Radiation Oncology Basics — MCQs

Radiation Oncology Basics Indian Medical PG Practice Questions and MCQs

Question 71: Which of the following is provided by a linear accelerator?

A. Electron (Correct Answer)
B. Neuron
C. Proton
D. Infrared rays

Explanation: ### Explanation **Correct Answer: A. Electron** **Underlying Medical Concept:** A **Linear Accelerator (LINAC)** is the most common device used for external beam radiation therapy. It works by using microwave technology to accelerate charged particles—specifically **electrons**—to high speeds through a waveguide. These high-energy electrons can be used in two ways: 1. **Electron Beam Therapy:** Used directly for treating superficial tumors (e.g., skin cancer, nodes) because electrons have a limited range of penetration. 2. **Photon Therapy (X-rays):** By hitting a high-atomic-number target (like Tungsten), the electrons produce high-energy X-rays (photons) via *Bremsstrahlung* radiation for treating deep-seated tumors. **Why Incorrect Options are Wrong:** * **B. Neuron:** Neurons are biological nerve cells and are not produced by radiotherapy machines. (Perhaps confused with *Neutrons*, which are used in specialized particle therapy but not by standard LINACs). * **C. Proton:** Proton therapy requires a **Cyclotron** or **Synchrotron**, not a standard LINAC. Protons are heavy charged particles characterized by the "Bragg Peak." * **D. Infrared rays:** These are low-energy electromagnetic waves used in thermotherapy or diagnostics, not for ionizing radiation therapy in a LINAC. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** LINAC uses a **Magnetron** or **Klystron** to generate microwaves that accelerate electrons. * **Flattening Filter:** Used in LINACs during photon mode to create a uniform beam intensity across the field. * **Bolus:** Often used in electron therapy to bring the maximum dose (Dmax) closer to the skin surface. * **Cobalt-60 vs. LINAC:** Unlike LINACs, Cobalt-60 machines use a radioactive source (Gamma rays) and do not require electricity to produce radiation.

Question 72: Which is the most radiosensitive bone tumor?

A. Ewing's sarcoma (Correct Answer)
B. Osteosarcoma
C. Aneurysmal bone cyst
D. Chondroblastoma

Explanation: **Explanation:** **1. Why Ewing’s Sarcoma is the Correct Answer:** Ewing’s sarcoma is a highly malignant, small round blue cell tumor. In radiation biology, tumors with high mitotic rates, poor differentiation, and high cellularity are generally more **radiosensitive** (Law of Bergonié and Tribondeau). Ewing’s sarcoma is exquisitely sensitive to ionizing radiation, which induces DNA damage and rapid apoptosis in these poorly differentiated cells. While surgery is the primary treatment, radiotherapy plays a crucial role in local control, especially for unresectable tumors or as adjuvant therapy. **2. Analysis of Incorrect Options:** * **Osteosarcoma:** This is a **radioresistant** tumor. It is a bone-forming tumor characterized by the production of osteoid; these mature, bone-producing cells do not respond well to standard doses of radiation. Management is primarily surgical with chemotherapy. * **Aneurysmal Bone Cyst (ABC):** This is a benign, reactive bone lesion. While radiation can be used in rare, recurrent cases where surgery is impossible, it is not the primary treatment and is generally avoided due to the risk of radiation-induced secondary sarcomas. * **Chondroblastoma:** This is a benign cartilaginous tumor. Like most cartilage-forming tumors, it is relatively radioresistant and is typically treated with curettage and bone grafting. **3. Clinical Pearls for NEET-PG:** * **Radiological Hallmark:** Ewing’s sarcoma typically presents with a "moth-eaten" lytic appearance and **"onion-skin"** periosteal reaction. * **Genetics:** Associated with the **t(11;22)** translocation (EWS-FLI1 fusion). * **Radiosensitivity Hierarchy:** Among bone tumors, Ewing’s is the most sensitive. Among all tumors, Lymphomas and Seminomas are even more radiosensitive. * **Phemister’s Triad:** Remember this for Tuberculous Arthritis, not Ewing’s (Juxta-articular osteopenia, peripheral erosions, and gradual joint space narrowing).

