Radiobiology Practice Questions

Q: Which among the following is the least radiosensitive cell?

Platelets. ***Platelets*** - Platelets are **anuclear cell fragments** derived from megakaryocytes, lacking a nucleus and DNA, which makes them highly resistant to the direct cytotoxic effects of radiation. - They are one of the **least radiosensitive** circulating components; platelet counts typically require very high radiation doses to fall significantly. *Monocytes* - Monocytes are generally considered **moderately radiosensitive**, more susceptible to radiation-induced death than mature neutrophils or platelets. - They exhibit susceptibility to **interphase death** upon irradiation, although less sensitive than lymphocytes. *Lymphocytes* - Lymphocytes are the **most radiosensitive** mature circulating blood cells, undergoing rapid apoptosis (programmed cell death) even at low radiation doses (sub-2 Gy). - Their high sensitivity relates to their dependence on DNA integrity for survival and their prompt initiation of the **apoptotic pathway** following damage. *Neutrophils* - Mature neutrophils are considered relatively radiosensitive, but significantly **less sensitive than lymphocytes** and moderately less sensitive than monocytes. - Their numbers drop less rapidly than lymphocytes primarily because of a large pre-formed **reserve pool** in the bone marrow, providing temporary protection.

Q: Most radiosensitive tumor is:

Dysgerminoma. ***Dysgerminoma*** - **Dysgerminoma** is known to be highly radiosensitive, meaning it responds very well to radiation therapy. - This characteristic is often leveraged in its treatment, particularly for localized or recurrent disease. *Teratoma* - **Teratomas** often contain various differentiated tissues, and their radiosensitivity varies depending on the specific cellular components. - Mature teratomas are generally considered **radioresistant**, while immature teratomas have some sensitivity but less than dysgerminomas. *Brenner's tumor* - **Brenner's tumors** are typically benign ovarian tumors that are not generally treated with radiation due to their usually indolent nature. - They are considered **radioresistant** if malignancy occurs, and surgical resection is the primary treatment. *Mucinous cystadenoma* - **Mucinous cystadenomas** are usually benign epithelial ovarian tumors primarily treated with surgical removal. - They are considered **radioresistant**, and radiation therapy is not a standard treatment modality for these tumors.

Q: Which of the following tissues is most radiosensitive?

Growing skin. ***Growing skin*** - Tissues with actively dividing cells, such as **growing skin**, are highly sensitive to radioactivity due to the disruption of DNA replication and cell division. - This vulnerability also applies to other rapidly proliferating tissues like the **bone marrow** and the **lining of the gastrointestinal tract**. *CNS* - The **central nervous system (CNS)** is generally considered less radiosensitive than rapidly dividing tissues. - While high doses can cause damage, its mature, non-dividing cells are more resistant to the immediate effects of radiation. *Adult bone* - Like the CNS, **adult bone** generally has a lower radiosensitivity because its cells divide much less frequently than those in growing tissues. - However, the bone marrow within the bone is highly radiosensitive due to its active cellular proliferation. *Pancreas* - The **pancreas** is also relatively radioresistant compared to rapidly growing tissues. - While it can be affected by high doses of radiation, chronic or acute pancreatitis due to radiation exposure is less common than damage to highly proliferative organs.

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

0.1 Gy. ***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.

Q: Most sensitive tissue to Radiation is

Gonads. ***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.

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

100 rad. ***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.

Q: Most Radiosensitive tumor of the following is –

cervical carcinoma. ***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.

Q: Cells are most sensitive to ionizing radiation during which phase?

G2M phase. ***G2M phase*** - Cells are most sensitive to ionizing radiation during the **G2 phase** and **M phase** (mitosis) due to the highly condensed chromatin structure and active DNA repair mechanisms being less efficient [2], [3]. - During G2, DNA synthesis is complete, and the cell is preparing for division, making DNA damage particularly detrimental and harder to repair without compromising cell viability [2]. *S phase* - Cells in the **S phase** (DNA synthesis phase) are relatively radioresistant because of active **DNA replication** and associated repair mechanisms. - These repair pathways are highly efficient at correcting DNA damage during replication, making the cell less susceptible to radiation-induced lethality. *G1 phase* - Cells in the **G1 phase** (first gap phase) show intermediate radiosensitivity. - While less sensitive than G2/M phases, G1 cells are more vulnerable than those in late S phase due to active metabolic preparation for DNA synthesis [1]. *G0 phase* - Cells in the **G0 phase** (quiescent phase) are generally **radioresistant** because they are not actively dividing or synthesizing DNA [3]. - They have ample time for DNA repair before re-entering the cell cycle, and their DNA structure is less vulnerable than during active division [3]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 302-303. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 37-38. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Central Nervous System Synapse, pp. 436-437.

Q: Which of the following is most radiosensitive tumor?

Medulloblastoma. ***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 1

Which among the following is the least radiosensitive cell?

Accepted Answer

Platelets

Answer

Lymphocytes

Answer

Neutrophils

Answer

Monocytes

Question 2

Most radiosensitive tumor is:

Accepted Answer

Dysgerminoma

Answer

Teratoma

Answer

Brenner's tumor

Answer

Mucinous cystadenoma

Question 3

Which of the following tissues is most radiosensitive?

Accepted Answer

Growing skin

Answer

CNS

Answer

Adult bone

Answer

Pancreas

Question 4

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

Accepted Answer

0.1 Gy

Answer

0.005 Gy

Answer

5 Gy

Answer

0.50 rads

Question 5

Radiosensitivity of tumour depends on:

Accepted Answer

Histology

Answer

Nucleus atypia

Answer

Blood supply

Answer

Number of cells

Question 6

Most sensitive tissue to Radiation is

Accepted Answer

Gonads

Answer

Skin

Answer

Spleen

Answer

Liver

Question 7

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

Accepted Answer

100 rad

Answer

200 rad

Answer

400 rad

Answer

50 rad

Question 8

Most Radiosensitive tumor of the following is –

Accepted Answer

cervical carcinoma

Answer

Ca Colon

Answer

Ca Kidney

Answer

Ca Pancreas

Question 9

Cells are most sensitive to ionizing radiation during which phase?

Accepted Answer

G2M phase

Answer

S phase

Answer

G0 phase

Answer

G1 phase

Question 10

Which of the following is most radiosensitive tumor?

Accepted Answer

Medulloblastoma

Answer

Teratoma

Answer

Craniopharyngioma

Answer

Astrocytoma

Radiobiology — MCQs

Radiobiology — MCQs

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