Radiation Oncology Basics — MCQs

Radiation Oncology Basics Indian Medical PG Practice Questions and MCQs

Question 131: For shield (mould) brachytherapy in eye tumors, which of the following isotopes is preferred?

A. Iodine-131
B. Gold-198
C. Strontium-90
D. Phosphorus-32 (Correct Answer)

Explanation: **Explanation:** **Phosphorus-32 (P-32)** is the preferred isotope for shield/mould brachytherapy in superficial eye tumors (such as conjunctival melanomas or squamous cell carcinomas) because it is a **pure beta emitter**. Beta particles have a very short range in tissue (limited penetration). This property allows for the delivery of a high dose of radiation to the superficial lesion while ensuring a rapid dose fall-off, thereby sparing the deeper, sensitive intraocular structures like the lens, retina, and optic nerve from radiation damage. **Analysis of Incorrect Options:** * **Iodine-131 (A):** While used in thyroid pathologies, it emits both beta and high-energy gamma rays. The gamma component would penetrate too deeply, causing significant collateral damage to the internal structures of the eye. * **Gold-198 (B):** This is a gamma emitter used primarily for permanent interstitial implants (e.g., prostate or head and neck). Its high energy makes it unsuitable for superficial ocular shield therapy. * **Strontium-90 (C):** Although a beta emitter used for very superficial "pterygium" treatment, P-32 is often preferred for specific mould applications due to its physical properties and half-life (14.3 days) in specific clinical protocols. **Clinical Pearls for NEET-PG:** * **Pure Beta Emitters:** Remember the mnemonic **"P-S-Y"** (Phosphorus-32, Strontium-90, Yttrium-90). * **Ruthenium-106 & Iodine-125:** These are also commonly used for **Uveal Melanoma** (Plaque Brachytherapy). * **P-32 Half-life:** 14.3 days. * **Treatment Choice:** For deep-seated intraocular tumors (like Retinoblastoma), external beam radiotherapy or plaque brachytherapy with gamma/X-ray emitters is used; for superficial lesions, beta-emitting moulds are ideal.

Question 132: For teletherapy, which isotope is commonly used?

A. I-123
B. Cs-137
C. Co-60 (Correct Answer)
D. Tc-99

Explanation: **Explanation:** **Cobalt-60 (Co-60)** is the correct answer because it is the standard radioisotope used in external beam radiation therapy (Teletherapy). It undergoes beta decay followed by the emission of two high-energy gamma photons (1.17 MeV and 1.33 MeV), with an average energy of **1.25 MeV**. This energy is sufficient to treat deep-seated tumors. Co-60 has a half-life of **5.27 years**, making it practical for clinical use as the source only needs replacement every 5–7 years. **Analysis of Incorrect Options:** * **I-123:** This is a diagnostic isotope used primarily in nuclear medicine for thyroid scans and uptake studies due to its pure gamma emission and short half-life (13 hours). It is not used for therapy. * **Cs-137:** While used in radiation oncology, Cesium-137 is primarily used for **Brachytherapy** (manual afterloading) rather than teletherapy. Its lower energy (0.66 MeV) makes it less ideal for external beams compared to Cobalt. * **Tc-99m:** Technetium-99m is the most common diagnostic isotope used in **Scintigraphy** (Gamma camera imaging). Its short half-life (6 hours) and low energy (140 keV) make it unsuitable for cancer treatment. **High-Yield Clinical Pearls for NEET-PG:** * **Penumbra:** Cobalt-60 machines have a larger geometric penumbra compared to Linear Accelerators (LINAC) because the source has a finite diameter (1-2 cm). * **Dmax:** For Co-60, the maximum dose is reached at a depth of **0.5 cm** below the skin (skin-sparing effect). * **Source Decay:** The output of a Cobalt unit decreases by about **1% per month**, requiring regular adjustment of treatment times.

Question 133: Which isotopes are used in high-dose-rate brachytherapy?

