Radiation Oncology Basics — MCQs

Radiation Oncology Basics Indian Medical PG Practice Questions and MCQs

Question 31: Which of the following is NOT a radiosensitizing drug?

A. 5-Fluorouracil (5-Fu)
B. Bromodeoxyuridine (BUDR)
C. Cyclophosphamide (Correct Answer)
D. Hydroxyurea

Explanation: ### Explanation **Radiosensitizers** are drugs that enhance the lethal effects of ionizing radiation on tumor cells, often by interfering with DNA repair or synchronizing cells into a sensitive phase of the cell cycle. **1. Why Cyclophosphamide is the correct answer:** Cyclophosphamide is a potent **alkylating agent** used primarily as a cytotoxic chemotherapy drug. While it kills cancer cells by cross-linking DNA, it is **not** classified as a radiosensitizer. In fact, combining cyclophosphamide with radiation can significantly increase systemic toxicity and local tissue damage (like hemorrhagic cystitis or lung fibrosis) without specifically sensitizing the tumor to the radiation dose. **2. Analysis of Incorrect Options (Radiosensitizers):** * **5-Fluorouracil (5-Fu):** A classic radiosensitizer. It inhibits thymidylate synthase, depleting the pool of nucleotides required for DNA repair after radiation damage. It is commonly used in gastrointestinal and head/neck cancers. * **Bromodeoxyuridine (BUDR):** A halogenated pyrimidine that incorporates into DNA in place of thymidine. This makes the DNA strand more susceptible to breakage when exposed to radiation. * **Hydroxyurea:** This drug inhibits ribonucleotide reductase and arrests cells in the **G1-S phase** boundary. Since cells are most radiosensitive in the G2 and M phases, and relatively sensitive in late G1/early S, hydroxyurea helps synchronize the cell population for more effective killing. ### NEET-PG High-Yield Pearls: * **Most Radiosensitive Phase:** M phase (followed by G2). * **Most Radioresistant Phase:** Late S phase. * **Hypoxic Cell Sensitizers:** Misonidazole and Nimorazole (mimic oxygen to fix radiation damage). * **Radioprotectors:** Amifostine (scavenges free radicals to protect normal tissue). * **Common Radiosensitizers (Mnemonic: "5-BHC"):** **5**-FU, **B**UDR, **H**ydroxyurea, **C**isplatin.

Question 32: Conformation radiotherapy uses which of the following?

A. Multileaf collimator (Correct Answer)
B. Single leaf collimator
C. Cone
D. Cylinder

Explanation: **Explanation:** **Conformal Radiotherapy (3D-CRT)** is a technique designed to deliver a radiation dose that "conforms" precisely to the three-dimensional shape of the tumor, thereby sparing the surrounding healthy tissues. 1. **Why Multileaf Collimator (MLC) is correct:** The MLC is the key component of modern linear accelerators (LINACs) used to achieve this conformation. It consists of a large number of individual, computer-controlled tungsten "leaves" that can move independently. These leaves can be positioned to create a custom-shaped aperture that matches the projection of the tumor from any given angle, effectively shielding normal tissue while allowing the beam to hit the target. 2. **Why the other options are incorrect:** * **Single leaf collimator:** This does not exist in clinical practice. Standard collimators use two pairs of jaws (X and Y) to create rectangular fields, which cannot conform to irregular tumor shapes. * **Cone:** These are circular attachments used primarily in Stereotactic Radiosurgery (SRS) or electron therapy for small, circular fields. They lack the flexibility to conform to complex, irregular volumes. * **Cylinder:** These are typically used as applicators in brachytherapy (e.g., vaginal cylinders) rather than as beam-shaping devices in external beam radiotherapy. **High-Yield Clinical Pearls for NEET-PG:** * **IMRT (Intensity Modulated Radiotherapy):** An advanced form of conformal therapy where the MLC leaves move *during* treatment to vary the intensity of the beam. * **MLC Material:** Usually made of **Tungsten** due to its high atomic number and density, providing excellent radiation attenuation. * **Goal of Conformal Therapy:** To increase the **Therapeutic Index** (maximizing tumor kill while minimizing Normal Tissue Complication Probability).

