Radiation Oncology Basics — MCQs

Radiation Oncology Basics Indian Medical PG Practice Questions and MCQs

Question 161: Which of the following radioisotopes is MOST commonly used in brachytherapy for cervical cancer?

A. Iridium-192 (Correct Answer)
B. Cobalt-60
C. Cesium-137
D. Radium-226

Explanation: ***Iridium-192*** - **Iridium-192** is the most commonly used radioisotope for **high-dose-rate (HDR)** brachytherapy in cervical cancer due to its optimal energy and half-life for this application. - Its **higher dose rate** allows for shorter treatment times and outpatient procedures, making it widely preferred in modern radiation oncology. *Cobalt-60* - **Cobalt-60** was historically used in brachytherapy but has largely been replaced by other isotopes due to its **higher energy** and longer half-life, which pose greater shielding challenges. - While still used in some external beam radiation therapy units, it is less common for intracavitary brachytherapy in cervical cancer compared to Iridium-192. *Cesium-137* - **Cesium-137** is primarily used in **low-dose-rate (LDR)** brachytherapy, which involves continuous irradiation over several days. - While effective, LDR brachytherapy with Cesium-137 requires inpatient admission and prolonged immobilization, making it less commonly used than **HDR brachytherapy** with Iridium-192 in many contemporary practices for cervical cancer. *Radium-226* - **Radium-226** was the first radioisotope used for brachytherapy in cervical cancer and was widely employed historically. - However, due to **radiation safety concerns** (long half-life of 1600 years) and the availability of safer alternatives like Iridium-192 and Cesium-137, it has been largely phased out in modern practice.

Question 162: Mantle field radiation is primarily used in the treatment of which condition?

A. Hodgkin's lymphoma (Correct Answer)
B. Mantle cell lymphoma
C. Multiple myeloma
D. Cervical cancer

Explanation: ***Hodgkin's lymphoma*** - **Mantle field radiation** is a historical technique primarily used for treating **supradiaphragmatic lymph node involvement** in Hodgkin's lymphoma. - This technique covers lymphatic regions such as the **neck**, **axillae**, and **mediastinum** in a single field due to the predictable spread patterns of the disease. *Mantle cell lymphoma* - This is a type of **non-Hodgkin lymphoma** that typically involves widespread disease at diagnosis, making regional radiation like mantle field therapy less suitable. - Treatment usually involves systemic chemotherapy and sometimes **stem cell transplant**. *Multiple myeloma* - Multiple myeloma is a **plasma cell malignancy** that primarily affects the **bone marrow** throughout the body. - Its widespread nature requires systemic therapies, and radiation is typically used for **localized pain control** or specific skeletal lesions, not wide-field regional approaches. *Cervical cancer* - Cervical cancer is a **gynecologic malignancy** typically treated with a combination of **surgery**, **pelvic radiation**, and **chemotherapy**. - The anatomy and lymphatic drainage patterns are distinct, making mantle field radiation irrelevant for this condition.

Question 163: A woman with endometrial carcinoma is undergoing radiotherapy. Which of the following statements about radiation therapy is true?

A. Small intestinal mucosa is radioresistant.
B. Rapidly proliferating cells are radioresistant.
C. Intensity is inversely proportional to the square of the distance from the source. (Correct Answer)
D. Small blood vessels are radioresistant.

Explanation: ***Intensity is inversely proportional to the square of the distance from the source.*** - This statement accurately describes the **inverse square law**, a fundamental principle in radiation physics, meaning radiation intensity decreases rapidly as the distance from the source increases. - This principle is crucial in **radiotherapy planning** to ensure precise dose delivery to the tumor while minimizing exposure to surrounding healthy tissues. *Small blood vessels are radioresistant.* - **Small blood vessels** (capillaries and arterioles) are actually **radiosensitive** and are often damaged by radiation, leading to late effects such as fibrosis and atrophy. - Damage to the vascular endothelium can cause **vascular insufficiency**, contributing to long-term tissue damage in irradiated areas. *Rapidly proliferating cells are radioresistant.* - Cells that are **rapidly proliferating** (have a high mitotic rate) are generally **radiosensitive**, making them more susceptible to radiation-induced damage. - This is the basis for using radiation therapy to target fast-growing cancers, as the radiation effectively destroys cells during their division phase. *Small intestinal mucosa is radioresistant.* - The **small intestinal mucosa** is composed of rapidly dividing cells and is therefore among the **most radiosensitive tissues** in the body. - This radiosensitivity often leads to common side effects of abdominal and pelvic radiotherapy, such as **nausea, vomiting, and diarrhea**.

