Radiobiology Practice Questions

Q: Which of the following can cause congenital malformations in the fetus?

CT abdomen. **Explanation:** The correct answer is **A. CT abdomen**. **1. Why CT Abdomen is Correct:** The risk of congenital malformations (teratogenesis) is directly related to the type and dose of radiation. CT scans of the abdomen and pelvis involve **ionizing radiation**, which can cause DNA damage. The most sensitive period for radiation-induced malformations is during **organogenesis** (weeks 2 to 8 post-conception). A standard CT abdomen delivers a fetal radiation dose (typically 10–25 mGy) that, while often below the 50 mGy threshold for deterministic effects, is significantly higher than conventional X-rays and poses a theoretical risk of malformation and childhood malignancy. **2. Why Other Options are Incorrect:** * **B. MRI (Magnetic Resonance Imaging):** MRI uses strong magnetic fields and radiofrequency pulses, which are **non-ionizing**. There is no documented evidence that MRI causes congenital malformations, though it is generally avoided in the first trimester as a precaution unless necessary. * **C. Doppler Ultrasound:** Ultrasound uses high-frequency sound waves (**non-ionizing**). While Doppler involves higher energy levels than B-mode ultrasound (potential thermal effects), it does not cause structural malformations. **3. High-Yield Clinical Pearls for NEET-PG:** * **Threshold Dose:** The risk of malformations is significantly increased only at fetal doses **>100 mGy**. Most diagnostic procedures (X-rays, CTs) fall below this. * **Most Sensitive Period:** Organogenesis (2–8 weeks) for malformations; 8–15 weeks for severe intellectual disability. * **All-or-None Phenomenon:** Exposure during the first 2 weeks (pre-implantation) usually results in either death of the conceptus or normal survival. * **Safe Modalities:** Ultrasound and MRI are the preferred imaging modalities in pregnancy.

Q: What is the optimal timing for oxygen administration to be effective during radiotherapy?

Just before starting. **Explanation:** The effectiveness of radiotherapy relies heavily on the **Oxygen Enhancement Ratio (OER)**. Oxygen acts as a potent radiosensitizer by reacting with free radicals produced by ionizing radiation. This reaction "fixes" the damage to the DNA (the **Oxygen Fixation Hypothesis**), making it permanent and lethal to the cancer cell. **1. Why "Just before starting" is correct:** For oxygen to sensitize cells, it must be present at the exact moment of irradiation. Administering oxygen just before starting ensures that the tumor microenvironment is sufficiently oxygenated and that the gas has diffused into the hypoxic core of the tumor before the first beam is delivered. **2. Analysis of incorrect options:** * **During and within microseconds:** While oxygen must be present during the radiation pulse, waiting until the treatment has started to administer it is too late. The chemical reactions (free radical formation) occur in fractions of a second ($10^{-5}$ seconds); if oxygen isn't already present in the tissue, the damage remains repairable. * **After 5 or 10 minutes:** Administering oxygen after the radiation dose is useless. Once the radiation beam stops, the free radicals have already either been repaired by intracellular thiols or have caused non-fixable damage. Oxygen has no "rescue" or delayed sensitizing effect. **Clinical Pearls for NEET-PG:** * **OER Value:** The OER for X-rays and gamma rays is typically **2.5 to 3.0**. * **Hypoxic Cells:** These are **3 times more resistant** to radiation than oxygenated cells. * **Hyperbaric Oxygen:** Historically used to overcome tumor hypoxia, though now largely replaced by hypoxic cell sensitizers (e.g., Nimorazole) or advanced fractionation. * **LET Relationship:** The oxygen effect is maximal with **Low-LET radiation** (X-rays) and minimal/absent with **High-LET radiation** (Alpha particles, Neutrons).

Q: Which of the following tumors is most radioresistant?

Osteosarcoma. **Explanation:** The radiosensitivity of a tumor is primarily determined by its cell of origin, growth fraction, and inherent DNA repair mechanisms. In clinical radiotherapy, tumors are classified on a spectrum from **radiosensitive** (easily destroyed by low doses) to **radioresistant** (requiring extremely high, often toxic, doses for control). **1. Why Osteosarcoma is the Correct Answer:** Osteosarcoma is a bone-forming malignant tumor characterized by the production of osteoid. It is classically considered **radioresistant**. The tumor cells have highly efficient DNA repair mechanisms and often exist in a dense, hypoxic matrix, making them less susceptible to the free radical damage induced by ionizing radiation. Therefore, the primary treatment for Osteosarcoma is surgical resection and chemotherapy, rather than primary radiotherapy. **2. Analysis of Incorrect Options:** * **Lymphoma (Option D):** These are **highly radiosensitive**. Lymphocytes are among the most sensitive cells in the human body (Bergonie-Tribondeau law), and low doses of radiation can induce rapid apoptosis. * **Ewing Sarcoma (Option A):** Unlike Osteosarcoma, Ewing sarcoma is **radiosensitive**. While surgery is often preferred, radiotherapy is a standard component of management for local control. * **Cervical Carcinoma (Option C):** This is **radiosensitive/radiocurable**. Radiotherapy (External Beam + Brachytherapy) is a definitive treatment modality for advanced stages. **Clinical Pearls for NEET-PG:** * **Most Radiosensitive Tumor:** Seminoma (Testis) and Dysgerminoma (Ovary). * **Most Radiosensitive Cell in the Body:** Small Lymphocyte. * **Most Radioresistant Phase of Cell Cycle:** Late S-phase. * **Most Radiosensitive Phase of Cell Cycle:** M-phase (followed by G2). * **Mnemonic for Radioresistant Tumors:** "MOP" — **M**elanoma, **O**steosarcoma, **P**ancreatic Cancer/Renal Cell Carcinoma.