Question 33: Which of the following statements is true about medulloblastoma?

A. Highly radiosensitive (Correct Answer)
B. Surgery is the primary treatment modality
C. Predominantly occurs in the adult age group
D. Chemotherapy is effective

Explanation: **Explanation:** **Medulloblastoma** is a highly malignant Grade IV embryonal tumor arising from the cerebellum. It is one of the most **radiosensitive** tumors in the central nervous system. 1. **Why Option A is correct:** Medulloblastoma belongs to the "Small Round Blue Cell Tumor" family. These tumors are characterized by high cellularity and rapid cell division, making them exquisitely sensitive to ionizing radiation. Because of its tendency to spread via cerebrospinal fluid (CSF), the standard of care involves **Craniospinal Irradiation (CSI)** to treat the entire neuraxis. 2. **Why Option B is incorrect:** While surgery is the initial step for maximal safe resection and diagnosis, it is rarely curative alone. The primary treatment is a **multimodal approach** combining surgery, radiation, and chemotherapy. 3. **Why Option C is incorrect:** Medulloblastoma is a pediatric tumor. It is the most common malignant brain tumor in children, typically peaking between ages 3 and 9. It is rare in adults. 4. **Why Option D is incorrect:** While chemotherapy is used (especially in infants to delay radiation or in high-risk groups), it is an **adjunct**. The question asks for the most defining characteristic; its extreme radiosensitivity is a classic radiological and oncological hallmark. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Arises from the **roof of the 4th ventricle** (vermis) in children. * **Imaging:** Appears hyperdense on NCCT (due to high cellularity) and shows "drop metastases" in the spinal subarachnoid space. * **Homer-Wright Rosettes:** A classic histopathological finding. * **Zuckerman’s Point:** A critical area in radiation planning to avoid overlapping doses during CSI.

Question 34: Which of the following is a radio-resistant tumor?

A. Ewing's sarcoma
B. Malignant fibrous histiocytoma (Correct Answer)
C. Seminoma
D. Dysgerminoma

Explanation: Radiosensitivity refers to the relative susceptibility of cells and tissues to the ionizing effects of radiation. In clinical oncology, tumors are categorized based on their response to radiotherapy. **Explanation of the Correct Answer:** **Malignant Fibrous Histiocytoma (MFH)**, now more commonly classified as Undifferentiated Pleomorphic Sarcoma (UPS), is a high-grade soft tissue sarcoma. Most soft tissue sarcomas (with the exception of Rhabdomyosarcoma and Ewing’s) are inherently **radio-resistant**. They possess efficient DNA repair mechanisms and often contain hypoxic zones that make them less susceptible to the free radicals generated by ionizing radiation. Therefore, the primary treatment for MFH is surgical excision rather than radiotherapy. **Explanation of Incorrect Options:** * **Ewing’s Sarcoma:** This is a highly **radiosensitive** small round blue cell tumor. While surgery and chemotherapy are primary, radiotherapy is frequently used for local control. * **Seminoma:** This germ cell tumor of the testis is exquisitely **radiosensitive**. Even low doses of radiation can effectively treat microscopic nodal disease. * **Dysgerminoma:** The ovarian counterpart to the seminoma, this is also highly **radiosensitive**. **NEET-PG High-Yield Pearls:** 1. **Bergonie-Tribondeau Law:** Cells are more radiosensitive if they have a high mitotic rate, a long mitotic future, and are undifferentiated. 2. **Most Radiosensitive Tumors:** Lymphoma, Leukemia, Seminoma, Dysgerminoma, and Wilms’ tumor. 3. **Most Radio-resistant Tumors:** Osteosarcoma, Malignant Melanoma, Pancreatic Carcinoma, and Renal Cell Carcinoma. 4. **Cell Cycle Sensitivity:** Cells are most sensitive in the **M and G2 phases** and most resistant in the **S phase**.