Question 164: Which is the treatment of choice for irradiation in Chordoma?

A. Protons (Correct Answer)
B. Electron therapy
C. Gamma knife surgery
D. 3D conformal radiation therapy

Explanation: ***Protons*** - **Proton therapy** is the treatment of choice for chordoma due to its ability to deliver a high dose of radiation precisely to the tumor while sparing surrounding healthy tissues, crucial for tumors located near critical structures like the brainstem or spinal cord. - The **Bragg peak** associated with proton beams allows for a sharp drop-off in dose beyond the target, minimizing damage to adjacent normal tissue. *Electron therapy* - **Electron beams** have a limited penetration depth and are primarily used for superficial tumors. - Chordomas are typically deep-seated tumors, making electron therapy ineffective for delivering a sufficient dose to the entire tumor volume. *Gamma knife surgery* - **Gamma knife surgery** is a form of stereotactic radiosurgery primarily used for small, well-defined tumors, often in the brain. - Chordomas are often larger, irregularly shaped, and can be located in areas not suitable for a single-fraction, high-dose treatment like Gamma Knife radiosurgery. *3D conformal radiation therapy* - While **3D conformal radiation therapy (3D-CRT)** can shape radiation beams to the tumor, it delivers a continuous dose through the entire path of the beam, leading to a higher dose to surrounding healthy tissues compared to proton therapy. - The limitations of 3D-CRT in sparing adjacent critical structures make it generally less favorable than proton therapy for chordomas, especially in complex anatomical locations.

Question 165: What is the primary indication for stereotactic radiosurgery?

A. Brain metastases (Correct Answer)
B. Medulloblastoma
C. Primary brain tumors
D. Meningioma

Explanation: ***Brain metastases*** - Stereotactic radiosurgery (SRS) is a highly effective primary or adjuvant treatment for patients with a limited number of **brain metastases**, especially those smaller than 3-4 cm. - It offers precise, high-dose radiation to the tumor with minimal damage to surrounding healthy brain tissue, making it ideal for treating multiple lesions without whole-brain radiation side effects. *Primary brain tumors (general category)* - While SRS can be used for some primary brain tumors, the overarching category is too broad to be the *primary* indication, as many primary brain tumors, like **glioblastoma multiforme**, require more extensive treatment. - The use of SRS for primary tumors depends heavily on their type, size, location, and whether they are **recurrent** or **inoperable**. *Medulloblastoma (a type of brain tumor)* - Medulloblastoma is typically treated with a combination of **surgery, craniospinal radiation**, and **chemotherapy** due to its propensity for CSF spread. - SRS is not a primary treatment for medulloblastoma and is generally reserved for rare, localized recurrences rather than initial management. *Meningioma (a type of benign brain tumor)* - SRS is a viable option for **meningiomas**, particularly those that are small, inoperable, or recurrent after surgery. - However, brain metastases represent a more common and frequent indication where SRS is often the first-line radiation treatment due to their distinct characteristics and radioresponsiveness.

Question 166: 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 167: What is the standard radiation dose to point A in the cervix for brachytherapy in the treatment of cervical cancer?

