. Granulomatosis:** This is a chronic inflammatory response characterized by granulomas (e.g., in Sarcoidosis or TB). It is not a standard pathological feature of radiation injury, which is characterized by fibrosis and atrophy. **High-Yield NEET-PG Pearls:** * **Early/Acute Effects:** Mucositis, erythema, and taste loss (occurs in rapidly dividing cells). * **Late Effects:** Atrophy, fibrosis, xerostomia (due to salivary gland destruction), and **Osteoradionecrosis** (most common in the mandible). * **Key Histology:** Look for "atypical fibroblasts" and "vascular endarteritis" in late radiation damage descriptions. * **Radiosensitivity:** The oral mucosa is highly radiosensitive due to its high cell turnover rate.

Question 33: Which of the following agents is a radioprotector?

A. Colony stimulating factor
B. Amifostine (Correct Answer)
C. Cisplatin
D. Methotrexate

Explanation: **Explanation:** **Amifostine (Option B)** is the correct answer. It is a prodrug that is dephosphorylated by alkaline phosphatase in tissues to its active metabolite, **WR-1065**. This active form acts as a potent free radical scavenger, neutralizing the reactive oxygen species (ROS) generated by ionizing radiation. It is specifically designed to protect normal tissues (like salivary glands) while sparing tumor cells, primarily because normal cells have higher alkaline phosphatase activity and better vascularity compared to the acidic, hypoxic tumor microenvironment. **Analysis of Incorrect Options:** * **Colony Stimulating Factors (Option A):** These are **radiomitigators**. Unlike protectors (given before radiation), mitigators are administered *after* exposure but before symptoms appear to accelerate recovery of the hematopoietic system. * **Cisplatin (Option C) and Methotrexate (Option D):** These are **radiosensitizers** or cytotoxic agents. They enhance the lethal effects of radiation on cells by inhibiting DNA repair or synchronized cell cycling, thereby increasing the therapeutic ratio against tumors. **High-Yield Clinical Pearls for NEET-PG:** * **FDA Approval:** Amifostine is specifically FDA-approved to reduce the incidence of **xerostomia** (dry mouth) in patients undergoing radiotherapy for head and neck cancers. * **The "Oxygen Effect":** Radioprotectors like sulfhydryl compounds (Amifostine) work best in well-oxygenated conditions by competing with oxygen to interact with free radicals. * **Dose Reduction Factor (DRF):** This is the ratio of the radiation dose required to produce a specific biological effect in the presence of a protector versus its absence. Amifostine has one of the highest DRFs. * **Side Effects:** The most common dose-limiting side effect of Amifostine is **hypotension**.

Question 34: Which of the following is a radioprotective drug?

A. Amifostine (Correct Answer)
B. Cyclophosphamide
C. Methotrexate
D. Paclitaxel

Explanation: **Explanation:** **Amifostine (Option A)** is the correct answer. It is a cytoprotective agent (specifically a **free radical scavenger**) used in clinical radiotherapy. It is a prodrug that is converted by alkaline phosphatase into its active thiol metabolite, **WR-1065**. This active form scavenges free radicals generated by ionizing radiation (indirect effect) and provides a source of hydrogen atoms to repair DNA damage. Crucially, it selectively protects normal tissues because alkaline phosphatase activity is higher, and vascularity is better in normal cells compared to the acidic, hypoxic environment of tumor cells. **Why the other options are incorrect:** * **Cyclophosphamide (Option B):** This is an alkylating chemotherapy agent. It is often a **radiosensitizer**, meaning it makes cells more sensitive to radiation, rather than protecting them. * **Methotrexate (Option C):** An antimetabolite (folate antagonist) that inhibits DNA synthesis. Like most chemotherapeutic agents, it can enhance radiation toxicity (e.g., radiation recall dermatitis). * **Paclitaxel (Option D):** A taxane that stabilizes microtubules. It arrests cells in the **G2/M phase** of the cell cycle, which is the most radiosensitive phase, thereby acting as a potent radiosensitizer. **High-Yield Clinical Pearls for NEET-PG:** * **FDA Approval:** Amifostine is primarily used to reduce the incidence of xerostomia (dry mouth) in patients undergoing radiotherapy for head and neck cancer. * **Radiosensitizers vs. Radioprotectors:** Radiosensitizers (e.g., Oxygen, Nimorazole, Cisplatin) increase the cell-killing effect of radiation, while radioprotectors (e.g., Amifostine, Cysteine) decrease it. * **The Oxygen Effect:** Oxygen is the most potent natural radiosensitizer. Radioprotectors like Amifostine work best in well-oxygenated environments by competing with oxygen for free radical interactions.