Question 35: Which of the following radioisotopes is used for interstitial therapy?

A. Phosphorus 32
B. Iodine 131
C. Iridium 191 (Correct Answer)
D. Gold 198

Explanation: **Explanation:** **Interstitial Brachytherapy** involves placing radioactive sources directly into the tumor tissue. The ideal isotope for this must have a high specific activity, a manageable half-life, and emit gamma radiation with sufficient energy to cover the tumor volume while sparing distant organs. **Why Iridium-192 (Option C) is correct:** *Note: While the question mentions Iridium-191, the clinically used isotope is **Iridium-192** (likely a typographical error in the source question). It is the most commonly used isotope for interstitial brachytherapy today. It is available as small "seeds" or wires, making it ideal for temporary implants in breast, head and neck, and soft tissue cancers. It has a half-life of **74 days** and emits gamma rays.* **Why the other options are incorrect:** * **Phosphorus-32 (A):** A pure beta-emitter used primarily for **intracavitary** therapy (e.g., malignant effusions) or systemic treatment of polycythemia vera. It lacks the penetration required for interstitial therapy. * **Iodine-131 (B):** Primarily used for **systemic** therapy (unsealed source) in thyroid cancer and hyperthyroidism. It is not used for interstitial implants. * **Gold-198 (D):** While historically used for permanent interstitial implants (seeds), it has been largely replaced by Iridium-192 and Iodine-125 due to its short half-life (2.7 days) and radiation safety concerns. **High-Yield Clinical Pearls for NEET-PG:** 1. **Common Interstitial Isotopes:** Iridium-192 (most common), Cesium-137, and Iodine-125 (Permanent seeds for Prostate). 2. **Intracavitary Therapy (Cervix):** Most commonly uses **Cesium-137** or **Cobalt-60**. 3. **Surface Molds:** Used for skin or mucosal lesions; often uses Iridium-192. 4. **Teletherapy:** The standard source is **Cobalt-60** (Half-life: 5.26 years).

Question 36: Recall phenomenon is seen in:

A. Radiotherapy following chemotherapy
B. Surgery after radiotherapy
C. Radiotherapy after surgery
D. Chemotherapy following radiotherapy (Correct Answer)

Explanation: **Explanation:** **Radiation Recall Phenomenon** is an acute inflammatory reaction that occurs in a previously irradiated area when a patient is subsequently administered certain systemic chemotherapy agents. **1. Why Option D is Correct:** The phenomenon is strictly defined by its sequence: **Chemotherapy following Radiotherapy**. After a patient has completed radiation and the initial skin reaction has healed, the administration of a "triggering" cytotoxic drug reactivates the inflammatory process in the exact same field that was irradiated. It is essentially a "latent" radiation injury being unmasked by chemotherapy. Common triggers include Doxorubicin, Paclitaxel, and Methotrexate. **2. Why Other Options are Incorrect:** * **Option A:** Radiotherapy following chemotherapy is not "recall"; it is simply standard sequential therapy. If chemotherapy sensitizes the tissue *during* radiation, it is called "radiosensitization." * **Options B & C:** Surgery does not trigger a "recall" of radiation effects. While radiotherapy can complicate surgical healing (due to fibrosis or poor vascularity), the specific inflammatory "recall" reaction is unique to pharmacological triggers. **3. Clinical Pearls for NEET-PG:** * **Most Common Site:** Skin (Radiation Recall Dermatitis), appearing like a severe sunburn or even blistering. * **Internal Organs:** It can also occur in the lungs (pneumonitis) or GI tract. * **Time Interval:** It can occur weeks, months, or even years after the original radiotherapy. * **Common Triggering Agents:** **Doxorubicin** (most common), Actinomycin-D, Gemcitabine, and Taxanes. * **Management:** Withdrawal of the triggering agent and administration of corticosteroids.