Question 73: Point B in the treatment of carcinoma cervix receives what dose of radiation?

A. 7000 cGy
B. 6000 cGy (Correct Answer)
C. 5000 cGy
D. 10,000 cGy

Explanation: **Explanation:** In the Manchester system for Brachytherapy in cervical cancer, **Point B** represents the dose to the **pelvic wall lymph nodes** (specifically the obturator nodes). It is anatomically located 2 cm superior to the external cervical os and **5 cm lateral** to the midline. The standard therapeutic dose for Point B is approximately **6000 cGy (60 Gy)**. This dose is achieved through a combination of External Beam Radiation Therapy (EBRT), which treats the whole pelvis, and Brachytherapy. While Point A receives the full radical dose, Point B receives roughly **one-third to one-fourth** of the Point A dose from the brachytherapy source itself, supplemented by EBRT to reach the 60 Gy target. **Analysis of Options:** * **Option A (7000 cGy):** This is closer to the prescribed dose for Point A (traditionally 70–85 Gy), which represents the paracervical triangle where the uterine artery crosses the ureter. * **Option B (6000 cGy):** **Correct.** This is the standard target dose for the pelvic side wall (Point B) to ensure sterilization of regional lymph nodes. * **Option C (5000 cGy):** While 45–50 Gy is a common dose for initial EBRT, it does not account for the total cumulative dose (EBRT + Brachytherapy) required for Point B. * **Option D (10,000 cGy):** This dose exceeds the tolerance of pelvic normal tissues (rectum/bladder) and would cause severe radiation necrosis. **High-Yield Clinical Pearls for NEET-PG:** * **Point A:** 2 cm superior to external os and **2 cm lateral** to the midline. It represents the crossing of the **uterine artery and ureter**. * **Point B:** 2 cm superior to external os and **5 cm lateral** to the midline. It represents the **obturator nodes**. * **Inverse Square Law:** The rapid dose fall-off between Point A and Point B is due to the inverse square law, protecting the pelvic side wall from the high intensity of the central brachytherapy source.

Question 74: Whole body radiation is indicated in which of the following conditions?

A. Advanced head & neck tumours
B. Bone marrow transplantation (Correct Answer)
C. Medulloblastoma
D. Mycosis fungoides

Explanation: **Explanation:** **Total Body Irradiation (TBI)** is a form of radiotherapy where the entire body is exposed to ionizing radiation. The primary clinical indication for TBI is as part of the **conditioning regimen for Bone Marrow Transplantation (BMT)**, particularly in hematologic malignancies like leukemia or lymphoma. **Why BMT is correct:** TBI serves two critical purposes in BMT: 1. **Myeloablation:** It destroys the patient's own diseased bone marrow and residual cancer cells. 2. **Immunosuppression:** It suppresses the host’s immune system to prevent the rejection of the donor’s stem cells (Graft-versus-Host prevention). **Why other options are incorrect:** * **Advanced Head & Neck Tumors:** These are treated with **Localized External Beam Radiation Therapy (EBRT)** or Brachytherapy to spare surrounding healthy tissues like the spinal cord and salivary glands. * **Medulloblastoma:** This requires **Craniospinal Irradiation (CSI)**. Because this tumor spreads via CSF, the entire brain and the complete length of the spinal cord are irradiated, but not the whole body. * **Mycosis Fungoides:** This is a cutaneous T-cell lymphoma treated with **Total Electron Beam Skin Irradiation (TSEI)**. Electrons are used because they have low tissue penetration, treating only the skin surface while sparing internal organs. **High-Yield Clinical Pearls for NEET-PG:** * **Dose:** TBI is usually delivered in fractions (12–15 Gy total) to reduce toxicity. * **Critical Organs:** The **lungs** are the dose-limiting organs in TBI; shields are used to prevent radiation pneumonitis. * **Late Complication:** Cataract formation is a common long-term side effect of TBI.

