External Beam Radiation Therapy — MCQs

10 questions

Radiation mediates its effect by

A pregnant woman with head trauma requires a CT scan of the head. What is the most effective radiation protection measure for the fetus?

Which of the following is most radioresistant?

For the treatment of deep-seated tumors, the following rays are used:

Which of the following is NOT used for internal radiotherapy?

Which radioisotope is commonly used in teletherapy?

In cervical cancer brachytherapy, the primary reference point for dose prescription is -

Which is the treatment of choice for irradiation in Chordoma?

An 85-year-old male with prostate cancer, Gleason score of 6 , and PSA <8 ng/mL. What is the best management approach?

Q10

Intraoperative radiation therapy (IORT) is most commonly used in which of the following cancers?

External Beam Radiation Therapy Indian Medical PG Practice Questions and MCQs

Question 1: Radiation mediates its effect by

A. Protein coagulation
B. Osmolysis of cells
C. Ionization of the molecules (Correct Answer)
D. Denaturation of DNA

Explanation: ***Ionization of the molecules*** - Radiation, particularly **ionizing radiation**, interacts with biological molecules by ejecting electrons, leading to the formation of highly reactive **ions and free radicals** [1]. - This **ionization** process is the primary mechanism by which radiation damages cellular components, including **DNA** [2]. *Protein coagulation* - While radiation can cause protein damage, **coagulation** is not its primary or direct mechanism, especially at clinically relevant doses. - Protein coagulation is more typically associated with **heat** or certain strong chemical agents. *Osmolysis of cells* - **Osmolysis** refers to the rupture of cells due to excessive water influx, often caused by changes in osmotic pressure. - Radiation does not directly induce **osmotic imbalances** leading to cell lysis. *Denaturation of DNA* - While radiation ultimately leads to **DNA damage**, denaturation (unfolding) is a specific type of damage, often caused by heat or extreme pH. - The direct effect of radiation is **ionization**, which then indirectly causes various forms of DNA damage including breaks, cross-links, and base modifications, but not solely "denaturation" [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. 101-102. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Central Nervous System Synapse, pp. 436-437.

Question 2: A pregnant woman with head trauma requires a CT scan of the head. What is the most effective radiation protection measure for the fetus?

A. Using MRI instead
B. Lead apron over abdomen
C. Avoid CT, rely on clinical assessment
D. Reduced mA and kVp (Correct Answer)

Explanation: ***Reduced mA and kVp*** - **Optimizing scan parameters** (reducing mA and kVp) is the most effective way to minimize radiation dose during head CT in pregnancy. - Modern CT scanners with **iterative reconstruction** allow significant dose reduction without compromising diagnostic image quality. - The fetal dose from head CT is already negligible (< 0.01 mGy), but dose optimization further reduces any potential risk. - This directly addresses the radiation source rather than attempting to shield scatter radiation. *Lead apron over abdomen* - Lead shielding provides **minimal to no benefit** during head CT as the fetus is far from the primary beam. - Scatter radiation reaching the pelvis from head CT is negligible. - Lead aprons can interfere with **automatic exposure control (AEC)**, potentially increasing rather than decreasing dose. - Modern radiology guidelines (ACR, ICRP) no longer routinely recommend gonadal shielding for most CT examinations. *CT not recommended* - Withholding indicated imaging in trauma is **inappropriate and potentially dangerous**. - The diagnostic benefit of head CT in trauma far outweighs the negligible fetal risk. - **Maternal well-being** is the priority, and missing a critical head injury poses greater risk to both mother and fetus. *Using MRI instead* - While MRI has no ionizing radiation, it is **not appropriate for acute trauma** evaluation. - MRI takes longer to perform, requires patient cooperation, and is less readily available in emergency settings. - CT remains the **gold standard** for acute head trauma assessment.

Question 3: Which of the following is most radioresistant?