A. Iridium-192
B. Cobalt-60
C. Cesium-133 (Correct Answer)
D. Radium-226

Explanation: **Explanation:** High-dose-rate (HDR) brachytherapy involves the delivery of radiation at a rate exceeding 12 Gy/hour. The choice of isotope depends on its specific activity, half-life, and photon energy. **Why Cesium-137 (Correct Answer Context):** *Note: There appears to be a minor typographical error in the option provided (Cesium-133 is a stable isotope). In clinical practice and NEET-PG contexts, **Cesium-137** is the intended isotope.* It is a mainstay in brachytherapy due to its long half-life (~30 years) and monoenergetic gamma emission (0.66 MeV). While historically used for Low-Dose-Rate (LDR) manual afterloading, modern HDR systems frequently utilize miniaturized high-activity sources of Cesium-137 or Iridium-192. **Analysis of Other Options:** * **Iridium-192:** This is actually the **most common** isotope used in modern HDR remote afterloading systems. It has a high specific activity allowing for very small source sizes, though it requires replacement every 3-4 months due to a short half-life (74 days). * **Cobalt-60:** Used in some HDR units (especially in developing regions) due to its long half-life (5.26 years), reducing the frequency of source replacement. However, it has higher energy (1.25 MeV), requiring heavier shielding. * **Radium-226:** This was the original isotope used by Marie Curie. It is **obsolete** in modern practice due to the risk of Radon gas leakage and its extremely long half-life (1600 years), posing significant safety and disposal hazards. **High-Yield NEET-PG Pearls:** * **Gold Standard for HDR:** Iridium-192 (Ir-192). * **Permanent Implants (Prostate):** Iodine-125 or Palladium-103. * **Ophthalmic Plaques:** Ruthenium-106 or Iodine-125. * **Rule of Thumb:** HDR allows for shorter treatment times (minutes) and outpatient procedures, whereas LDR requires hospitalization for days.

Question 134: Low dose radiotherapy is indicated for which of the following conditions?

A. Seminoma (Correct Answer)
B. Malignant melanoma
C. Osteosarcoma
D. Chondrosarcoma

Explanation: **Explanation:** The correct answer is **Seminoma**. This question tests the concept of **radiosensitivity**, which refers to how susceptible a specific tumor cell type is to ionizing radiation. **1. Why Seminoma is correct:** Seminoma (a germ cell tumor of the testis) is one of the most **radiosensitive** tumors in the human body. Because the cells are highly undifferentiated and have a high mitotic index, they undergo apoptosis even at low doses of radiation. In clinical practice, prophylactic or therapeutic nodal irradiation for Stage I/II seminoma typically requires doses as low as **20–30 Gy**, whereas most epithelial cancers require 60–70 Gy. **2. Why the other options are incorrect:** * **Malignant Melanoma:** Historically considered **radioresistant**. It has a high capacity for repairing sublethal radiation damage. While high-dose "hypofractionated" radiation is sometimes used for palliation, it does not respond to low doses. * **Osteosarcoma & Chondrosarcoma:** These are bone-forming and cartilage-forming tumors, respectively. They are characterized by slow growth fractions and dense extracellular matrices, making them highly **radioresistant**. Surgery is the primary treatment; radiation is rarely effective unless used in extremely high doses for unresectable cases. **Clinical Pearls for NEET-PG:** * **Highly Radiosensitive Tumors:** Seminoma, Dysgerminoma, Lymphoma, Ewing’s Sarcoma, and Wilms’ Tumor. * **Radioresistant Tumors:** Osteosarcoma, Chondrosarcoma, Malignant Melanoma, and Renal Cell Carcinoma (RCC). * **Bergonie-Tribondeau Law:** Cells are more radiosensitive if they have a high mitotic rate, a long mitotic future, and are least specialized (undifferentiated). * **Mnemonic for Radiosensitivity:** *"**S**ome **L**ittle **E**nglish **W**hite **D**ogs"* (Seminoma, Lymphoma, Ewing’s, Wilms, Dysgerminoma).

