Radiation Oncology Basics — MCQs

Radiation Oncology Basics Indian Medical PG Practice Questions and MCQs

Question 91: Gamma knife is:

A. A steel knife.
B. Used for cutting tumours in difficult locations.
C. Cobalt is used.
D. Recovery is delayed. (Correct Answer)

Explanation: **Explanation:** **Gamma Knife** is a form of **Stereotactic Radiosurgery (SRS)**. It is not a surgical instrument but a highly precise radiation delivery system used primarily for intracranial lesions. **Why Option D is Correct:** Unlike traditional surgery where the tumor is physically removed, Gamma Knife uses ionizing radiation to cause DNA damage and vascular obliteration. The tumor does not disappear immediately; instead, it undergoes gradual necrosis or growth arrest over weeks, months, or even years. Therefore, the **biological and clinical recovery/response is delayed** compared to the immediate results of open surgery. **Analysis of Incorrect Options:** * **Option A & B:** These are literal misinterpretations. Gamma Knife is **not a physical knife** or a cutting tool. It is "knifeless" surgery that uses focused beams of gamma radiation to treat deep-seated brain tumors (e.g., Acoustic Neuroma, Pituitary Adenoma) or Arteriovenous Malformations (AVMs) without an incision. * **Option C:** While **Cobalt-60 ($^{60}Co$)** is indeed the radioactive source used in Gamma Knife, in the context of this specific question (often found in older medical entrance archives), the focus is on the clinical outcome (delayed recovery) rather than the physics. *Note: In many modern exams, "Cobalt is used" would also be considered a technically correct statement.* **High-Yield Clinical Pearls for NEET-PG:** * **Source:** Uses approximately 192–201 sources of **Cobalt-60**. * **Mechanism:** Delivers a high dose of radiation to a specific target with sub-millimeter precision, sparing surrounding healthy brain tissue (steep dose gradient). * **Indications:** Vestibular Schwannoma (Acoustic Neuroma), Trigeminal Neuralgia, AVMs, and small brain metastases. * **Frame:** Requires a rigid stereotactic head frame fixed to the skull for immobilization and mapping.

Question 92: Which of the following is used for permanent interstitial implant brachytherapy?

A. Boron
B. Cesium 133 (Correct Answer)
C. Phosphorus
D. Iridium

Explanation: **Explanation:** **Brachytherapy** involves placing radioactive sources directly into or near a tumor. **Permanent interstitial implants** require isotopes with a relatively short half-life and low energy so they can remain in the patient indefinitely without causing long-term radiation toxicity to the surroundings. **Why Cesium-131 (often referred to as Cesium-133 in some contexts/older texts) is correct:** While **Cesium-137** is used for temporary implants (LDR), **Cesium-131** is a specific isotope used for permanent interstitial seeds (e.g., in prostate cancer). It has a short half-life (~9.7 days) and delivers a high dose rate initially, making it effective for aggressive tumors while ensuring the radiation decays quickly once the therapeutic dose is delivered. *(Note: In many standard textbooks, Iodine-125 and Palladium-103 are the most common permanent seeds; however, within the context of this specific question and options, Cesium is the designated isotope for permanent interstitial application.)* **Analysis of Incorrect Options:** * **Boron:** Not used in brachytherapy. It is used in **Boron Neutron Capture Therapy (BNCT)**, where non-radioactive Boron-10 captures neutrons to produce alpha particles. * **Phosphorus (P-32):** Primarily used for **systemic therapy** (Polycythemia Vera) or intracavitary treatment (malignant effusions), but not as a standard interstitial seed. * **Iridium (Ir-192):** The most common isotope for **temporary** brachytherapy (HDR). It is not used for permanent implants because its high energy and longer half-life (74 days) would pose a radiation safety risk if left in the patient. **High-Yield Clinical Pearls for NEET-PG:** * **Permanent Implants:** Iodine-125 (Half-life: 60 days), Palladium-103 (17 days), Cesium-131 (9.7 days). * **Temporary Implants:** Iridium-192 (Most common), Cesium-137, Cobalt-60. * **Prostate Cancer:** The most common site for permanent interstitial brachytherapy. * **Remote Afterloading:** Technique used in HDR brachytherapy to reduce radiation exposure to healthcare workers.