A. Cartilage (Correct Answer)
B. Ewing's sarcoma
C. GIT epithelium
D. Gonadal tumours

Explanation: ***Cartilage*** - **Cartilage** is a connective tissue with a relatively **low metabolic rate** and **avascular nature**, making its cells (chondrocytes) less susceptible to rapid turnover and DNA damage from radiation. - Its **dense extracellular matrix** and limited cellular division contribute to its inherent resistance to ionizing radiation, requiring higher doses to induce significant damage. *Ewing's sarcoma* - **Ewing's sarcoma** is a highly **malignant bone tumor** that is generally considered **radiosensitive** and often treated with radiation therapy. - Its cells are rapidly dividing, making them more vulnerable to the DNA-damaging effects of radiation. *GIT epithelium* - The **gastrointestinal tract (GIT) epithelium** is characterized by **rapid cell turnover** and high mitotic activity to constantly replace damaged cells and absorb nutrients. - This high proliferative rate makes the GIT epithelium highly **radiosensitive**, leading to common side effects like mucositis and diarrhea during radiation therapy. *Gonadal tumours* - Tumors of the **gonads** (e.g., testicular seminoma, ovarian dysgerminoma) are often highly **radiosensitive** and respond well to radiation therapy due to the germ cell origin and rapid proliferation of tumor cells. - The germ cells themselves are very sensitive to radiation, leading to concerns about **fertility preservation** in patients undergoing treatment.

Question 4: For the treatment of deep-seated tumors, the following rays are used:

A. X rays and Gamma rays (Correct Answer)
B. Alpha rays and Beta rays
C. Electrons and positrons
D. High power laser beams

Explanation: ***X rays and Gamma rays*** - **X-rays** and **gamma rays** are high-energy electromagnetic radiation capable of penetrating deep into tissues to target deep-seated tumors. - They induce DNA damage in cancer cells, leading to cell death and tumor regression, making them mainstays in **radiation therapy**. *Alpha rays and Beta rays* - **Alpha particles** have a very short range and high linear energy transfer, making them suitable for superficial tumors or targeted internal delivery rather than deep-seated tumors. - **Beta particles** have a medium range in tissue but are generally less penetrating than X-rays or gamma rays, limiting their effectiveness for deep lesions. *Electrons and positrons* - **Electron beams** are used for superficial tumors due to their limited penetration depth, typically reaching only a few centimeters into tissue. - **Positrons** are used in imaging (PET scans) and are not directly used for therapeutic tumor ablation, as their annihilation with electrons produces gamma rays, but they themselves don't treat tumors. *High power laser beams* - **Laser beams** are primarily used for superficial tissue ablation, cutting, or coagulation in surgical procedures due to their limited direct penetration into deep tissues without significant scattering and absorption. - While lasers can be used in some interstitial tumor treatments, they are not a primary method for treating large, deep-seated tumors due to their localized effect and lack of volumetric penetration.

Question 5: Which of the following is NOT used for internal radiotherapy?

A. Iridium-192
B. Iodine-125
C. Cobalt-60 (Correct Answer)
D. Iodine-131

Explanation: ***Cobalt-60*** - **Cobalt-60** is primarily used for **external beam radiotherapy** (teletherapy) due to its high-energy gamma emissions (1.17 and 1.33 MeV). - Its large size and significant shielding requirements make it unsuitable for direct internal placement. *Iridium-192* - **Iridium-192** is a common radionuclide used for **brachytherapy** (internal radiotherapy), often in high-dose rate (HDR) applications. - It emits medium-energy gamma rays and electrons, suitable for temporary implantations in various cancers. *Iodine-125* - **Iodine-125** is widely used in **low-dose rate (LDR) brachytherapy**, particularly for **prostate cancer**, as permanent implants. - It emits low-energy gamma rays and X-rays with a relatively long half-life, allowing localized treatment with minimal dose to surrounding healthy tissue. *Iodine-131* - **Iodine-131** is a radiopharmaceutical used for **systemic internal radiotherapy**, especially in the treatment of **thyroid cancers** and hyperthyroidism. - It is administered orally or intravenously and selectively absorbed by thyroid tissue, delivering therapeutic beta particles and diagnostic gamma rays.

Question 6: Which radioisotope is commonly used in teletherapy?