Question 35: Which of the following is a radioprotective drug?

A. Paclitaxel
B. Vincristine
C. Amifostine (Correct Answer)
D. Etoposide

Explanation: ### Explanation **Correct Answer: C. Amifostine** **Mechanism and Rationale:** Amifostine is a sulfur-containing compound (organic thiophosphate) that acts as a **free radical scavenger**. It is a "prodrug" that is converted by the enzyme alkaline phosphatase into its active metabolite, **WR-1065**. Radiation therapy causes damage primarily through the radiolysis of water, creating reactive oxygen species (ROS). Amifostine protects normal tissues by: 1. Scavenging these free radicals. 2. Donating hydrogen atoms to DNA radicals. 3. Inducing cellular hypoxia. Crucially, it selectively protects normal tissues because alkaline phosphatase activity is higher in normal cells than in tumor cells, and normal cells have better vascularity, allowing for higher drug uptake. **Analysis of Incorrect Options:** * **A. Paclitaxel:** This is a microtubule stabilizer (taxane) used in chemotherapy. It is actually a **radiosensitizer**, meaning it makes cells more sensitive to radiation by arresting them in the G2/M phase of the cell cycle (the most radiosensitive phase). * **B. Vincristine:** A vinca alkaloid that inhibits microtubule polymerization. Like Paclitaxel, it is a chemotherapeutic agent, not a radioprotector. * **C. Etoposide:** A topoisomerase II inhibitor used to treat various cancers. It does not possess radioprotective properties. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Use:** Amifostine is FDA-approved to reduce **xerostomia** (dry mouth) in patients undergoing radiotherapy for head and neck cancers and to reduce renal toxicity from cisplatin. * **Side Effects:** The most common dose-limiting side effect is **hypotension**. Others include nausea, vomiting, and hypocalcemia. * **Radiosensitizers vs. Radioprotectors:** Remember that most chemotherapy drugs (like 5-FU, Cisplatin, and Taxanes) act as *sensitizers*, whereas Amifostine is the classic *protector*.

Question 36: Salivary secretion becomes zero at what radiation dose?

A. >30 Gray
B. >60 Gray (Correct Answer)
C. >40 Gray
D. >50 Gray

Explanation: **Explanation:** Salivary glands are highly sensitive to ionizing radiation, despite being composed of relatively slow-dividing cells. The correct answer is **>60 Gray** because this threshold represents the dose at which permanent, irreversible destruction of the acinar cells occurs, leading to a complete cessation of salivary flow (xerostomia). * **Why >60 Gray is correct:** While salivary flow begins to decrease significantly at doses as low as 20 Gy, the total secretory function typically reaches zero (or near-zero) when the cumulative dose exceeds 60 Gy. At this level, the serous acini are replaced by extensive fibrosis and fatty degeneration, making the damage permanent. * **Why other options are wrong:** * **>30 Gray:** At this dose, patients experience a 50-60% reduction in salivary flow, but secretion is still present. * **>40-50 Gray:** These doses cause severe xerostomia and significant changes in saliva viscosity and pH, but some residual function may remain in the mucous glands. Total cessation is not guaranteed until the 60 Gy threshold is crossed. **High-Yield Clinical Pearls for NEET-PG:** 1. **Serous vs. Mucous:** Serous acini (predominant in the Parotid gland) are much more radiosensitive than mucous acini (Sublingual gland). Therefore, the Parotid gland is the most affected by radiation. 2. **Early Changes:** Salivary flow can drop by 50% within the first week of radiotherapy (after only 10 Gy). 3. **Radiation Caries:** The loss of the buffering capacity and cleansing action of saliva after high-dose radiation (>60 Gy) leads to rampant "radiation caries," typically affecting the cervical margins of the teeth. 4. **Pilocarpine:** This cholinergic agonist is often used to stimulate saliva in patients with residual functioning glandular tissue.