Question 75: Electron Beam Radiotherapy is used in the treatment of which condition?

A. Deep Seated Brain Tumors
B. Skull Base Chordomas
C. Mycosis Fungoides (Correct Answer)
D. Lung Cancer

Explanation: **Explanation:** **Electron Beam Radiotherapy (EBRT)** is characterized by a **finite range** and a **rapid dose fall-off** beyond a specific depth. Unlike high-energy X-rays (photons) which penetrate deep into the body, electrons deposit their energy superficially, making them ideal for treating skin and subcutaneous lesions while sparing underlying deep structures. **Why Mycosis Fungoides is Correct:** Mycosis Fungoides is a cutaneous T-cell lymphoma that primarily involves the skin. **Total Skin Electron Beam Therapy (TSEBT)** is the gold standard radiotherapeutic approach for generalized disease. It utilizes the low-penetrating power of electrons to treat the entire skin surface to a uniform depth (usually 5–10 mm) without causing toxicity to internal organs like the lungs or bone marrow. **Why Other Options are Incorrect:** * **Deep Seated Brain Tumors & Lung Cancer:** These require high-energy **Photon beams** (X-rays) or Gamma rays. Photons have high penetrability and a "skin-sparing effect," allowing them to reach deep-seated visceral organs. * **Skull Base Chordomas:** These are typically treated with **Proton Beam Therapy** or Stereotactic Radiosurgery (SRS). Protons offer superior dose localization (Bragg Peak) for tumors located near critical structures like the brainstem. **High-Yield Clinical Pearls for NEET-PG:** * **Electron Beam Uses:** Skin cancers (BCC/SCC), lip cancer, chest wall irradiation (post-mastectomy), and nodal boosts. * **The "Rule of Thumb":** The depth of the 80% isodose line (therapeutic depth) for electrons is roughly **Energy (MeV) / 3**. * **Key Advantage:** Rapid dose fall-off protects deep-seated critical organs (e.g., protecting the lung during chest wall irradiation).

Question 76: High brachytherapy dose rate is:

A. 0.4-2 Gy/hour
B. 12 Gy/hour (Correct Answer)
C. 2-12 Gy/hour
D. 0.01-0.3 Gy/hour

Explanation: **Explanation:** Brachytherapy is classified based on the rate at which the radiation dose is delivered to the target tissue. This classification is standardized by the **ICRU (International Commission on Radiation Units and Measurements)**. * **Correct Answer (B):** **High Dose Rate (HDR)** is defined as the delivery of radiation at a rate exceeding **12 Gy/hour** (or >0.2 Gy/min). HDR is commonly used in modern clinical practice (e.g., for cervical or prostate cancer) because it allows for short treatment sessions (minutes), outpatient management, and better radiation safety for staff. **Analysis of Incorrect Options:** * **Option A (0.4–2 Gy/hour):** This defines **Low Dose Rate (LDR)**. Historically, this involved manual loading of radioactive sources (like Cesium-137) where the patient remained hospitalized for several days. * **Option C (2–12 Gy/hour):** This defines **Medium Dose Rate (MDR)**. This is an intermediate category and is less commonly used in modern protocols compared to LDR and HDR. * **Option D (0.01–0.3 Gy/hour):** This range is characteristic of **Ultra-Low Dose Rate (ULDR)**, typically associated with permanent seed implants (e.g., Iodine-125 seeds in prostate brachytherapy). **High-Yield Clinical Pearls for NEET-PG:** * **Radiobiology:** HDR is radiobiologically less efficient than LDR for sparing normal tissues (due to less time for sublethal damage repair); therefore, HDR is usually delivered in multiple small fractions. * **Common Isotopes:** **Iridium-192** is the most common source used in HDR units due to its high specific activity and small source size. * **PDR (Pulsed Dose Rate):** A hybrid technique that mimics LDR by delivering a short HDR pulse (e.g., once every hour).

Question 77: Radiotherapy is the treatment of choice in which of the following malignancies?

