Principles of Radiation Therapy — MCQs
Precisely directed high dose radiation is used in which of the following therapies?
The somatic non-stochastic effect of radiation is seen
Cells are most sensitive to ionizing radiation during which phase?
In the fetus, deterministic effects due to radiation are less likely to occur below the dose of?
Radiation causes cell death by:
Which radiotherapy technique involves the use of remote afterloading to deliver radiation directly to the tumor?
In the context of medical imaging, which parameter of electromagnetic radiation remains constant?
Which of the following is/are most radioresistant?
In cervical cancer brachytherapy, the primary reference point for dose prescription is -
Which is the treatment of choice for irradiation in Chordoma?
Principles of Radiation Therapy Indian Medical PG Practice Questions and MCQs
Question 1: Precisely directed high dose radiation is used in which of the following therapies?
- A. EBRT
- B. IMRT
- C. Brachytherapy
- D. Stereotactic radiosurgery (Correct Answer)
Explanation: ***Stereotactic radiosurgery*** - **Stereotactic radiosurgery (SRS)** is a highly precise radiation therapy that uses focused, high-dose radiation beams to target small tumors or abnormalities with **sub-millimeter accuracy**. - It delivers **very high doses per fraction** (typically 15-24 Gy in a single session) using stereotactic guidance systems. - Commonly used for **brain metastases, AVMs, acoustic neuromas**, and other small intracranial targets. *IMRT* - **Intensity-modulated radiation therapy (IMRT)** is an advanced form of 3D-conformal radiation therapy that modulates beam intensity to conform to tumor shape. - While IMRT is precise, it uses **conventional fractionation** (1.8-2 Gy per fraction over many treatments), not the high-dose approach of SRS. *EBRT* - **External beam radiation therapy (EBRT)** is a general term for radiation delivered from outside the body. - It encompasses various techniques but *does not specifically indicate the **stereotactic precision and high-dose per fraction** characteristic of SRS*. *Brachytherapy* - **Brachytherapy** involves placing radioactive sources **directly inside or next to the tumor**. - While it delivers high doses locally, it is not "precisely directed high-dose radiation" from external beams like SRS.
Question 2: 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 3: Cells are most sensitive to ionizing radiation during which phase?
- A. S phase
- B. G2M phase (Correct Answer)
- C. G0 phase
- D. G1 phase
Explanation: ***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.
Question 4: 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 5: Radiation causes cell death by:
- A. Charring of nucleoproteins
- B. Ionization (Correct Answer)
- C. Disruption of cytosol
- D. Destroying their mitochondria
Explanation: ***Ionization*** - Radiation, particularly **ionizing radiation**, causes cell death by directly or indirectly damaging cellular components through the process of **ionization**. [1] - This involves the removal of electrons from atoms or molecules, leading to the formation of highly reactive **free radicals** (especially hydroxyl radicals from water radiolysis) that can damage DNA, proteins, and lipids. [1] - The most critical lethal lesion is **DNA double-strand breaks**, which are difficult to repair and trigger apoptosis or mitotic catastrophe. [1] *Charring of nucleoproteins* - **Charring** typically refers to the combustion or burning of organic matter, which is not the mechanism of cell death caused by therapeutic radiation doses. - While radiation can cause protein denaturation, it does not lead to the macroscopic charring of nucleoproteins within cells. *Disruption of cytosol* - While severe radiation damage can impact the entire cell, direct and selective **disruption of the cytosol** is not the primary or most impactful mechanism of radiation-induced cell death. - The critical targets for radiation-induced cell death are primarily the **nucleus** and its DNA, not the cytoplasm. [2] *Destroying their mitochondria* - Although radiation can induce **mitochondrial dysfunction** and contribute to cell death through apoptosis, it is not the initial or primary mechanism of cell destruction. - The most critical and direct damage leading to cell death is inflicted upon the **DNA** in the nucleus, particularly causing double-strand breaks. [1] **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 100-102. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Central Nervous System Synapse, pp. 438-439.
Question 6: Which radiotherapy technique involves the use of remote afterloading to deliver radiation directly to the tumor?
- A. Brachytherapy (Correct Answer)
- B. External Beam Radiotherapy
- C. Stereotactic Radiotherapy
- D. Proton Beam Radiotherapy
Explanation: ***Correct: Brachytherapy*** - **Remote afterloading** is a hallmark of modern brachytherapy, where radioactive sources are automatically advanced into catheters placed within or near the tumor. - This technique allows for the delivery of a **high dose of radiation directly to the tumor** while sparing surrounding healthy tissues. - Examples include **intracavitary** (cervical cancer), **interstitial** (prostate cancer), and **intraluminal** (esophageal cancer) brachytherapy. *Incorrect: External Beam Radiotherapy* - This technique involves delivering radiation from a machine **outside the body** to target a tumor. - It does not involve the direct placement of radioactive sources within the patient or the use of **remote afterloading**. *Incorrect: Stereotactic Radiotherapy* - While a precise form of external beam radiotherapy using focused beams, it still involves an **external source** of radiation. - It does not utilize internal radioactive sources or **afterloading techniques**. *Incorrect: Proton Beam Radiotherapy* - This is an advanced form of external beam radiotherapy that uses **protons instead of photons** to deliver radiation with high precision. - It does not involve the placement of radioactive sources within the patient or the use of **remote afterloading**.