Q: Radiation caries typically appears after what period of time?

3 months. **Explanation:** **Radiation caries** is a rapid and highly destructive form of dental decay that occurs as a secondary complication of radiotherapy for head and neck cancers. 1. **Why 3 Months is Correct:** The primary cause of radiation caries is not the direct effect of radiation on the teeth, but rather the **permanent damage to the salivary glands** (specifically the serous acini), leading to severe **xerostomia** (dry mouth). Without the buffering capacity, minerals, and antimicrobial properties of saliva, the oral pH drops, and plaque accumulation accelerates. Clinical evidence shows that significant demineralization and the onset of radiation caries typically become clinically apparent approximately **3 months (90 days)** after the completion of radiotherapy. 2. **Analysis of Incorrect Options:** * **3 weeks & 6 weeks:** These timeframes are too early for the structural breakdown of dental hard tissues. During this period, patients are more likely to experience acute mucositis or initial changes in taste (dysgeusia). * **6 months:** While caries certainly persist and worsen at 6 months, the *initial appearance* and rapid onset are documented much earlier, typically around the 3-month mark. **High-Yield Clinical Pearls for NEET-PG:** * **Most Common Site:** Radiation caries typically starts at the **cervical (neck) region** of the teeth, often leading to amputation of the crown. * **Osteoradionecrosis (ORN):** This is a more severe late complication involving the bone (usually the mandible). The risk of ORN is highest if extractions are performed *after* radiotherapy. * **Prevention:** Daily application of **1.1% Neutral Sodium Fluoride** gel in custom trays is the gold standard for prevention. * **Threshold Dose:** Salivary gland dysfunction can begin at doses as low as **10-15 Gy**, but permanent xerostomia usually occurs at doses exceeding **26-30 Gy**.

Q: Which of the following organs is most sensitive to radiation-induced cancer?

Female breast. **Explanation:** The sensitivity of an organ to radiation-induced cancer is determined by its **tissue weighting factor ($W_T$)**, which reflects the relative stochastic risk (cancer and hereditary effects) of an organ compared to the whole body. **Why Female Breast is Correct:** According to the **ICRP (International Commission on Radiological Protection) 103** guidelines, the female breast is assigned a high tissue weighting factor of **0.12**. It is one of the most radiosensitive organs for stochastic effects, particularly in younger women. Rapidly dividing mammary epithelial cells are highly susceptible to DNA damage from ionizing radiation, leading to a higher relative risk of carcinogenesis compared to most other tissues. **Why Other Options are Incorrect:** * **Skin:** Has a very low tissue weighting factor (**0.01**). While it can suffer deterministic effects (like erythema), it is relatively resistant to radiation-induced malignancy compared to the breast. * **Thyroid:** While sensitive (especially in children), its weighting factor is **0.04**, which is significantly lower than that of the breast. * **Nerve:** Nerve cells are permanent, non-dividing cells. They are highly **radioresistant** and among the least likely tissues to develop radiation-induced cancer. **NEET-PG High-Yield Pearls:** * **Highest $W_T$ (0.12):** Red bone marrow, Colon, Lung, Stomach, and **Female Breast**. * **Gonads:** Previously considered most sensitive, their $W_T$ was reduced from 0.20 (ICRP 60) to **0.08** (ICRP 103). * **Law of Bergonie and Tribondeau:** Radiosensitivity is directly proportional to the reproductive activity (mitosis) and inversely proportional to the degree of differentiation of the cells. * **Most sensitive phase of cell cycle:** M phase (Mitosis), followed by G2. * **Least sensitive phase:** Late S phase.