Question 37: Low dose therapeutic radiation will cause all of the following effects in less time except?

A. Mutagenesis
B. Carcinogenesis
C. Teratogenesis
D. Mucositis (Correct Answer)

Explanation: This question tests the fundamental distinction between **Stochastic** and **Deterministic** effects of radiation. ### **Explanation** The correct answer is **Mucositis**. Radiation effects are classified into two categories: 1. **Stochastic Effects (Options A, B, and C):** These are "all-or-none" phenomena where the **probability** of occurrence increases with dose, but the **severity** does not. There is **no threshold dose**. Even a single low-dose exposure can cause DNA damage leading to **Mutagenesis** (germ cell mutations), **Carcinogenesis** (cancer induction), or **Teratogenesis** (fetal malformations). These effects can manifest after a single exposure in a very short timeframe or after a long latency period. 2. **Deterministic (Non-stochastic) Effects (Option D):** These occur only after a specific **threshold dose** is exceeded. The severity increases with the dose. **Mucositis** is a deterministic effect resulting from the killing of rapidly dividing mucosal stem cells. Low-dose therapeutic radiation typically falls below the threshold required to cause acute tissue reactions like mucositis; therefore, it is the least likely to occur compared to the random nature of stochastic risks. ### **Why other options are wrong:** * **Mutagenesis & Carcinogenesis:** These are classic stochastic effects. Theoretically, a single photon can cause a mutation; thus, they can be initiated by low doses. * **Teratogenesis:** While often having a threshold, it is considered a stochastic-like risk in the context of low-dose fetal exposure (especially during organogenesis), where even minor radiation can lead to permanent structural defects. ### **High-Yield Clinical Pearls for NEET-PG:** * **Stochastic:** No threshold, severity independent of dose (e.g., Leukemia, Genetic mutations). * **Deterministic:** Threshold exists, severity is dose-dependent (e.g., Cataract, Mucositis, Erythema, Sterility). * **Most sensitive phase of cell cycle:** M phase (followed by G2). * **Most radioresistant phase:** Late S phase. * **Law of Bergonie and Tribondeau:** Radiosensitivity is directly proportional to the reproductive activity and inversely proportional to the degree of differentiation of the cell.

Question 38: A single whole-body radiation dose of how many rads can result in death?

A. 100 rads
B. 200 rads
C. 300 rads (Correct Answer)
D. 500 rads

Explanation: **Explanation:** The biological effect of radiation depends on the dose, the area of the body exposed, and the duration of exposure. In the context of a **single whole-body radiation dose**, the concept of **LD50/60** (Lethal Dose for 50% of the population within 60 days) is the standard clinical benchmark. **Why 300 rads is correct:** For humans, the LD50/60 is approximately **300 to 450 rads (3–4.5 Gy)** without intensive medical intervention. At 300 rads, the primary cause of death is **Hematopoietic Syndrome** (Bone Marrow Syndrome). The radiation destroys rapidly dividing stem cells in the bone marrow, leading to severe pancytopenia, subsequent hemorrhage, and fatal opportunistic infections. **Analysis of Incorrect Options:** * **100 rads (1 Gy):** This is the threshold for **Acute Radiation Syndrome (ARS)**. While it causes mild symptoms like nausea and vomiting (prodromal stage) and a temporary drop in lymphocyte count, it is rarely fatal. * **200 rads (2 Gy):** This dose causes significant illness and moderate bone marrow suppression, but with supportive care, the survival rate is very high. * **500 rads (5 Gy):** While this dose is certainly lethal (approaching LD100), the question asks for the dose that *can* result in death. 300 rads is the established clinical threshold where mortality becomes a significant statistical reality. **High-Yield Clinical Pearls for NEET-PG:** * **LD50/60 for humans:** 3–4.5 Gy (300–450 rads). * **Gastrointestinal (GI) Syndrome:** Occurs at doses **>10 Gy (1000 rads)**; death occurs within 3–10 days due to mucosal denudation. * **Cerebrovascular/CNS Syndrome:** Occurs at doses **>50 Gy (5000 rads)**; death occurs within 24–48 hours. * **Unit Conversion:** 1 Gray (Gy) = 100 rads; 1 Sievert (Sv) = 100 rem.