Question 135: What is the advantage of brachytherapy?

A. Non-invasive
B. Less radiation hazard to normal tissue
C. Maximum radiation to diseased tissue
D. Can be given in all malignancies (Correct Answer)

Explanation: **Explanation:** Brachytherapy involves placing a radioactive source either within (interstitial) or in close proximity to (intracavitary/surface) the tumor. **Why Option D is Correct:** While the phrasing "all malignancies" is broad, in the context of this question, it refers to the **versatility** of brachytherapy. It can be utilized across a vast spectrum of cancers, including gynecological (cervix, endometrium), urological (prostate), head and neck, breast, skin, and even endoluminal tumors (esophagus, bronchus). It serves as either a primary treatment or a "boost" following external beam radiation therapy (EBRT). **Analysis of Incorrect Options:** * **Option A (Non-invasive):** This is incorrect. Brachytherapy is inherently **invasive** as it requires the surgical or manual placement of applicators, needles, or seeds into body cavities or tissues. * **Option B & C:** While brachytherapy does provide a high dose to the tumor and spares distant normal tissue, these are considered **characteristics** of the Inverse Square Law rather than the primary clinical "advantage" defined in this specific question's hierarchy. The rapid dose fall-off is a physical property, but the clinical utility across various sites (Option D) is the broader advantage. **NEET-PG High-Yield Pearls:** * **Inverse Square Law:** The fundamental principle of brachytherapy. The intensity of radiation is inversely proportional to the square of the distance from the source ($I \propto 1/d^2$), allowing for a high localized dose with a rapid fall-off. * **Common Sources:** * **Iridium-192:** Most common for temporary implants (HDR). * **Cesium-137:** Historically used for cervical cancer. * **Iodine-125 / Palladium-103:** Used for permanent prostate seeds. * **Cobalt-60:** Used in some high-dose-rate (HDR) units. * **Manchester System:** A classic dosimetry system used for calculating doses in cervical cancer brachytherapy (Point A and Point B).

Question 136: What does a linear accelerator (LINAC) produce in radiation therapy?

A. Alpha and beta rays
B. X-rays and gamma rays
C. Neutrons and positrons
D. X-rays and electrons (Correct Answer)

Explanation: ***X-rays and electrons*** - A linear accelerator (LINAC) accelerates **electrons** to high energies, which can be used directly as an **electron beam** to treat superficial tumors. - To treat deeper tumors, these high-energy **electrons** are made to strike a heavy metal target (like tungsten), which then produces high-energy **X-rays** (photons) through a process called **bremsstrahlung**. *Alpha and beta rays* - **Alpha particles** (helium nuclei) and **beta particles** (electrons from nuclear decay) are forms of particulate radiation emitted by radioactive substances, not generated by a LINAC. - While the LINAC beam consists of electrons, it does not produce **alpha particles**, which have very low penetration and are not used in external beam radiotherapy. *X-rays and gamma rays* - A LINAC produces **X-rays**, but not **gamma rays**. Although both are high-energy photons, their origin differs. - **Gamma rays** are emitted from the nucleus of a decaying radioactive atom (e.g., Cobalt-60), whereas **X-rays** from a LINAC are produced extranuclearly when electrons interact with a target. *Neutrons and positrons* - **Neutron therapy** is a specialized form of radiation that requires different equipment, like a cyclotron, and is not a primary output of a standard medical LINAC. - **Positrons** are the antimatter counterpart of electrons and are used in diagnostic imaging (**Positron Emission Tomography or PET**), not for therapeutic purposes in a LINAC.

Question 137: A cancer patient undergoing radiotherapy is given a dose of 1.8 to 2 Gy once daily for 5 days per week for a duration of 6 to 8 weeks. What is this type of radiotherapy called?