Question 7: In the context of medical imaging, which parameter of electromagnetic radiation remains constant?
- A. Intensity
- B. Wavelength
- C. Velocity
- D. Frequency (Correct Answer)
Explanation: ***Frequency*** - The **frequency** of electromagnetic radiation is an intrinsic property determined by the **source** and remains constant regardless of the medium it travels through. - Energy of a photon is directly proportional to its frequency (E=hν), therefore, **energy** also remains constant. *Intensity* - **Intensity** is the power per unit area and is dependent on the **amplitude** of the wave, which can change as the radiation interacts with matter. - As electromagnetic radiation passes through different media or encounters obstacles, its intensity often **decreases** due to absorption or scattering. *Wavelength* - The **wavelength** of electromagnetic radiation changes as it passes from one medium to another because the **velocity** of the wave changes. - This change in wavelength is described by the refractive index of the medium, while the **frequency** remains constant. *Velocity* - The **velocity** of electromagnetic radiation is maximum in a **vacuum** (speed of light, c) and **decreases** as it passes through a medium. - This change in velocity is due to interactions with the atoms and molecules of the medium, affecting how quickly the wave propagates.
Question 8: Which of the following is/are most radioresistant?
- A. Neurons
- B. Muscle cells
- C. Erythrocytes (Correct Answer)
- D. All of the options
Explanation: ***Erythrocytes*** - Erythrocytes are **anucleated** and terminally differentiated cells, meaning they do not divide. Cells that do not divide are generally **radioresistant**. - Their primary function is oxygen transport, and they have a limited metabolic capacity, making them less susceptible to the genetic damage that typically leads to radiation-induced cell death. *Neurons* - While neurons are **post-mitotic** and generally radioresistant compared to rapidly dividing cells, they are still more susceptible than mature erythrocytes. - High doses of radiation can lead to neuronal damage and death through mechanisms like **apoptosis** and indirect effects from damage to surrounding glial cells and vasculature. *Muscle cells* - Muscle cells (myocytes) are **terminally differentiated** and have a low mitotic rate, making them relatively radioresistant. - However, they are still more sensitive to radiation than erythrocytes, and high doses can cause muscle degeneration and fibrosis. *All of the options* - This option is incorrect because while neurons and muscle cells are relatively radioresistant, **erythrocytes are demonstrably the most radioresistant** among the choices due to their complete lack of a nucleus and inability to divide.
Question 9: In cervical cancer brachytherapy, the primary reference point for dose prescription is -
- A. Point A (Correct Answer)
- B. Point B
- C. Side walls of pelvis
- D. Point H
Explanation: ***Point A*** - **Point A** is defined as 2 cm lateral to the central canal of the uterus and 2 cm superior to the external os, representing a dose estimation to the **parametrium** and a critical reference for tumoricidal dose. - This point serves as the **primary prescription and reporting point** for brachytherapy in cervical cancer, as it is highly correlated with treatment outcomes and complications. - Established by **ICRU Report 38** as the standard reference point for dose prescription. *Point B* - **Point B** is located 5 cm from the midline (3 cm lateral to Point A) at the level of Point A, and is primarily used to estimate the dose received by the **pelvic side wall** and regional lymphatics. - It provides an indication of dose to structures further from the applicator but is **not the primary prescription point** for the target volume in brachytherapy. *Side walls of pelvis* - The dose to the **side walls of the pelvis** is relevant for assessing potential toxicity to structures like the obturator nerve and external iliac vessels, and for ensuring adequate coverage of pelvic lymph nodes. - While critical for treatment planning, the side walls themselves are not a primary dose prescription point but rather a **region of interest** for dose constraints and coverage. *Point H* - **Point H** represents the reference point for estimating the dose to the **rectum** in brachytherapy, located at the posterior vaginal wall. - While important for assessing **rectal toxicity** and as a dose-limiting structure, Point H is used for reporting organ-at-risk doses, not for primary tumor dose prescription.
Question 10: Which is the treatment of choice for irradiation in Chordoma?
- A. Protons (Correct Answer)
- B. Electrons
- C. Gamma radiation
- D. 3D - CRT
Explanation: ***Protons*** - Proton therapy is the treatment of choice for **chordoma** due to its ability to deliver a high dose of radiation directly to the tumor while minimizing dose to surrounding healthy tissues. - This precision is critical for tumors located near **sensitive structures**, such as the brainstem, spinal cord, or optic nerves, common sites for chordomas. *Electrons* - **Electron therapy** is typically used for superficial tumors because electrons rapidly deposit their energy within the first few centimeters of tissue. - Chordomas are often deeply seated tumors, making electron therapy an unsuitable option for comprehensive treatment. *Gamma radiation* - **Gamma radiation**, as delivered by techniques like **Gamma Knife radiosurgery**, is primarily used for smaller, well-circumscribed intracranial lesions. - While precise, it may not be ideal for the larger, often irregularly shaped chordomas found in the skull base or sacrum, and it lacks the dose-sparing capabilities of proton beams at depth. *3D - CRT* - **3D Conformal Radiation Therapy (3D-CRT)** uses multiple beams to shape the radiation dose to the tumor, offering better conformity than conventional radiation. - However, compared to proton therapy, 3D-CRT still deposits a significant amount of radiation in tissues both distal and proximal to the tumor, leading to a higher risk of side effects, which is particularly concerning for chordomas given their proximity to critical structures.
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