A. 8000 rad (Correct Answer)
B. 6000 rad
C. 10000 rad
D. 4000 rad

Explanation: ***8000 rad*** - The standard **total cumulative radiation dose** to **Point A** in the cervix for the treatment of cervical cancer is approximately **8000 rad (80 Gy)**. - This represents the **combined dose** from external beam radiation therapy (EBRT, typically 45-50 Gy) plus intracavitary brachytherapy (typically 30-40 Gy to Point A). - Point A is a classical reference point defined as **2 cm superior to the external cervical os and 2 cm lateral to the uterine canal**, representing the location where the uterine artery crosses the ureter. - This total dose aims to provide adequate tumor control while minimizing toxicity to surrounding organs like the bladder and rectum. *6000 rad* - A total dose of **6000 rad** is insufficient for definitive local control of cervical cancer. - This dose is below the therapeutic threshold and would result in significantly higher rates of local recurrence and treatment failure. - Adequate doses are essential for curative intent in cervical cancer management. *10000 rad* - A dose of **10000 rad** to Point A would be excessively high and significantly increase the risk of severe acute and late toxicities to surrounding tissues. - Such a high dose could lead to serious complications including **rectovaginal or vesicovaginal fistulas, proctitis, cystitis, bowel strictures, and tissue necrosis**. - The therapeutic window would be exceeded, causing more harm than benefit. *4000 rad* - A dose of **4000 rad** would be substantially lower than the standard therapeutic dose for cervical cancer. - This suboptimal dose would likely result in **inadequate tumor control and increased risk of local recurrence**. - It is far below the dose required for curative treatment of cervical cancer.

Question 168: 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 169: Which of the following radioisotopes is commonly used as a source for external beam radiotherapy in the treatment of cancer patients?

A. Cobalt - 59
B. Cobalt - 60 (Correct Answer)
C. Strontium - 89
D. Radium - 226

Explanation: ***Cobalt - 60*** - **Cobalt-60** is a radioisotope that emits high-energy gamma rays, making it suitable for deep tissue penetration in **external beam radiotherapy**. - Its decay produces gamma rays of 1.17 and 1.33 MeV, which are effective in **damaging cancer cells' DNA**. *Strontium - 89* - **Strontium-89** is a **beta-emitter** primarily used for **palliative treatment of bone metastases**, not external beam radiotherapy. - It accumulates in areas of increased bone turnover, providing localized pain relief. *Radium - 226* - **Radium-226** was historically used in brachytherapy but is now largely replaced due to its long half-life and production of **alpha particles** and **radon gas**, posing significant safety concerns. - Its primary therapeutic use involved internal placement in tumors. *Cobalt - 59* - **Cobalt-59** is the only **stable isotope of cobalt** and is not radioactive, thus it cannot be used as a source for radiotherapy. - It is used in various applications, but not for radiation therapy.

Question 170: Which is used in both teletherapy and brachytherapy?

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

Explanation: ***Cobalt 60*** - **Cobalt-60** is the correct answer as it is used in both **teletherapy and brachytherapy**. - **Teletherapy**: Cobalt-60 teletherapy machines (cobalt units) have been widely used for external beam radiation therapy and remain in use globally. - **Brachytherapy**: Cobalt-60 is used in both low-dose rate (LDR) and high-dose rate (HDR) brachytherapy applications, with Co-60 seeds and sources for various tumor sites. - Its high energy (1.17 and 1.33 MeV gamma rays) and suitable half-life (5.27 years) make it versatile for both modalities. *Iridium 192* - **Iridium-192** is predominantly used in **HDR brachytherapy** as a temporary implant source. - It is the most common radioisotope for modern HDR brachytherapy systems. - While it emits gamma radiation, it is **not routinely used for teletherapy** in clinical practice due to its lower energy and shorter half-life (74 days). *Palladium* - **Palladium-103** is used exclusively in **low-dose rate (LDR) brachytherapy**, particularly for permanent seed implants in prostate cancer. - Its lower energy photons and short half-life (17 days) make it unsuitable for teletherapy. *Iodine 131* - **Iodine-131** is primarily used in **nuclear medicine** for thyroid cancer treatment and hyperthyroidism (targeted radionuclide therapy). - It is not used for conventional external beam teletherapy or brachytherapy implants.

Practice by Chapter

Principles of Radiation Therapy

Practice Questions

Radiation Therapy Equipment

Practice Questions

Treatment Planning Process

Practice Questions

External Beam Radiation Therapy

Practice Questions

Brachytherapy

Practice Questions

3D Conformal Radiation Therapy

Practice Questions

Intensity-Modulated Radiation Therapy

Practice Questions

Image-Guided Radiation Therapy

Practice Questions

Stereotactic Radiosurgery

Practice Questions

Total Body Irradiation

Practice Questions

Palliative Radiation Therapy

Practice Questions

Combined Modality Treatments

Practice Questions

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