Q: The short-term effects of radiation on a tissue are determined by the sensitivity of its:

Parenchymal cells. ### Explanation The biological effects of radiation on tissues are categorized into early (short-term) and late (long-term) effects, depending on which cell population is primarily affected. **Why Parenchymal Cells are Correct:** The **short-term (acute) effects** of radiation are determined by the death of **parenchymal cells**, which are the functional cells of an organ. According to the **Law of Bergonie and Tribondeau**, cells that are rapidly dividing (high mitotic index) and undifferentiated are the most radiosensitive. In tissues with high turnover (e.g., bone marrow, intestinal epithelium, skin), radiation kills these parenchymal stem cells, leading to rapid depletion and clinical symptoms within days to weeks. **Analysis of Incorrect Options:** * **Mesenchymal cells:** These include cells of the connective tissue and blood vessels (endothelium). Damage to the mesenchymal/vascular compartment is primarily responsible for **late (long-term) effects**, such as fibrosis and scarring, which manifest months or years after exposure. * **Ectodermal cells:** While some ectodermal derivatives (like skin) show early effects, "ectodermal" is a germ layer classification, not a functional one. Radiation sensitivity is determined by **mitotic activity**, not embryonic origin. **High-Yield Clinical Pearls for NEET-PG:** * **Most Radiosensitive Phase:** **M phase** (Mitosis) of the cell cycle; the **G2 phase** is the second most sensitive. * **Most Radioresistant Phase:** **S phase** (DNA synthesis). * **Radiosensitivity Hierarchy:** Lymphocytes (most sensitive) > Erythroblasts > Myeloblasts > Epithelial cells > Endothelial cells > Connective tissue > Bone > Nerve/Muscle (most resistant). * **Late Effects:** Primarily due to damage to **stromal/vascular** components and non-dividing parenchymal cells.

Q: What is observed in the periosteum of a bone that has received radiotherapy?

More destruction of bone. **Explanation:** The correct answer is **C. More destruction of bone.** **Underlying Medical Concept:** Bone is a relatively radioresistant tissue in terms of cell division, but its **vascular supply** is highly sensitive to radiation. The periosteum is a dense layer of vascular connective tissue enveloping the bones. Radiotherapy causes **endarteritis obliterans** (inflammation and narrowing of small arteries), leading to chronic ischemia. When the periosteum is irradiated, its osteogenic potential is lost, and the blood supply to the underlying bone is compromised. This leads to **osteoradionecrosis (ORN)**, where the bone fails to heal, becomes devitalized, and undergoes progressive destruction rather than repair. **Analysis of Incorrect Options:** * **A & B (Good response/Fast healing):** These are incorrect because radiation damages the osteoblasts and the microvasculature. Instead of healing, the bone becomes brittle and prone to non-healing fractures or secondary infections. * **D (Tumour regression is not affected):** This is incorrect in the context of the periosteum's structural integrity. While the tumor might regress, the collateral damage to the periosteum significantly alters the bone's ability to maintain its matrix, often complicating the clinical outcome. **High-Yield Clinical Pearls for NEET-PG:** * **Osteoradionecrosis (ORN):** Most commonly affects the **mandible** (due to its high density and limited collateral blood supply) following radiotherapy for head and neck cancers. * **Pathogenesis:** Often described by the "3-H" principle: **Hypocellular, Hypovascular, and Hypoxic** tissue. * **Radiographic features:** Irregular lucent areas (moth-eaten appearance), cortical destruction, and sequestration without significant involucrum formation. * **Management:** Hyperbaric oxygen (HBO) therapy is often used to stimulate angiogenesis in irradiated tissues.

Question 1

Which of the following can cause congenital malformations in the fetus?

Accepted Answer

CT abdomen

Answer

MRI

Answer

Doppler ultrasound

Answer

None of the above

Question 2

What is the optimal timing for oxygen administration to be effective during radiotherapy?

Accepted Answer

Just before starting

Answer

During and within microseconds of starting

Answer

After 5 minutes

Answer

After 10 minutes

Question 3

Which of the following tumors is most radioresistant?

Accepted Answer

Osteosarcoma

Answer

Ewing sarcoma

Answer

Cervical carcinoma

Answer

Lymphoma

Question 4

Functional basis of ionizing radiation depends on:

Accepted Answer

Linear energy transfer

Answer

Removal of orbital electron

Answer

Adding orbital electron

Answer

Production of free radicals

Question 5

Radiation caries typically appears after what period of time?

Accepted Answer

3 months

Answer

3 weeks

Answer

6 months

Answer

6 weeks

Question 6

Which of the following organs is most sensitive to radiation-induced cancer?

Accepted Answer

Female breast

Answer

Skin

Answer

Thyroid

Answer

Nerve

Question 7

What is the amount of radiation that causes injury to the CVS or CNS?

Accepted Answer

50 Gy

Answer

7 Gy

Answer

15 Gy

Answer

30 Gy

Question 8

The short-term effects of radiation on a tissue are determined by the sensitivity of its:

Accepted Answer

Parenchymal cells

Answer

Mesenchymal cells

Answer

Ectodermal cells

Answer

None of the above

Question 9

High dose radiotherapy to the pancreas most commonly causes deficiency of which of the following structures?

Accepted Answer

Acinar cells

Answer

Islets of Langerhans

Answer

Both acinar cells and islets of Langerhans

Answer

None of the above

Question 10

What is observed in the periosteum of a bone that has received radiotherapy?

Accepted Answer

More destruction of bone

Answer

Response to radiotherapy is good

Answer

Fast healing

Answer

Tumour regression is not affected

Radiobiology — MCQs

Radiobiology — MCQs

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