Radiobiology — MCQs

Radiobiology Indian Medical PG Practice Questions and MCQs

Question 91: Half-life of Cobalt-60:

A. 4.3 years
B. 5.3 years (Correct Answer)
C. 3.3 years
D. 6.3 years

Explanation: ***5.3 years*** - The **half-life of Cobalt-60** is approximately 5.27 years, often rounded to 5.3 years. - This specific half-life is crucial for its applications in **radiotherapy** and **sterilization**, as it determines the rate of decay and activity. *4.3 years* - This value is not the correct half-life for **Cobalt-60**. - A slightly different half-life would significantly impact calculations in medical physics and nuclear applications. *3.3 years* - This value is not accurate for the **half-life of Cobalt-60**. - Such a shorter half-life would mean a faster decay rate, affecting its utility in continuous radiation sources. *6.3 years* - This value is not the correct half-life for **Cobalt-60**. - A longer half-life would imply a slower decay, altering its radioactive properties and safe handling protocols.

Question 92: In the fetus, deterministic effects due to radiation are less likely to occur below the dose of?

A. 0.005 Gy
B. 0.1 Gy (Correct Answer)
C. 5 Gy
D. 0.50 rads

Explanation: ***0.1 Gy*** - For the fetus, **deterministic effects** (e.g., malformations, mental retardation) are generally considered unlikely to occur below a threshold dose of **0.1 Gy** (100 mGy). - This threshold represents a dose below which the probability of observing these effects is very low, although it's important to remember there is no truly "safe" level of radiation exposure. *0.005 Gy* - This dose (5 mGy) is significantly lower than the established threshold for deterministic effects in a fetus. - While it still carries a very small risk of **stochastic effects** (e.g., cancer) over a lifetime, it is not the threshold for deterministic effects. *5 Gy* - A dose of **5 Gy** is an extremely high dose of radiation for a fetus and would almost certainly result in severe **deterministic effects**, including major congenital anomalies, growth restriction, and fetal death, depending on the gestational age. - This dose is far above the threshold for deterministic effects. *0.50 rads* - To compare, 0.50 rads is equal to 0.005 Gy (since 1 rad = 0.01 Gy), which is a very low dose. - As with 0.005 Gy, this dose is below the threshold for deterministic effects in the fetus, but carries a negligible risk of stochastic effects.

Question 93: Most sensitive tissue to Radiation is

A. Skin
B. Gonads (Correct Answer)
C. Spleen
D. Liver

Explanation: ***Gonads*** - **Gonadal cells (spermatogonia and oocytes)** are highly radiosensitive due to their rapid proliferation and differentiation, making them very vulnerable to radiation-induced damage. - Exposure to radiation can lead to **sterility** or genetic mutations in germ cells, which can be passed on to future generations. *Skin* - While skin is a moderately radiosensitive tissue, showing effects like **erythema** and desquamation at certain doses, it is not the most sensitive. - Its regenerative capacity allows for recovery from moderate radiation damage, unlike germ cells. *Spleen* - The **spleen**, as a lymphoid organ, contains rapidly dividing cells, particularly lymphocytes, which are radiosensitive. - However, its sensitivity is generally lower than that of germ cells in the gonads. *Liver* - The **liver** is generally considered a radioresistant organ, requiring much higher doses of radiation to exhibit significant damage. - Hepatocytes have a relatively slow turnover rate compared to other tissues like gonads or bone marrow.

Question 94: The Hematopoietic syndrome results when acute whole-body radiation exposure is above:

A. 200 rad
B. 400 rad
C. 50 rad
D. 100 rad (Correct Answer)

Explanation: ***100 rad*** - The **hematopoietic syndrome** is consistently observed in individuals exposed to whole-body radiation doses of **100 rad (1 Gy)** or higher. - This dose causes significant damage to the **bone marrow**, leading to the suppression of blood cell production and increased susceptibility to infection and hemorrhage. *200 rad* - While a dose of **200 rad** would certainly cause the hematopoietic syndrome, its onset is typically observed at a lower threshold of **100 rad**. - At 200 rad, the syndrome would be more severe, with prolonged pancytopenia and higher mortality if left untreated. *400 rad* - Exposure to **400 rad** is generally considered the **lethal dose for 50% of the population (LD50/60)** without medical intervention, signifying a very severe form of the hematopoietic syndrome. - At this dose, individuals would experience profound bone marrow suppression and are at very high risk for life-threatening infections and bleeding within weeks. *50 rad* - Exposure to **50 rad** typically causes only mild, temporary changes in blood counts, such as a transient decrease in lymphocytes, but generally does not lead to the full clinical picture of the **hematopoietic syndrome**. - While some subtle effects on bone marrow might occur, significant clinical symptoms requiring aggressive intervention are usually not seen at this dose.

Question 95: Most Radiosensitive tumor of the following is –

A. Ca Colon
B. cervical carcinoma (Correct Answer)
C. Ca Kidney
D. Ca Pancreas

Explanation: ***cervical carcinoma*** - **Cervical carcinoma** is generally considered one of the more radiosensitive gynecological malignancies, particularly **squamous cell carcinoma**, which is the most common type. - Its high radiosensitivity means that **radiation therapy** is a primary and highly effective treatment modality, often used alone or in combination with chemotherapy, achieving significant tumor regression and cure rates. *Ca Colon* - **Colorectal cancer** is typically considered to be **radioresistant** compared to many other epithelial cancers. - While radiation therapy can be used in certain settings (e.g., rectal cancer before surgery), it is generally less effective as a primary standalone treatment for the tumor itself due to its inherent resistance. *Ca Kidney* - **Renal cell carcinoma (RCC)**, especially the clear cell type, is well-known for its significant **radioresistance**. - Radiation therapy is therefore not a primary treatment for localized RCC and is usually reserved for palliative care in advanced or metastatic settings. *Ca Pancreas* - **Pancreatic adenocarcinoma** is also known for being a very **radioresistant** tumor. - While radiation therapy is often used in combination with chemotherapy for locally advanced pancreatic cancer, its effectiveness is limited by the tumor's inherent resistance and the proximity of vital organs.

Question 96: Which of the following is most radiosensitive tumor?

A. Medulloblastoma (Correct Answer)
B. Teratoma
C. Craniopharyngioma
D. Astrocytoma

Explanation: ***Medulloblastoma*** - **Medulloblastomas** are highly radiosensitive tumors, meaning they are very responsive to **radiation therapy**. - They originate in the **cerebellum** and are often treated with craniospinal irradiation. *Teratoma* - **Teratomas** contain a variety of tissues from all three germ layers and have variable radiosensitivity depending on their composition. - Mature teratomas are generally **radioresistant**, while immature teratomas can be more sensitive, but typically less so than medulloblastomas. *Craniopharyngioma* - **Craniopharyngiomas** are benign tumors that are generally less radiosensitive than medulloblastomas. - While radiation therapy can be part of their treatment, they often require **surgical resection** due to their location near vital structures. *Astrocytoma* - The radiosensitivity of **astrocytomas** varies significantly with their grade; low-grade astrocytomas are generally less radiosensitive. - High-grade astrocytomas (e.g., glioblastoma) are often treated with radiation, but their overall prognosis remains challenging due to their infiltrative nature and inherent **radioresistance** compared to medulloblastomas.

Question 97: The somatic non-stochastic effect of radiation is seen

A. As effect of mutation
B. As effects on the body irradiated (Correct Answer)
C. As hereditary phenomenon
D. All of the options

Explanation: ***As effects on the body irradiated*** - **Non-stochastic effects** (also called deterministic effects) have a threshold dose below which they do not occur, and their severity increases with dose. - **Somatic effects** refer to effects on the irradiated individual's body cells, rather than germ cells. Thus, this option correctly describes a direct, dose-dependent effect on the exposed individual. *As effect of mutation* - Mutations are typically associated with **stochastic effects** of radiation, which are random in nature and have no threshold dose. - While radiation can cause mutations, the prompt specifies "non-stochastic effect," which refers to predictable, dose-dependent changes. *As hereditary phenomenon* - **Hereditary phenomena** relate to genetic effects passed down to offspring, involving germ cell mutations. - The question specifically asks about **somatic effects**, which refer to effects on the individual's body, not inherited effects. *All of the options* - This option is incorrect because the other choices do not accurately describe the **somatic non-stochastic effect** of radiation. - Only "As effects on the body irradiated" specifically refers to the direct, dose-dependent effects on the exposed organism's body cells.