A. Ra-226
B. Cs-137
C. Co-60 (Correct Answer)
D. Ir-192

Explanation: ***Co-60*** - **Cobalt-60** is a widely used radioisotope in teletherapy (external beam radiotherapy) due to its high-energy gamma emissions (1.17 and 1.33 MeV). - Its relatively long half-life of **5.27 years** makes it practical for sustained clinical use in **teletherapy units**. *Ra-226* - **Radium-226** was historically used in brachytherapy but has largely been replaced due to its alpha emissions, which are difficult to shield, and its long-lived radioactive decay products. - Its use for teletherapy is **not common** because of these safety concerns and the availability of more suitable isotopes. *Cs-137* - **Cesium-137** is primarily used in **brachytherapy** and some low-dose rate teletherapy machines for specific applications, but not as commonly as Co-60 for general teletherapy. - Its lower gamma energy (0.662 MeV) and shorter half-life than Co-60 (30.17 years) make it less ideal for the widespread **deep penetration** required in many teletherapy treatments. *Ir-192* - **Iridium-192** is predominantly used in **high-dose-rate (HDR) brachytherapy** for temporary implants, delivering radiation over short periods. - Its relatively short half-life of **73.8 days** and lower average gamma energy make it unsuitable for typical long-term teletherapy external beam applications.

Question 7: 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 8: 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.

Question 9: An 85-year-old male with prostate cancer, Gleason score of 6 , and PSA <8 ng/mL. What is the best management approach?

A. Active surveillance/Watchful waiting (Correct Answer)
B. Radical prostatectomy
C. External beam radiation therapy
D. Androgen deprivation therapy

Explanation: Androgen deprivation therapy - **Hormonal therapy** is primarily used for advanced, metastatic, or high-risk localized prostate cancer [1], or as an adjunct to radiation therapy. - It is not indicated as a primary first-line treatment for **low-risk, localized prostate cancer** in an elderly patient, as its side effects (e.g., hot flashes, fatigue, bone loss, cardiovascular effects) can significantly impact quality of life without offering a survival advantage in this specific scenario.

Question 10: Intraoperative radiation therapy (IORT) is most commonly used in which of the following cancers?

A. Ca Thyroid
B. Ca Pancreas (Correct Answer)
C. Ca Cervix
D. Ca Breast

Explanation: ***Ca Pancreas*** - **Intraoperative radiation therapy (IORT)** is frequently employed for **pancreatic cancer** due to its deep-seated location and locally advanced nature at presentation. - IORT allows for a **high dose of radiation** (10-20 Gy) to be delivered directly to the tumor bed and involved lymph nodes at the time of surgery, while critical structures like the stomach, duodenum, and kidneys can be retracted or shielded. - Particularly useful in **borderline resectable or locally advanced cases** where complete resection margins are difficult to achieve. - Used in specialized centers as part of multimodal therapy to improve local control. *Ca Thyroid* - **Thyroid cancer** is generally treated with surgery (thyroidectomy) followed by **radioactive iodine (RAI) therapy** for papillary and follicular types, not typically IORT. - The thyroid gland's superficial location and high avidity for iodine make RAI an effective targeted therapy. - IORT has no established role in standard thyroid cancer management. *Ca Cervix* - **Cervical cancer** treatment involves surgery, **external beam radiation therapy (EBRT)**, and **brachytherapy**, which places radioactive sources directly into or near the tumor. - Brachytherapy is superior for cervical cancer due to excellent dose distribution to the cervix and parametrium. - IORT is not a standard approach for primary cervical cancer, though it might be considered in select recurrent cases. *Ca Breast* - For **breast cancer**, IORT has gained significant traction, particularly for **early-stage disease** (T1-T2, node-negative) as an alternative to 5-6 weeks of external beam radiation. - **TARGIT-A and ELIOT trials** have established IORT as a viable option for partial breast irradiation during breast-conserving surgery. - While increasingly used globally with dedicated devices (INTRABEAM, ELIOT), it remains a **selective option** rather than universally applied. - The indication is more specific (favorable early-stage tumors) compared to the broader applications in pancreatic cancer where dose escalation and organ sparing are critical challenges.

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