Radiobiology Practice Questions

Q: Which of the following is a radio-resistant tumor?

Malignant fibrous histiocytoma. Radiosensitivity refers to the relative susceptibility of cells and tissues to the ionizing effects of radiation. In clinical oncology, tumors are categorized based on their response to radiotherapy. **Explanation of the Correct Answer:** **Malignant Fibrous Histiocytoma (MFH)**, now more commonly classified as Undifferentiated Pleomorphic Sarcoma (UPS), is a high-grade soft tissue sarcoma. Most soft tissue sarcomas (with the exception of Rhabdomyosarcoma and Ewing’s) are inherently **radio-resistant**. They possess efficient DNA repair mechanisms and often contain hypoxic zones that make them less susceptible to the free radicals generated by ionizing radiation. Therefore, the primary treatment for MFH is surgical excision rather than radiotherapy. **Explanation of Incorrect Options:** * **Ewing’s Sarcoma:** This is a highly **radiosensitive** small round blue cell tumor. While surgery and chemotherapy are primary, radiotherapy is frequently used for local control. * **Seminoma:** This germ cell tumor of the testis is exquisitely **radiosensitive**. Even low doses of radiation can effectively treat microscopic nodal disease. * **Dysgerminoma:** The ovarian counterpart to the seminoma, this is also highly **radiosensitive**. **NEET-PG High-Yield Pearls:** 1. **Bergonie-Tribondeau Law:** Cells are more radiosensitive if they have a high mitotic rate, a long mitotic future, and are undifferentiated. 2. **Most Radiosensitive Tumors:** Lymphoma, Leukemia, Seminoma, Dysgerminoma, and Wilms’ tumor. 3. **Most Radio-resistant Tumors:** Osteosarcoma, Malignant Melanoma, Pancreatic Carcinoma, and Renal Cell Carcinoma. 4. **Cell Cycle Sensitivity:** Cells are most sensitive in the **M and G2 phases** and most resistant in the **S phase**.

Q: Which of the following tumors is most sensitive to radiation?

Seminoma. **Explanation:** The sensitivity of a tumor to ionizing radiation depends on its cellular kinetics, differentiation, and inherent repair mechanisms. According to the **Bergonie-Tribondeau law**, cells that are rapidly dividing, undifferentiated, and have a high metabolic rate are the most radiosensitive. **1. Why Seminoma is the Correct Answer:** Seminoma is classified as **highly radiosensitive**. It is the classic example of a "radiocurable" tumor. Even at low doses of radiation (20–25 Gy), seminoma cells undergo rapid apoptosis. This extreme sensitivity makes radiotherapy a primary treatment modality for early-stage seminoma. **2. Analysis of Incorrect Options:** * **Small Cell Lung Cancer (SCLC):** While SCLC is considered **radiosensitive** and responds well initially, it is generally less sensitive than germ cell tumors like seminoma. It often requires higher doses and is prone to early recurrence. * **Soft Tissue Sarcoma:** These are generally **radioresistant**. They are bulky, slow-growing, and well-differentiated mesenchymal tumors that require surgical excision as the primary treatment; radiation is typically used only as an adjuvant. * **Osteosarcoma:** This is a classic example of a **highly radioresistant** tumor. The osteoid matrix and the nature of the malignant osteoblasts make radiation ineffective for primary control. **3. NEET-PG High-Yield Pearls:** * **Most Radiosensitive Cells:** Lymphocytes (exception to the rule as they are non-dividing) and Germ cells (Spermatogonia). * **Radiosensitivity Hierarchy:** 1. **High:** Lymphoma, Leukemia, Seminoma, Dysgerminoma, Wilms’ tumor. 2. **Moderate:** Squamous cell carcinoma (e.g., Cervix, H&N), Adenocarcinoma (Breast, GI). 3. **Low (Resistant):** Osteosarcoma, Pancreatic cancer, Malignant Melanoma, Glioblastoma Multiforme. * **Cell Cycle:** Cells are most sensitive in the **M (Mitosis)** and **G2 phases**, and most resistant in the **S (Synthesis) phase**.

Q: Which type of cancer is commonly associated with radiation exposure?