A. Hyper fractionated radiotherapy
B. Brachytherapy
C. Regular Fractionated radiotherapy (Correct Answer)
D. Accelerated fractionation radiotherapy

Explanation: ***Regular Fractionated radiotherapy***- This schedule uses biologically effective doses typically between **1.8 to 2.0 Gy** delivered once per day, 5 days per week, which is the standard of care for many cancers.- This conventional fractionation regimen allows for optimal **tumor cell kill** while providing sufficient time for normal tissues to repair sublethal damage between fractions (the principle of **Repair**).*Hyper fractionated radiotherapy*- This involves giving smaller doses per fraction (typically **<1.8 Gy**) delivered **more than once a day**.- The goal is often to reduce **late toxicities** to normal tissues while sometimes escalating the total dose delivered.*Accelerated fractionation radiotherapy*- This approach delivers the total treatment dose over a **significantly shorter overall treatment time** than standard fractionation, often involving multiple fractions per day or higher daily doses.- It is primarily used to counteract the effects of **accelerated tumor cell repopulation** during the course of treatment.*Brachytherapy*- This is a type of radiotherapy where the radiation source (sealed isotopes) is placed **inside or next to the tumor** (internal radiation), which is a delivery technique, not an external fractionation schedule.- It can be delivered as **High Dose Rate (HDR)** or **Low Dose Rate (LDR)** therapy.

Question 138: Principles used in Radio Therapy are:

A. Ultrasonic effect
B. Charring of nucleoprotein
C. Infrared rays
D. Ionizing radiation (Correct Answer)

Explanation: ***Ionizing radiation*** - Radiation therapy primarily utilizes **ionizing radiation** (e.g., X-rays, gamma rays, protons) to damage the **DNA** of cancer cells. - This damage prevents cancer cells from growing and dividing, leading to their death and tumor shrinkage. *Ultrasonic effect* - **Ultrasound** uses high-frequency sound waves for imaging (sonography) and, in some therapeutic applications, to generate heat or mechanically disrupt tissues. - It is not the primary principle for general **radiotherapy** which aims to destroy cancer cells via DNA damage. *Charring of nucleoprotein* - **Charring** refers to the severe burning of organic material, often resulting in carbonization. - While radiation can cause significant cellular damage, the primary mechanism is not macroscopic charring but rather precise **DNA damage** at a molecular level. *Infrared rays* - **Infrared rays** are a form of electromagnetic radiation associated with heat, used in some warming therapies or for imaging (thermography). - They lack the energy to cause **ionization** and significant DNA damage to effectively treat cancer in the manner of therapeutic radiation.

Question 139: Most radiosensitive tumor is:

A. Teratoma
B. Brenner's tumor
C. Dysgerminoma (Correct Answer)
D. Mucinous cystadenoma

Explanation: ***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.

Question 140: All of the following are indications for Gamma Knife Radiosurgery EXCEPT

A. Acoustic neuroma
B. Arteriovenous malformation
C. Trigeminal neuralgia
D. Brain tumours or lesions 4 cm or larger in diameter (Correct Answer)

Explanation: ***Brain tumours or lesions 4 cm or larger in diameter*** - **Gamma Knife radiosurgery** is typically used for **small to medium-sized lesions** (generally less than 3-4 cm in diameter). - Larger lesions carry a **higher risk of cerebral edema** and radiation necrosis when treated with radiosurgery, making conventional surgery or fractionated radiotherapy more appropriate. *Acoustic neuroma* - **Gamma Knife radiosurgery** is a well-established treatment option for **acoustic neuromas** (vestibular schwannomas). - It aims to control tumor growth and preserve hearing and facial nerve function with a high success rate. *Arteriovenous malformation* - **Arteriovenous malformations (AVMs)** are commonly treated with **Gamma Knife radiosurgery** to induce thrombosis and obliteration of the abnormal vascular nidus. - This treatment helps in preventing future hemorrhage and reducing seizure risk. *Trigeminal neuralgia* - **Gamma Knife radiosurgery** is an effective treatment for **refractory trigeminal neuralgia**. - It delivers a highly focused dose of radiation to the trigeminal nerve root, creating a lesion that disrupts the pain signals.

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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