Radiobiology — MCQs
On this page
Which brain tumor is the most radiosensitive?
At which gestational period is the fetus most radiosensitive?
The cell most sensitive to radiotherapy is:
Most radio-resistant phase in the cell cycle:
Radiobiology Indian Medical PG Practice Questions and MCQs
Question 121: Which brain tumor is the most radiosensitive?
- A. Glioblastoma multiforme
- B. Astrocytoma
- C. Ependymoma
- D. Medulloblastoma (Correct Answer)
Explanation: ***Medulloblastoma*** - **Medulloblastoma** is highly **radiosensitive** due to its rapid cell proliferation and immature cellular characteristics, making radiation therapy a cornerstone of treatment. - This tumor commonly originates in the **cerebellum** and is one of the most common malignant brain tumors in children. *Ependymoma* - **Ependymomas** are generally only moderately **radiosensitive**; while radiation is used, it is often delivered in higher doses directly to the tumor bed. - These tumors arise from **ependymal cells** lining the ventricles and spinal cord. *Glioblastoma multiforme* - **Glioblastoma multiforme (GBM)** is known for its marked **radioresistance**, requiring high doses of radiation often in combination with chemotherapy, and still having a poor prognosis. - It is the most aggressive and common type of primary **brain tumor in adults**, characterized by rapid growth and extensive infiltration. *Astrocytoma* - The **radiosensitivity** of astrocytomas varies significantly by grade; **low-grade astrocytomas** are relatively radioresistant, while **anaplastic astrocytomas** have intermediate radiosensitivity. - These tumors originate from **astrocytes**, a type of glial cell, and can occur in various parts of the brain and spinal cord.
Question 122: At which gestational period is the fetus most radiosensitive?
- A. 8-15 weeks (Correct Answer)
- B. 10-15 weeks
- C. >20 weeks
- D. 15-20 weeks
Explanation: ***8-15 weeks*** - The period between **8-15 weeks of gestation** is considered the most radiosensitive for the fetus, as the **central nervous system** is undergoing rapid development and cell differentiation. - Exposure to radiation during this time significantly increases the risk of severe **developmental abnormalities**, including **intellectual disability** and **microcephaly**. *10-15 weeks* - While falling within the highly sensitive period, this option is slightly less precise as the most critical window of vulnerability begins earlier, around **8 weeks**. - The peak sensitivity for radiation-induced **severe mental retardation** is specifically stated to be between 8-15 weeks gestational age. *15-20 weeks* - Sensitivity to radiation-induced severe developmental effects, such as **intellectual disability**, decreases significantly after **15 weeks gestation**. - While some risks still exist, the likelihood of major malformations or severe neurological damage is considerably lower compared to the earlier period. *>20 week* - Beyond **20 weeks of gestation**, the fetus is much less susceptible to **teratogenic effects** from radiation exposure, as most organogenesis is complete. - Risks during this later period are more commonly related to **growth restriction**, **premature birth**, or an increased lifetime risk of **childhood cancer**, rather than major developmental anomalies.
Question 123: The cell most sensitive to radiotherapy is:
- A. Neutrophils
- B. Lymphocytes (Correct Answer)
- C. Monocytes
- D. Platelets
Explanation: ***Lymphocytes*** - **Lymphocytes** are highly sensitive to radiation due to their rapid turnover and extensive interphase death, making them particularly vulnerable to DNA damage. - They undergo both **apoptosis** and replicative death even at relatively low doses of radiation, leading to their rapid depletion. *Neutrophils* - **Neutrophils** are relatively less sensitive to radiation compared to lymphocytes. - While radiation can affect neutrophil counts, their life span is shorter, and their precursors in the bone marrow are more radiosensitive than mature circulating neutrophils. *Monocytes* - **Monocytes** are also less radiosensitive than lymphocytes, with counts declining more slowly after radiation exposure. - Their primary role in tissue repair and immune response makes them more resilient to radiation-induced damage. *Platelets* - **Platelets** are highly resistant to direct radiation damage because they are anuclear cell fragments. - Their decline in number after radiotherapy is primarily due to the radiosensitivity of their precursors, **megakaryocytes**, in the bone marrow.
Question 124: Most radio-resistant phase in the cell cycle:
- A. G2
- B. Late S (Correct Answer)
- C. Early S
- D. G1
Explanation: ***Late S*** - The **late S phase** is the **most radio-resistant phase** of the cell cycle due to high levels of DNA repair enzymes and mechanisms active during this period. - During late S phase, chromatin is **tightly packed** and DNA synthesis is accompanied by robust **homologous recombination repair** capacity, making cells less vulnerable to radiation-induced damage. - This is a well-established principle in radiobiology, with cells in late S showing **2-3 times more resistance** compared to G2/M phases. *Early S* - While S phase overall is relatively radio-resistant, **early S is less resistant than late S** because repair mechanisms are not yet fully upregulated. - DNA replication has just begun, and the cell has moderate levels of repair enzymes compared to late S phase. *G1* - Cells in G1 phase have **moderate radio-sensitivity**, more sensitive than S phase but less sensitive than G2/M phases. - While cells have time for repair in G1, they lack the enhanced repair enzyme levels present during S phase. - G1 cells are in an **active metabolic state** preparing for DNA synthesis. *G2* - G2 phase is one of the **most radio-sensitive phases** along with mitosis (M phase). - Cells have **duplicated DNA** (4n content) and are preparing for division, making them highly vulnerable to radiation damage. - Any unrepaired DNA damage will be transmitted to both daughter cells during the upcoming mitosis.
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
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free