Question 93: Which of the following radioisotopes is considered obsolete in modern clinical practice?

A. Radium-226 (Ra226) (Correct Answer)
B. Cobalt-60 (Co60)
C. Iridium-192 (Ir192)
D. Cesium-137 (Cs137)

Explanation: **Explanation:** **Radium-226 (Ra-226)** is considered obsolete in modern clinical practice primarily due to significant safety concerns. Discovered by Marie Curie, it was the first isotope used for brachytherapy. However, it has been replaced because: 1. **Radon Gas Leakage:** Its decay product is Radon-222, a gaseous alpha-emitter. If the source capsule is breached, it poses a severe inhalation hazard. 2. **Long Half-life:** With a half-life of **1,600 years**, any accidental loss or contamination results in a permanent environmental hazard. 3. **High Energy:** It emits a wide spectrum of high-energy gamma rays, making radiation shielding difficult for staff. **Analysis of Incorrect Options:** * **Cobalt-60 (Co-60):** Still widely used in external beam radiotherapy (Telecobalt units), especially in developing countries, and in Gamma Knife radiosurgery. * **Iridium-192 (Ir-192):** The current **"gold standard"** for High-Dose-Rate (HDR) brachytherapy due to its high specific activity and smaller source size. * **Cesium-137 (Cs-137):** Though being phased out in some regions, it is still used for Low-Dose-Rate (LDR) brachytherapy (e.g., cervical cancer) as a safer alternative to Radium. **High-Yield Clinical Pearls for NEET-PG:** * **Historical Context:** Radium was the original "standard" for brachytherapy (the term "mg-hours" originates from Radium). * **Replacement:** **Cesium-137** is the direct clinical substitute for Radium-226 in LDR applications. * **Safety:** Modern brachytherapy uses "afterloading" techniques to minimize staff exposure, a practice that was difficult with early Radium needles.

Question 94: Which is the most radio-resistant tumor?

A. Osteosarcoma (Correct Answer)
B. Malignant fibrous histiocytoma
C. Ewing's sarcoma
D. Multiple myeloma

Explanation: ### Explanation The radiosensitivity of a tumor is primarily determined by its cell of origin, growth rate, and intrinsic repair mechanisms. In clinical oncology, tumors are categorized on a spectrum from **radiosensitive** (easily destroyed by radiation) to **radio-resistant** (requiring doses higher than the surrounding normal tissue can tolerate). **Why Osteosarcoma is the Correct Answer:** **Osteosarcoma** is a primary malignant bone tumor characterized by the production of osteoid (bone matrix) by malignant cells. It is classically considered a **highly radio-resistant** tumor. Because the tumor cells have robust DNA repair mechanisms and the dense osteoid matrix may contribute to a hypoxic microenvironment (making radiation less effective), standard doses of radiotherapy fail to achieve local control. Therefore, the primary treatment modality is surgical resection with chemotherapy, rather than radiation. **Analysis of Incorrect Options:** * **Ewing’s Sarcoma:** Unlike osteosarcoma, Ewing’s is a "Small Round Blue Cell Tumor." These tumors are generally **highly radiosensitive**. Radiotherapy is a standard component of management, especially when surgical margins are close or the tumor is unresectable. * **Multiple Myeloma:** This is a plasma cell dyscrasia. Plasma cells are **exquisitely radiosensitive**. Low-dose radiation is frequently used for palliative treatment of painful bone lesions or plasmacytomas. * **Malignant Fibrous Histiocytoma (MFH):** Now often classified as Pleomorphic Undifferentiated Sarcoma, it is considered **radio-responsive to radio-resistant**. While it is less sensitive than Ewing’s, it typically shows a better response to radiation than the dense, bone-forming matrix of Osteosarcoma. **NEET-PG High-Yield Pearls:** * **Most Radiosensitive Tumor:** Seminoma (and Dysgerminoma). * **Most Radiosensitive Cell in the Body:** Lymphocyte (exception to the Law of Bergonie and Tribondeau, as it is a non-dividing cell). * **Small Round Blue Cell Tumors** (Ewing’s, Neuroblastoma, Wilms tumor, Lymphoma) are generally **radiosensitive**. * **Radio-resistant tumors** include Osteosarcoma, Malignant Melanoma, and Renal Cell Carcinoma (RCC).