A. Ewing's sarcoma (Correct Answer)
B. Osteosarcoma
C. Osteoclastoma
D. Synovial sarcoma

Explanation: **Explanation:** The sensitivity of a tumor to radiation is a key determinant in selecting radiotherapy (RT) as a primary treatment modality. **1. Why Ewing’s Sarcoma is the Correct Answer:** Ewing’s sarcoma is a **highly radiosensitive** tumor belonging to the "Small Round Blue Cell Tumor" family. While the current standard of care is a multimodal approach (Neoadjuvant Chemotherapy followed by local control), RT is the treatment of choice for local control when the tumor is **surgically unresectable** or located in sites where surgery would cause morbid functional loss (e.g., spine or pelvis). Historically, it was treated with RT alone, and it remains the most radioresponsive bone malignancy among the options provided. **2. Analysis of Incorrect Options:** * **Osteosarcoma:** This is a **radioresistant** tumor. The primary treatment is surgical resection (limb-salvage surgery) with chemotherapy. RT is only used for palliation or in rare cases of positive surgical margins. * **Osteoclastoma (Giant Cell Tumor):** This is generally treated with **extended curettage**. RT is avoided because it carries a high risk of inducing **malignant transformation** (post-radiation sarcoma) in the residual cells. * **Synovial Sarcoma:** While more sensitive than other soft tissue sarcomas, the primary treatment is **wide local excision**. RT is used as an adjuvant (supplementary) therapy rather than the primary treatment of choice. **Clinical Pearls for NEET-PG:** * **Most Radiosensitive Bone Tumor:** Ewing’s Sarcoma. * **Most Radioresistant Bone Tumor:** Osteosarcoma. * **Radiological Hallmark:** Ewing’s sarcoma typically shows an "onion-skin" periosteal reaction and a "moth-eaten" appearance on X-ray. * **Genetic Marker:** Ewing’s is associated with translocation **t(11;22)** involving the EWS-FLI1 gene.

Question 78: For which malignancy is dog leg field radiotherapy primarily indicated?

A. Lymphoma
B. Seminoma (Correct Answer)
C. Bronchogenic cancer
D. Cancer cervix

Explanation: **Explanation:** The **Dog-leg field** is a classic radiotherapy technique used primarily for **Stage I and IIa Seminoma**. The name is derived from its shape, which resembles the hind leg of a dog. **1. Why Seminoma is correct:** Seminomas are highly radiosensitive. The primary lymphatic drainage of the testis is to the **para-aortic nodes** (at the level of L1-L3). The dog-leg field is designed to cover these primary nodes along with the **ipsilateral iliac nodes**. * **The "Vertical" part:** Covers the para-aortic nodes. * **The "Angled" part:** Extends inferiorly and laterally to cover the ipsilateral common, external, and internal iliac lymph nodes. * *Note:* In modern practice, "Para-aortic only" fields are often used for Stage I to reduce toxicity, but the dog-leg remains the classic association for exams. **2. Why other options are incorrect:** * **Lymphoma:** Historically treated with "Mantle fields" (above the diaphragm) or "Inverted-Y fields" (below the diaphragm). The dog-leg is essentially a modified half of an inverted-Y field. * **Bronchogenic cancer:** Treated with localized thoracic fields or prophylactic cranial irradiation (in SCLC), not pelvic/abdominal fields. * **Cancer Cervix:** Treated with "Four-field box" techniques or IMRT to cover the central pelvis and bilateral parametria/nodes. **3. High-Yield Clinical Pearls for NEET-PG:** * **Inverted-Y Field:** Covers para-aortic, bilateral iliac, inguinal, and femoral nodes (used historically for Hodgkin’s). * **Hockey Stick Field:** Another name often used interchangeably with the dog-leg field in some texts. * **Dose:** Seminomas require relatively low doses (approx. 20-25 Gy) due to extreme radiosensitivity. * **Shielding:** During dog-leg irradiation, the contralateral testis must be shielded to preserve fertility.

Question 79: For teletherapy setup, all are used except?