Leukemia. **Explanation:** Radiation-induced carcinogenesis is a **stochastic effect**, meaning the probability of occurrence increases with dose, but the severity does not. Among all malignancies, **Leukemia** (specifically Acute Myeloid Leukemia, Chronic Myeloid Leukemia, and Acute Lymphoblastic Leukemia) is the most common and earliest cancer to develop following significant radiation exposure. 1. **Why Leukemia is Correct:** Bone marrow is one of the most radiosensitive tissues in the body due to its high rate of cell turnover (Bergonié-Tribondeau law). Leukemia has the **shortest latent period** (approximately 2–5 years) compared to solid tumors, making it the most frequently cited "early" malignancy in post-radiation epidemiological studies (e.g., atomic bomb survivors). 2. **Why other options are incorrect:** * **Bronchogenic, Thyroid, and Breast Carcinomas:** While these are indeed radiation-induced solid tumors, they have much longer latent periods (often 10–40 years). While Thyroid cancer is highly associated with radiation in children, Leukemia remains the classic "textbook" answer for the most common overall association in general populations. **High-Yield Clinical Pearls for NEET-PG:** * **Radiosensitivity:** Lymphocytes are the most radiosensitive cells in the body (exception to the rule that mature cells are radioresistant). * **Latent Period:** Leukemia (2–5 years) vs. Solid tumors (>10 years). * **Chronic Lymphocytic Leukemia (CLL):** This is the only leukemia **NOT** associated with radiation exposure. * **Radiosensitive Organs:** Bone marrow, GI tract epithelium, and gonads are highly sensitive; Nerve and Muscle cells are the most radioresistant.

Q: Acute radiation sickness is characterised by:

All of the above. **Explanation:** Acute Radiation Syndrome (ARS), also known as radiation sickness, occurs following exposure of the whole body (or a large portion of it) to a high dose of penetrating ionizing radiation over a very short period. It is characterized by three distinct sub-syndromes that manifest depending on the dose received. 1. **Hematological (Bone Marrow) Syndrome:** Occurs at doses of **2–10 Gy**. It is characterized by a drop in blood cell counts (pancytopenia) due to bone marrow suppression, leading to infection and hemorrhage. 2. **Gastrointestinal (GI) Syndrome:** Occurs at doses of **10–50 Gy**. It results from the destruction of the intestinal mucosal stem cells, leading to severe diarrhea, dehydration, and electrolyte imbalance. 3. **Central Nervous System (CNS) Syndrome:** Occurs at very high doses (**>50 Gy**). It is characterized by cerebral edema, seizures, and coma, usually resulting in death within hours to days. **Why "All of the above" is correct:** Since ARS is a dose-dependent spectrum, a patient may exhibit symptoms from one or all categories depending on the severity of exposure. Therefore, hematological, GI, and CNS symptoms are all recognized components of the syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Most sensitive cell to radiation:** Lymphocyte (first to drop in ARS). * **Most sensitive phase of cell cycle:** M phase (followed by G2). * **Least sensitive phase:** S phase. * **LD 50/60 for humans:** Approximately **3.5 to 4.5 Gy** (dose that kills 50% of the population in 60 days without medical intervention). * **Prodromal Stage:** The initial phase (nausea, vomiting, anorexia) occurring shortly after exposure before the main syndromes manifest.

Q: Sublethal damage to DNA is caused by which type of radiation effect?

Stochastic effect. ### Explanation The correct answer is **Stochastic effect**. **1. Why Stochastic Effect is Correct:** Stochastic effects are "random" or "probabilistic" in nature. They occur when radiation causes **sublethal damage** to the DNA of a cell. Instead of killing the cell, the radiation induces a mutation that is not perfectly repaired. This damaged cell remains viable, survives, and proliferates, eventually leading to outcomes like **carcinogenesis** (cancer) or **genetic mutations** in offspring. Crucially, stochastic effects have **no threshold dose**; even a single photon could theoretically cause the sublethal damage necessary to trigger the effect. **2. Why Other Options are Incorrect:** * **Deterministic Effect (Tissue Reactions):** These occur due to **lethal damage** (cell death). When a large number of cells in a tissue die, organ function is impaired (e.g., radiation burns, cataracts, sterility). These effects have a **threshold dose** below which the effect does not occur, and the severity increases with the dose. * **Both:** Incorrect because the mechanisms of damage (sublethal vs. lethal) and the dose-response relationships are fundamentally different. **3. High-Yield Clinical Pearls for NEET-PG:** | Feature | Stochastic Effect | Deterministic Effect | | :--- | :--- | :--- | | **Damage Type** | **Sublethal** (Mutation) | **Lethal** (Cell Death) | | **Threshold** | No threshold (Linear Non-Threshold) | Has a specific threshold | | **Severity** | Independent of dose | Increases with dose | | **Probability** | Increases with dose | 100% above threshold | | **Examples** | Cancer, Genetic mutations | Cataracts, Skin erythema, Sterility | * **Key Concept:** In diagnostic radiology (X-rays, CT scans), we are primarily concerned with **stochastic effects**, as doses are usually below deterministic thresholds.

Q: Which ocular structure is most radiosensitive?