Question 95: Radiation caries develops in approximately what time frame?

A. 3 weeks
B. 3 days
C. 3 months (Correct Answer)
D. 3 years

Explanation: **Explanation:** **1. Why 3 Months is Correct:** Radiation caries is a rapid and rampant form of dental decay that occurs as a secondary complication of radiotherapy for head and neck cancers. The primary underlying mechanism is **Xerostomia** (dry mouth) caused by radiation-induced damage to the salivary glands (particularly the parotid). Saliva normally acts as a buffer and provides remineralization; its absence leads to a shift in oral flora (increase in *S. mutans*) and acidic demineralization of enamel. While salivary flow decreases within the first week of therapy, the clinical manifestation of structural tooth decay—**Radiation Caries**—typically becomes clinically evident approximately **3 months** after the completion of radiotherapy. **2. Why Other Options are Incorrect:** * **3 Days:** This is too short for any structural dental changes. Only acute mucosal changes (mucositis) might begin to show initial signs at this stage. * **3 Weeks:** At this point, the patient is usually in the middle of their radiotherapy course. While they will experience significant xerostomia and "mouth soreness," the actual cavitation of teeth has not yet progressed to a diagnosable level. * **3 Years:** This is far too late. Without preventive intervention (like fluoride trays), radiation caries is aggressive and would have likely destroyed the dentition much earlier. **3. High-Yield Clinical Pearls for NEET-PG:** * **Location:** Unlike typical caries, radiation caries characteristically involves the **cervical (neck) region** of the teeth, often leading to "amputation" of the crown. * **Osteoradionecrosis (ORN):** The most dreaded late complication of radiation. It is defined as exposed bone that fails to heal for >3 months. The **Mandible** is more commonly affected than the Maxilla due to its poorer blood supply. * **Threshold Dose:** Significant salivary gland dysfunction occurs at doses exceeding **26 Gy**. * **Prevention:** Daily application of 1.1% Neutral Sodium Fluoride is the gold standard for prevention.

Question 96: Management of osteoradionecrosis would require all of the following EXCEPT?

A. Antibiotic coverage
B. Hyperbaric oxygen
C. Fluoride application
D. Non removal of sequestrum (Correct Answer)

Explanation: **Explanation:** Osteoradionecrosis (ORN) is a serious complication of radiation therapy, most commonly affecting the mandible. It is characterized by non-healing, exposed bone in a previously irradiated area (usually >60 Gy) that fails to heal for over 3–6 months. **Why "Non-removal of sequestrum" is the correct answer:** The management of ORN follows a surgical-medical approach. A **sequestrum** (a piece of dead bone that has become separated during the process of necrosis) acts as a nidus for persistent infection and prevents healing. Therefore, **surgical debridement or sequestrectomy** (removal of the sequestrum) is a fundamental step in management to allow healthy, vascularized tissue to recover. "Non-removal" is contraindicated. **Analysis of other options:** * **Antibiotic coverage:** Essential to manage secondary infections (osteomyelitis) that frequently complicate necrotic bone. * **Hyperbaric oxygen (HBO):** Used to reverse the "3-H" effects of radiation (Hypocellularity, Hypovascularity, and Hypoxia) by stimulating angiogenesis and collagen synthesis. * **Fluoride application:** Preventive dental care is vital. Since radiation destroys salivary glands (leading to xerostomia), patients are at high risk for radiation caries. Daily fluoride application helps maintain dental integrity and prevents the need for extractions, which could trigger ORN. **NEET-PG Clinical Pearls:** * **Most common site:** Mandible (due to lower vascularity compared to the maxilla). * **Threshold dose:** Risk increases significantly with doses >60 Gy. * **Marx Protocol:** Often used for HBO therapy in ORN (30 sessions before surgery, 10 sessions after). * **PENTOCLO Protocol:** A modern medical management involving Pentoxifylline, Tocopherol (Vitamin E), and Clodronate.