Question 98: The most radiosensitive cells are

A. Nerve cells
B. Bone-marrow cells (Correct Answer)
C. Breast glandular cells
D. Muscle cells

Explanation: ***Bone-marrow cells*** - **Bone marrow cells**, including **hematopoietic stem cells** and their precursors, are highly radiosensitive due to their rapid proliferation and high mitotic activity. Cells that divide frequently are more susceptible to radiation-induced damage. - This radiosensitivity explains why **radiation therapy** can cause **myelosuppression**, leading to conditions like **anemia**, **leukopenia**, and **thrombocytopenia**. *Nerve cells* - **Mature nerve cells** are generally considered **radioresistant** because they are terminally differentiated and do not undergo mitosis. - While very high doses of radiation can cause neuronal damage, they are far less sensitive than rapidly dividing cells. *Breast glandular cells* - While breast tissue can be affected by radiation and is a concern in imaging, **breast glandular cells** are not among the most radiosensitive cells in the body. - Their radiosensitivity is intermediate compared to bone marrow cells and nerve cells; fast-dividing cells are more sensitive. *Muscle cells* - Similar to nerve cells, **mature muscle cells** are generally **radioresistant** because they are post-mitotic and have a low rate of cell division. - Significant damage to muscle cells typically requires very high doses of radiation.

Question 99: Skin erythema dose is:

A. 300-400 R
B. 400-500 R
C. 200-300 R (Correct Answer)
D. 100-200 R

Explanation: ***200-300 R*** - The **skin erythema dose** refers to the amount of radiation exposure, expressed in Roentgens (R), that typically causes reddening of the skin. - This range historically served as a basic measure for assessing acute radiation effects and was an early practical limit for radiation exposure in medical imaging. *300-400 R* - While within the broader range of doses that can cause skin effects, **300-400 R** is generally considered a higher dose than the threshold for a noticeable, transient erythema. - Exposure at this level might lead to more pronounced or persistent skin reactions. *400-500 R* - Doses of **400-500 R** are significantly high and would typically cause more severe skin reactions, such as blistering or moist desquamation, rather than just transient erythema. - This level of exposure is well beyond what is considered the skin erythema dose threshold. *100-200 R* - A dose of **100-200 R** is generally considered to be below the threshold for reliably inducing noticeable skin erythema in most individuals. - While some mild, transient redness might occur in very sensitive individuals, it is not the commonly accepted range for the skin erythema dose.

Question 100: When is oxygen effective during radiotherapy?

A. During and within microseconds of starting (Correct Answer)
B. Just before starting the therapy
C. After 5 minutes
D. After 10 minutes

Explanation: ***During and within microseconds of starting*** - Oxygen is effective during radiotherapy primarily due to the **oxygen enhancement ratio (OER)**, which describes the increased radiosensitivity of cells in the presence of oxygen. - This effect is almost instantaneous, as oxygen acts as a **radical sensitizer** by stabilizing DNA damage caused by radiation, making it irreparable by cellular repair mechanisms. *Just before starting the therapy* - While having oxygen present just before therapy is important, the actual sensitization effect requires oxygen to be present **during** the radiation exposure itself. - Simply having oxygen before without its presence during treatment will not maximize the therapeutic benefit. *After 5 minutes* - The critical period for oxygen's radiosensitizing effect is during and immediately after the ionization events caused by radiation, which occur over **microseconds**. - Oxygen administered 5 minutes after radiation exposure would be too late to impact the initial damage fixation process. *After 10 minutes* - Similar to the 5-minute mark, oxygen delivered 10 minutes after radiation would have **no significant impact** on the immediate radiation-induced cellular damage. - The window of opportunity for oxygen to enhance radiosensitivity is extremely short, occurring at the moment of radiation interaction with biological molecules.

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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