A. Iridium 127 (Correct Answer)
B. Cobalt 60
C. Simulator
D. Computer

Explanation: **Explanation:** The core concept here is the distinction between **Teletherapy** (radiation source at a distance from the patient) and **Brachytherapy** (radiation source placed inside or very close to the tumor). * **Why Iridium 127 is the correct answer:** There is no isotope called Iridium-127 used in clinical practice. The commonly used isotope is **Iridium-192**. Furthermore, Iridium-192 is primarily used in **Brachytherapy** (interstitial or intracavitary) rather than teletherapy. It is the most common source for High Dose Rate (HDR) brachytherapy due to its high specific activity and small source size. * **Why other options are incorrect:** * **Cobalt-60:** This is the classic radioisotope used in **Teletherapy** units. It emits high-energy gamma rays (1.17 and 1.33 MeV) and has a half-life of 5.26 years. * **Simulator:** A simulator is a diagnostic X-ray machine that mimics the geometry of the teletherapy unit. It is essential for treatment planning and verifying the treatment fields before the actual radiation is delivered. * **Computer:** Modern teletherapy (like IMRT, VMAT, or Gamma Knife) relies entirely on Treatment Planning Systems (TPS). Computers are used for dose calculation, contouring, and controlling the Linear Accelerator (LINAC). **High-Yield Facts for NEET-PG:** * **Teletherapy Sources:** Cobalt-60 (Isotope) and Linear Accelerators (LINAC - uses electrons/photons). * **Brachytherapy Sources:** Iridium-192 (most common), Cesium-137, Gold-198, Iodine-125 (Permanent seeds for prostate). * **Cobalt-60 Half-life:** 5.26 years (Requires source change every 5–7 years). * **Gamma Knife:** Uses 192–201 sources of Cobalt-60 for stereotactic radiosurgery.

Question 80: A questionable tooth is removed before radiotherapy to avoid all except:

A. Radiation pulpitis
B. Radiation caries
C. Osteoradionecrosis
D. None of the above (Correct Answer)

Explanation: ### Explanation The primary goal of pre-radiotherapy dental evaluation is to identify and extract teeth with a poor prognosis (e.g., advanced caries, periodontal disease, or periapical lesions) to prevent complications arising from impaired tissue healing and reduced vascularity. **Why "None of the above" is correct:** The question asks which condition is **NOT** avoided by removing a questionable tooth. However, extracting a compromised tooth *before* starting radiotherapy is a standard prophylactic measure specifically designed to prevent **all** the listed complications. Since all options (A, B, and C) are valid reasons for pre-radiation extraction, "None of the above" is the correct choice. **Analysis of Options:** * **Osteoradionecrosis (ORN):** This is the most dreaded complication. Radiation causes "3H" changes: Hypocellularity, Hypovascularity, and Hypoxia. If a tooth is extracted *after* radiotherapy, the bone cannot heal, leading to exposed necrotic bone. Extracting "questionable" teeth beforehand allows for primary healing while the blood supply is still intact. * **Radiation Caries:** Radiotherapy to the head and neck often damages the salivary glands, causing xerostomia (dry mouth). This alters oral flora and pH, leading to rapid, rampant decay. Removing compromised teeth reduces the "niche" for decay and simplifies oral hygiene. * **Radiation Pulpitis:** High-dose radiation can cause inflammation of the dental pulp and loss of odontoblasts. Pre-emptive extraction removes teeth that are already vulnerable to such inflammatory changes. **Clinical Pearls for NEET-PG:** * **The "Golden Period":** Extractions should ideally be performed at least **10–14 days before** radiotherapy begins to allow for adequate mucosal healing. * **ORN Risk:** The risk is highest in the **mandible** (due to lower vascularity compared to the maxilla) and with doses exceeding **60 Gy**. * **Post-Radiation Care:** If an extraction is absolutely necessary *after* radiotherapy, **Hyperbaric Oxygen (HBO)** therapy is often used to stimulate angiogenesis and reduce the risk of ORN.

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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