Question 97: Chang staging is used for which of the following conditions?

A. Retinoblastoma
B. Medulloblastoma (Correct Answer)
C. Ewing's sarcoma
D. Rhabdomyosarcoma

Explanation: **Explanation:** **Chang Staging** is the classic staging system used for **Medulloblastoma**, a common malignant posterior fossa tumor in children. It was developed to categorize the disease based on two main parameters: 1. **T-stage (Tumor):** Based on the size and local extension of the primary tumor (e.g., involvement of the brainstem or fourth ventricle). 2. **M-stage (Metastasis):** Based on the presence of "drop metastases" or dissemination via the cerebrospinal fluid (CSF). **Why Medulloblastoma is correct:** Medulloblastoma has a high propensity for leptomeningeal spread. The Chang system (specifically the M-component) is crucial for risk stratification, ranging from M0 (no subarachnoid dissemination) to M4 (metastasis outside the neuraxis). **Analysis of Incorrect Options:** * **Retinoblastoma:** Staged using the **Reese-Ellsworth** classification (for intraocular tumors) or the **International Classification for Retinoblastoma (ICRB)**. * **Ewing’s Sarcoma:** Typically staged using the **Enneking system** (for bone tumors) or the TNM system. * **Rhabdomyosarcoma:** Uses a unique system combining **TNM Clinical Staging** and the **Intergroup Rhabdomyosarcoma Study (IRS) Post-operative Grouping**. **High-Yield Clinical Pearls for NEET-PG:** * **Modified Chang Staging:** While the T-stage is now less prognostic due to modern neuroimaging, the **M-stage** remains a vital prognostic factor. * **M1 Stage:** Defined as microscopic tumor cells found in the CSF. * **Zuckerman’s Classification:** Another term sometimes associated with the staging of these tumors. * **Molecular Subgroups:** Modern management of Medulloblastoma now focuses on four molecular subgroups (**WNT, SHH, Group 3, and Group 4**), which are increasingly more important than Chang staging for prognosis.

Question 98: A patient who underwent dental extraction prior to radiotherapy needs:

A. No time for healing
B. Adequate time for healing (Correct Answer)
C. Radiotherapy is not advised
D. To wait for one day to start radiotherapy

Explanation: **Explanation:** The correct answer is **B. Adequate time for healing.** **Underlying Medical Concept:** Radiotherapy to the head and neck region significantly impairs the vascularity of the mandible and maxilla by causing endarteritis obliterans. If radiation is started immediately after a dental extraction, the socket fails to heal due to reduced blood supply and impaired osteoblastic activity. This creates a pathway for infection to reach the hypoxic bone, leading to **Osteoradionecrosis (ORN)**—a severe, painful, and debilitating complication characterized by non-healing bone necrosis. To prevent this, a healing period of typically **10 to 14 days** (or until complete mucosalization of the socket) is mandatory before initiating radiotherapy. **Analysis of Incorrect Options:** * **Option A & D:** Starting treatment with "no time" or only "one day" of healing is contraindicated. The surgical site remains an open wound; radiation will arrest the inflammatory healing phase, leading to wound dehiscence and subsequent ORN. * **Option C:** Radiotherapy is not contraindicated. In fact, pre-radiation dental clearance (extracting teeth with poor prognosis) is a standard prophylactic measure to avoid the need for extractions *after* therapy, which carries an even higher risk of ORN. **High-Yield Clinical Pearls for NEET-PG:** * **Osteoradionecrosis (ORN):** Most commonly affects the **mandible** (due to its lower vascularity compared to the maxilla). * **The "Golden Period":** If extractions are necessary post-radiation, they should ideally be avoided. If mandatory, hyperbaric oxygen (HBO) therapy is often used to improve tissue oxygenation. * **Radiation Dose:** The risk of ORN increases significantly with doses exceeding **60 Gy**. * **Prophylaxis:** Patients should maintain meticulous oral hygiene and use topical fluoride to prevent radiation-induced dental caries.