Question 37: Which of the following isotopes is used in Brachytherapy?

A. Radium
B. Iodine-125
C. Iridium
D. All of the above (Correct Answer)

Explanation: **Explanation:** Brachytherapy is a form of radiotherapy where a sealed radioactive source is placed inside or in close proximity to the area requiring treatment. This allows for a high dose of radiation to be delivered locally to the tumor while sparing surrounding healthy tissues. **Why "All of the Above" is Correct:** All three isotopes listed are historically or currently significant in brachytherapy practice: * **Radium-226:** Historically, Radium was the first isotope used in brachytherapy (pioneered by Marie Curie). Although largely replaced by modern isotopes due to safety concerns (long half-life and radon gas production), it remains the "gold standard" against which other sources are compared. * **Iodine-125:** This is a low-energy gamma emitter commonly used for **permanent interstitial implants**, most notably in the treatment of **Prostate Cancer** (seed brachytherapy). * **Iridium-192:** This is the most widely used isotope in modern **High-Dose-Rate (HDR)** brachytherapy. It is preferred due to its high specific activity and small source size, making it ideal for temporary implants in breast, cervix, and head and neck cancers. **High-Yield Clinical Pearls for NEET-PG:** * **Cobalt-60:** Used in Teletherapy (External Beam Radiation) and Gamma Knife, but rarely in modern brachytherapy. * **Cesium-137:** Frequently used in the past for intracavitary brachytherapy in cervical cancer (Manual Afterloading). * **Gold-198:** Used for permanent implants (similar to I-125). * **Inverse Square Law:** The fundamental physical principle of brachytherapy, explaining why the dose falls off rapidly as distance from the source increases. * **Half-life (T 1/2) to remember:** Ir-192 (~74 days), I-125 (~60 days), Ra-226 (1600 years).

Question 38: Which of the following tumors is most sensitive to radiation?

A. Small cell lung cancer
B. Seminoma (Correct Answer)
C. Soft tissue sarcoma
D. Osteosarcoma

Explanation: **Explanation:** The sensitivity of a tumor to ionizing radiation depends on its cellular kinetics, differentiation, and inherent repair mechanisms. According to the **Bergonie-Tribondeau law**, cells that are rapidly dividing, undifferentiated, and have a high metabolic rate are the most radiosensitive. **1. Why Seminoma is the Correct Answer:** Seminoma is classified as **highly radiosensitive**. It is the classic example of a "radiocurable" tumor. Even at low doses of radiation (20–25 Gy), seminoma cells undergo rapid apoptosis. This extreme sensitivity makes radiotherapy a primary treatment modality for early-stage seminoma. **2. Analysis of Incorrect Options:** * **Small Cell Lung Cancer (SCLC):** While SCLC is considered **radiosensitive** and responds well initially, it is generally less sensitive than germ cell tumors like seminoma. It often requires higher doses and is prone to early recurrence. * **Soft Tissue Sarcoma:** These are generally **radioresistant**. They are bulky, slow-growing, and well-differentiated mesenchymal tumors that require surgical excision as the primary treatment; radiation is typically used only as an adjuvant. * **Osteosarcoma:** This is a classic example of a **highly radioresistant** tumor. The osteoid matrix and the nature of the malignant osteoblasts make radiation ineffective for primary control. **3. NEET-PG High-Yield Pearls:** * **Most Radiosensitive Cells:** Lymphocytes (exception to the rule as they are non-dividing) and Germ cells (Spermatogonia). * **Radiosensitivity Hierarchy:** 1. **High:** Lymphoma, Leukemia, Seminoma, Dysgerminoma, Wilms’ tumor. 2. **Moderate:** Squamous cell carcinoma (e.g., Cervix, H&N), Adenocarcinoma (Breast, GI). 3. **Low (Resistant):** Osteosarcoma, Pancreatic cancer, Malignant Melanoma, Glioblastoma Multiforme. * **Cell Cycle:** Cells are most sensitive in the **M (Mitosis)** and **G2 phases**, and most resistant in the **S (Synthesis) phase**.