Lens. **Explanation:** The **lens** is the most radiosensitive structure in the human eye. This sensitivity is due to the unique physiology of the lens epithelium. When exposed to ionizing radiation, the germinal cells at the lens equator undergo DNA damage. Because the lens is an encapsulated structure with no mechanism to shed dead or damaged cells, these abnormal cells migrate toward the posterior pole and form opacities, leading to **Radiation Cataract**. The threshold dose for cataract formation is remarkably low (as little as 0.5 to 2 Gy). **Analysis of Incorrect Options:** * **A. Cornea:** While high doses of radiation can cause keratitis or edema, the cornea is significantly more resistant than the lens. * **C. Sclera:** The sclera consists of dense collagenous tissue with low metabolic activity, making it one of the most radioresistant structures in the eye. * **D. Retina:** The retina is composed of highly differentiated neural tissue. While "Radiation Retinopathy" (a microangiopathy) can occur, it typically requires much higher doses (usually >30-35 Gy) compared to the lens. **High-Yield Clinical Pearls for NEET-PG:** * **Type of Cataract:** Radiation typically causes **Posterior Subcapsular Cataract (PSC)**. * **Latent Period:** There is often a latent period between exposure and cataract formation, which is inversely proportional to the dose received. * **Occupational Hazard:** Interventional cardiologists and radiologists are at high risk; hence, the use of lead-lined glasses is mandatory. * **Radiosensitivity Hierarchy:** Lens > Retina > Cornea > Sclera.

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

G2. **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).

Q: Months or years following radiotherapy, the irradiated oral mucosa:

Becomes atrophic. **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.

Q: Which of the following tumors is least sensitive to radiation?

Malignant melanoma. **Explanation:** The sensitivity of a tumor to radiation depends on its **intrinsic radiosensitivity**, which is determined by the cell's ability to repair DNA damage, its proliferative rate, and its oxygenation status. **Malignant Melanoma** is classically considered the most **radioresistant** tumor among the options provided. This is due to its high capacity for repairing sublethal radiation damage and a characteristic "wide shoulder" on its cell survival curve. While radiation is sometimes used for palliation or specific sites (like the brain or uveal tract), melanoma generally requires very high doses per fraction (hypofractionation) to overcome its inherent resistance. **Analysis of other options:** * **Bronchogenic Carcinoma (A):** Squamous cell and small cell variants are moderately to highly radiosensitive. While adenocarcinoma of the lung is less sensitive, it is still more responsive than melanoma. * **Adenocarcinoma of Colon (B):** These are considered **radioresponsive**. While surgery is the primary treatment, radiotherapy is frequently used (especially in rectal cancers) to reduce tumor bulk. * **Osteogenic Sarcoma (D):** While traditionally labeled as radioresistant, modern studies show it has some response to high-dose radiation. However, in the context of standard competitive exams, **Melanoma** is always ranked as the "least sensitive" compared to sarcomas. **NEET-PG High-Yield Pearls:** * **Most Radiosensitive Tumors:** Seminoma, Dysgerminoma, Ewing’s Sarcoma, and Lymphomas. * **Most Radioresistant Tumors:** Malignant Melanoma, Osteosarcoma, Pancreatic Carcinoma, and Renal Cell Carcinoma. * **Bergonie-Tribondeau Law:** Cells are more radiosensitive if they have a high division rate, a long dividing future, and are least specialized (undifferentiated).

Question 1

Which of the following is a radio-resistant tumor?

Accepted Answer

Malignant fibrous histiocytoma

Answer

Ewing's sarcoma

Answer

Seminoma

Answer

Dysgerminoma

Question 2

Which of the following tumors is most sensitive to radiation?

Accepted Answer

Seminoma

Answer

Small cell lung cancer

Answer

Soft tissue sarcoma

Answer

Osteosarcoma

Question 3

Which type of cancer is commonly associated with radiation exposure?

Accepted Answer

Leukemia

Answer

Bronchogenic carcinoma

Answer

Thyroid carcinoma

Answer

Breast cancer

Question 4

Acute radiation sickness is characterised by:

Accepted Answer

All of the above

Answer

Hematological symptoms

Answer

CNS symptoms

Answer

Gastrointestinal symptoms

Question 5

Sublethal damage to DNA is caused by which type of radiation effect?

Accepted Answer

Stochastic effect

Answer

Deterministic effect

Answer

Both

Answer

None

Question 6

Which ocular structure is most radiosensitive?

Accepted Answer

Lens

Answer

Cornea

Answer

Sclera

Answer

Retina

Question 7

Which of the following is the most radiosensitive stage of the cell cycle?

Accepted Answer

G2

Answer

G0

Answer

G1

Answer

S

Question 8

Months or years following radiotherapy, the irradiated oral mucosa:

Accepted Answer

Becomes atrophic

Answer

Undergoes necrosis

Answer

Develops candidiasis

Answer

Develops granulomatosis

Question 9

Which of the following tumors is least sensitive to radiation?

Accepted Answer

Malignant melanoma

Answer

Bronchogenic carcinoma

Answer

Adenocarcinoma of colon

Answer

Osteogenic sarcoma

Question 10

Which radiosensitizer drug is used in head and neck cancer treatment?

Accepted Answer

Paclitaxel

Answer

Cisplatin

Answer

Amikacin

Answer

Mitomycin-C

Radiobiology — MCQs

Radiobiology — MCQs

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