Radiation Oncology Basics Practice Questions

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

Phosphorus-32. **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.

Q: For teletherapy, which isotope is commonly used?

Co-60. **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.

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

Cesium-133. **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.

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

Seminoma. **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).

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

X-rays and electrons. ***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.

Q: 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?

Regular Fractionated radiotherapy. ***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.

Q: Principles used in Radio Therapy are:

Ionizing radiation. ***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.

Q: Most radiosensitive tumor is:

Dysgerminoma. ***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.

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

Brain tumours or lesions 4 cm or larger in diameter. ***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.

Question 1

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

Accepted Answer

Phosphorus-32

Answer

Iodine-131

Answer

Gold-198

Answer

Strontium-90

Question 2

For teletherapy, which isotope is commonly used?

Accepted Answer

Co-60

Answer

I-123

Answer

Cs-137

Answer

Tc-99

Question 3

Which isotopes are used in high-dose-rate brachytherapy?

Accepted Answer

Cesium-133

Answer

Iridium-192

Answer

Cobalt-60

Answer

Radium-226

Question 4

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

Accepted Answer

Seminoma

Answer

Malignant melanoma

Answer

Osteosarcoma

Answer

Chondrosarcoma

Question 5

What is the advantage of brachytherapy?

Accepted Answer

Can be given in all malignancies

Answer

Non-invasive

Answer

Less radiation hazard to normal tissue

Answer

Maximum radiation to diseased tissue

Question 6

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

Accepted Answer

X-rays and electrons

Answer

Alpha and beta rays

Answer

X-rays and gamma rays

Answer

Neutrons and positrons

Question 7

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?

Accepted Answer

Regular Fractionated radiotherapy

Answer

Hyper fractionated radiotherapy

Answer

Brachytherapy

Answer

Accelerated fractionation radiotherapy

Question 8

Principles used in Radio Therapy are:

Accepted Answer

Ionizing radiation

Answer

Ultrasonic effect

Answer

Charring of nucleoprotein

Answer

Infrared rays

Question 9

Most radiosensitive tumor is:

Accepted Answer

Dysgerminoma

Answer

Teratoma

Answer

Brenner's tumor

Answer

Mucinous cystadenoma

Question 10

All of the following are indications for Gamma Knife Radiosurgery EXCEPT

Accepted Answer

Brain tumours or lesions 4 cm or larger in diameter

Answer

Acoustic neuroma

Answer

Arteriovenous malformation

Answer

Trigeminal neuralgia

Radiation Oncology Basics — MCQs

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