Question 39: Regarding the Bragg peak effect of protons, all of the following are true, except:

A. It involves electromagnetic radiation. (Correct Answer)
B. It produces ionization in a biological system.
C. The dose of a single beam is 2-4 times greater than the path dose.
D. It is used in beam therapy for arteriovenous malformation of the brain.

Explanation: ### Explanation The **Bragg Peak** is a fundamental concept in particle therapy, specifically proton beam therapy. It describes the phenomenon where a charged particle deposits the majority of its energy at a specific depth (the end of its range) just before coming to rest, followed by a sharp drop-off to zero dose. **1. Why Option A is the correct answer (The Exception):** The Bragg peak is a characteristic of **heavy charged particles** (like protons, carbon ions, or alpha particles). It does **not** occur with electromagnetic radiation (X-rays or Gamma rays). Photons (electromagnetic radiation) follow an exponential attenuation pattern, depositing their maximum dose near the surface and continuing to deliver exit doses through the body. **2. Analysis of other options:** * **Option B:** Protons are ionizing radiation. As they travel through tissue, they interact with electrons, causing **ionization** which leads to DNA damage in biological systems. * **Option C:** A single proton beam delivers a "peak" dose at the target that is significantly higher (**2 to 4 times**) than the "plateau" dose (the dose delivered along the entry path). This allows for superior sparing of normal tissues compared to conventional radiotherapy. * **Option D:** Due to its precision and rapid dose fall-off, proton beam therapy is clinically utilized for treating deep-seated tumors and vascular abnormalities like **Arteriovenous Malformations (AVMs)**, especially those near critical structures like the brainstem. ### NEET-PG High-Yield Pearls: * **Spread-Out Bragg Peak (SOBP):** In clinical practice, multiple proton beams of varying energies are superimposed to create a "Spread-Out Bragg Peak" to cover the entire volume of a tumor. * **RBE (Relative Biological Effectiveness):** The RBE of protons is approximately **1.1**, meaning they are about 10% more effective at killing cells than X-rays. * **Clinical Advantage:** The primary advantage of protons is the **absence of an exit dose**, significantly reducing long-term side effects and secondary malignancies.

Question 40: What is true about the device shown below, except?

A. Used for vaginal brachytherapy after surgery for uterine cancer.
B. This treatment decreases the risk of vaginal cuff recurrence.
C. Indicated for only large tumor volumes. (Correct Answer)
D. It is placed in the proximal vagina.

Explanation: ***Indicated for only large tumor volumes.*** - **Vaginal cylinders** are actually used for **microscopic or small-volume residual disease**, not large tumor volumes. - Large tumor volumes would require **external beam radiation therapy** or **interstitial brachytherapy**, not intracavitary cylinders. *Used for vaginal brachytherapy after surgery for uterine cancer.* - The **vaginal cylinder** is specifically designed for **intracavitary brachytherapy** following hysterectomy. - Commonly used in **endometrial cancer** patients to deliver targeted radiation to the vaginal cuff. *This treatment decreases the risk of vaginal cuff recurrence.* - **Adjuvant vaginal brachytherapy** significantly reduces the risk of **local recurrence** at the vaginal cuff. - Studies show a **reduction in vaginal recurrence rates** from approximately 7% to 2-3% with this treatment. *It is placed in the proximal vagina.* - The cylinder is positioned in the **upper third of the vagina** (proximal vagina) near the vaginal cuff. - Proper placement ensures optimal **dose distribution** to the surgical margin while sparing surrounding organs.

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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