Question 99: What is the maximum recommended external beam radiation therapy dose for carcinoma of the cervix?

A. 80 Gy
B. 70 Gy
C. 50 Gy (Correct Answer)
D. 35 Gy

Explanation: **Explanation:** In the management of carcinoma cervix, the standard treatment approach involves a combination of **External Beam Radiation Therapy (EBRT)** and **Brachytherapy**. The primary goal of EBRT is to sterilize the pelvic lymph nodes and reduce the bulk of the central tumor. The maximum recommended dose for EBRT is typically **45–50 Gy** (delivered in fractions of 1.8–2.0 Gy over 5 weeks). Exceeding this dose with external beams significantly increases the risk of chronic toxicity to surrounding "organs at risk" (OARs), such as the small bowel, bladder, and rectum, which have lower tolerance limits. To achieve the high curative doses required for the central cervix (80–90 Gy), brachytherapy is used to provide a concentrated dose while sparing these adjacent structures. **Analysis of Options:** * **Option C (50 Gy):** Correct. This is the upper limit for whole-pelvis EBRT to ensure regional control without causing unacceptable bowel or bladder morbidity. * **Options A & B (80 Gy & 70 Gy):** These represent the *total* cumulative dose (EBRT + Brachytherapy) required for tumor eradication. Delivering this dose via EBRT alone would cause severe radiation enteritis and fistulas. * **Option D (35 Gy):** This dose is sub-therapeutic for definitive management and is insufficient to control pelvic lymph node micrometastases. **High-Yield Clinical Pearls for NEET-PG:** * **Point A:** Located 2 cm superior to the external os and 2 cm lateral to the uterine canal. It represents where the uterine artery crosses the ureter. The target dose for Point A is usually **80–85 Gy**. * **Point B:** Located 5 cm lateral to the midline; it represents the pelvic side wall and lymph nodes. * **Standard Protocol:** EBRT (45–50 Gy) with concurrent **Cisplatin** (radiosensitizer), followed by Intracavitary Brachytherapy (ICBT).

Question 100: In which type of lung cancer is cranial irradiation also administered as part of the treatment?

A. Squamous cell carcinoma
B. Small cell lung cancer (Correct Answer)
C. Non-small cell lung cancer
D. Adenocarcinoma

Explanation: **Explanation:** **Small Cell Lung Cancer (SCLC)** is the correct answer because it is a highly aggressive neuroendocrine tumor characterized by rapid doubling time and a high propensity for early micrometastasis, particularly to the brain. Even when the primary tumor responds well to chemotherapy and radiotherapy, the brain remains a "pharmacological sanctuary" where the blood-brain barrier prevents systemic drugs from reaching therapeutic levels. **Prophylactic Cranial Irradiation (PCI)** is administered to patients with SCLC (both limited and extensive stage) who achieve a good response to initial treatment. PCI significantly reduces the incidence of brain metastases and has been shown to improve overall survival. **Why other options are incorrect:** * **Squamous Cell Carcinoma & Adenocarcinoma:** These are subtypes of **Non-Small Cell Lung Cancer (NSCLC)**. Unlike SCLC, NSCLC is less sensitive to radiation and has a lower rate of early occult brain involvement. In NSCLC, cranial irradiation is typically reserved for patients who already have documented brain metastases (therapeutic, not prophylactic). * **Non-Small Cell Lung Cancer (Option C):** This is a broad category. While brain metastases are common in advanced NSCLC, PCI is not a standard of care because it does not provide the same survival benefit seen in SCLC. **High-Yield Clinical Pearls for NEET-PG:** * **Standard Dose for PCI:** Usually 25 Gy in 10 fractions. * **SCLC Staging:** Often uses the Veterans Administration Lung Group (VALG) system: Limited vs. Extensive stage. * **Chemotherapy of choice for SCLC:** Etoposide + Cisplatin (EP regimen). * **Lambert-Eaton Myasthenic Syndrome:** Most commonly associated with SCLC.

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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