Question 39: Which of the following cells is most radiosensitive?

A. Neutrophil
B. Lymphocyte (Correct Answer)
C. Megakaryocyte
D. Basophil

Explanation: **Explanation:** The radiosensitivity of cells is generally governed by the **Law of Bergonié and Tribondeau**, which states that cells are most sensitive to radiation when they are rapidly dividing (high mitotic rate), have a long dividing future, and are undifferentiated. **Why Lymphocytes are the Correct Answer:** The **lymphocyte** is a unique and notable exception to the Law of Bergonié and Tribondeau. Despite being a non-dividing (highly differentiated) cell, it is the **most radiosensitive cell in the human body**. Unlike most cells that die during mitosis (mitotic death), lymphocytes undergo **interphase death** (apoptosis) shortly after 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 marker. **Analysis of Incorrect Options:** * **Neutrophils (A) and Basophils (D):** These are mature granulocytes. While they are sensitive to radiation, they are less sensitive than lymphocytes. Their counts drop later than lymphocytes because their mature forms in the peripheral blood are relatively radioresistant compared to their precursors in the bone marrow. * **Megakaryocytes (C):** These are large bone marrow cells responsible for platelet production. While bone marrow stem cells are sensitive, the mature megakaryocyte is less sensitive than the small lymphocyte. **High-Yield Clinical Pearls for NEET-PG:** * **Most Radiosensitive Cell:** Lymphocyte. * **Most Radioresistant Cell:** Nerve cell (Neuron) / Mature Muscle cell. * **Most Radiosensitive Phase of Cell Cycle:** M phase (Mitosis), followed by G2. * **Most Radioresistant Phase of Cell Cycle:** Late S phase (due to high DNA repair activity). * **Order of sensitivity in blood:** Lymphocytes > Neutrophils > Platelets > RBCs (RBCs are the most radioresistant blood cells).

Question 40: Which cells are most susceptible to radiation?

A. Nucleic acids (Correct Answer)
B. Nerve cells
C. Muscle cells
D. Epithelial cells

Explanation: **Explanation:** The susceptibility of biological structures to radiation is governed by the target theory and the Law of Bergonie and Tribondeau. **Why Nucleic Acids are Correct:** At the molecular level, **DNA (Nucleic acids)** is the most radiosensitive macromolecule in the cell. It is considered the "critical target." Radiation causes direct ionization or indirect damage (via free radicals from water radiolysis), leading to single-strand or double-strand breaks. Since DNA carries the genetic blueprint required for cell division and protein synthesis, damage to it is the primary cause of radiation-induced cell death (mitotic death) and mutations. **Analysis of Incorrect Options:** * **Nerve Cells & Muscle Cells:** These are highly differentiated, non-dividing (permanent) cells. According to the Law of Bergonie and Tribondeau, mature, specialized cells that do not undergo frequent mitosis are the most **radioresistant**. * **Epithelial Cells:** While epithelial cells (like those in the GI tract or skin) are radiosensitive because they divide regularly, they are *cellular* structures. The question asks for the most susceptible component; the underlying *molecular* target within these cells that leads to their death is the nucleic acid. **NEET-PG High-Yield Pearls:** 1. **Law of Bergonie and Tribondeau:** Radiosensitivity is directly proportional to the reproductive activity (mitosis) and inversely proportional to the degree of differentiation. 2. **Most sensitive phase of Cell Cycle:** **M phase** (Mitosis) is the most sensitive, followed by G2. 3. **Most resistant phase:** Late **S phase**. 4. **Order of Cellular Radiosensitivity:** Lymphocytes (exception to the law as they are non-dividing but highly sensitive) > Erythroblasts > Spermatogonia > Intestinal crypt cells > Nerve/Muscle (least sensitive).

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

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Radiation in Pregnancy

Practice Questions

Biological Dosimetry

Practice Questions

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