Radiation Oncology Basics — MCQs

Radiation Oncology Basics Indian Medical PG Practice Questions and MCQs

Question 111: What is the preferred treatment modality for an exophytic, radiosensitive, and well-oxygenated tumor?

A. Surgery
B. Radiation therapy (Correct Answer)
C. Chemotherapy
D. Combined therapy

Explanation: **Explanation:** The correct answer is **Radiation Therapy (RT)**. This choice is based on the fundamental principles of radiobiology and tumor morphology: 1. **Radiosensitivity:** By definition, a radiosensitive tumor undergoes significant cell death when exposed to ionizing radiation. 2. **Oxygenation Status:** Oxygen is the most potent **radiosensitizer**. According to the **Oxygen Enhancement Ratio (OER)**, well-oxygenated tissues are 2.5 to 3 times more sensitive to radiation than hypoxic tissues. Oxygen "fixes" the damage caused by free radicals to the tumor DNA (Oxygen Fixation Hypothesis). 3. **Exophytic Growth:** Exophytic tumors grow outward from the surface and typically possess a robust blood supply compared to endophytic (infiltrative) tumors. This superior vascularity ensures high oxygen tension, making them ideal targets for RT. **Why other options are incorrect:** * **Surgery:** While surgery is a primary modality for many cancers, the question specifically highlights features (radiosensitivity and oxygenation) that make the tumor uniquely susceptible to radiation, making RT the most "preferred" based on the descriptors provided. * **Chemotherapy:** Generally used for systemic disease or as a sensitizer. It is not the primary modality for a localized, highly radiosensitive tumor where local control can be achieved via RT. * **Combined Therapy:** Usually reserved for advanced stages or poorly oxygenated/radioresistant tumors to improve local control. For a highly sensitive, well-oxygenated tumor, monotherapy with RT often suffices, minimizing toxicity. **High-Yield Clinical Pearls for NEET-PG:** * **Law of Bergonie and Tribondeau:** Cells are more radiosensitive if they have a high mitotic rate, a long mitotic future, and are undifferentiated. * **The 5 R’s of Radiobiology:** Repair, Redistribution, Repopulation, Reoxygenation, and Radiosensitivity. * **Hypoxic cells** are relatively radioresistant; hence, hyperbaric oxygen or radiosensitizers are sometimes used to enhance RT efficacy.

Question 112: What is the primary mechanism of action of radiotherapy?

A. DNA damage (Correct Answer)
B. Production of non-sense proteins
C. Depletion of ATP
D. Depletion of glucose

Explanation: **Explanation:** The primary target of ionizing radiation in radiotherapy is the **DNA** of the cell. Radiation induces cell death (mitotic catastrophe) by causing structural damage to the genetic material, which prevents the cell from replicating. **Mechanism of Action:** 1. **Direct Action:** The radiation photon directly strikes the DNA molecule, causing single-strand or double-strand breaks (DSBs). 2. **Indirect Action (Most Common):** Radiation interacts with water molecules in the cell (radiolysis) to produce **Free Radicals** (e.g., Hydroxyl radicals •OH). These radicals then attack the DNA. This process is oxygen-dependent, which is why hypoxic tumors are more resistant to radiation. **Analysis of Incorrect Options:** * **B. Production of non-sense proteins:** While radiation can affect protein synthesis indirectly, it is not the primary mechanism. Non-sense proteins are more commonly associated with specific genetic mutations or certain antibiotics (e.g., Aminoglycosides). * **C & D. Depletion of ATP/Glucose:** These relate to metabolic pathways. While a dying cell will eventually lose its metabolic functions, radiotherapy does not primarily target the energy production or glucose uptake of the cell. **High-Yield Clinical Pearls for NEET-PG:** * **Double-Strand Breaks (DSBs):** These are considered the most lethal form of damage caused by radiation. * **Cell Cycle Sensitivity:** Cells are most sensitive to radiation in the **G2 and M phases** and most resistant in the **S phase**. * **The 5 R’s of Radiobiology:** Repair, Redistribution, Repopulation, Reoxygenation, and Radiosensitivity. * **Free Radicals:** Indirect action accounts for approximately **70%** of the damage caused by low-LET radiation (like X-rays).

Question 113: Intracavitary radiation is indicated in which of the following conditions?

A. Carcinoma of the cervix (Correct Answer)
B. Carcinoma of the lung
C. Carcinoma of the oesophagus
D. Carcinoma of the oral cavity

Explanation: **Explanation:** **Intracavitary Brachytherapy** is a form of internal radiation where radioactive sources are placed directly into a body cavity near the tumor site. **Why Carcinoma of the Cervix is Correct:** The most classic and common indication for intracavitary radiation is **Carcinoma of the Cervix**. The anatomy of the vagina and the cervical canal allows for the insertion of specialized applicators (like Tandem and Ovoids or Tandem and Ring). This technique delivers a high dose of radiation directly to the tumor while rapidly "falling off" to spare adjacent critical structures like the bladder and rectum. It is a cornerstone of curative treatment in cervical cancer. **Why the Other Options are Incorrect:** * **Carcinoma of the Lung:** Usually treated with External Beam Radiation Therapy (EBRT). While *intraluminal* brachytherapy (via bronchoscopy) is possible for endobronchial lesions, it is not the standard "intracavitary" approach. * **Carcinoma of the Oesophagus:** Similar to the lung, this requires *intraluminal* brachytherapy. Intracavitary refers to natural body cavities (like the uterus/vagina), whereas intraluminal refers to the lumen of a tube. * **Carcinoma of the Oral Cavity:** This typically requires **Interstitial Brachytherapy**, where radioactive needles or seeds are implanted directly into the tongue or buccal tissues. **High-Yield Clinical Pearls for NEET-PG:** * **Types of Brachytherapy:** * **Intracavitary:** Cervix, Endometrium. * **Interstitial:** Breast, Tongue (Oral cavity), Prostate, Soft tissue sarcoma. * **Surface Mold:** Skin cancers, Hard palate. * **Intraluminal:** Oesophagus, Bronchus, Bile duct. * **Isotopes used:** **Iridium-192** (most common for HDR), Cesium-137, and Cobalt-60. * **Manchester System:** A classic dosimetry system used in cervical brachytherapy to define **Point A** (2cm lateral and 2cm superior to the external os).

Question 114: The Marx protocol for osteoradionecrosis is related to which of the following?

A. High dose antibiotic therapy
B. Hyperbaric oxygen therapy (Correct Answer)
C. Both high dose antibiotic therapy and hyperbaric oxygen therapy
D. None of the above

Explanation: **Explanation:** The **Marx Protocol** is the gold-standard therapeutic framework for managing **Osteoradionecrosis (ORN)** of the jaw. It is fundamentally based on the use of **Hyperbaric Oxygen (HBO) therapy**. **Why Option B is correct:** The underlying pathophysiology of ORN, as described by Marx, is the **"3-H" principle**: Hypovascularity, Hypocellularity, and Hypoxia. Radiation damages the microvasculature, leading to chronic tissue ischemia. HBO therapy works by increasing the partial pressure of oxygen in the tissues, which stimulates **angiogenesis** and **collagen synthesis**, thereby promoting healing in the necrotic bone. The protocol typically involves "20 dives" (sessions) of HBO before surgery and "10 dives" after surgery (the 20/10 rule). **Why other options are incorrect:** * **Option A & C:** While antibiotics may be used to manage secondary superficial infections in ORN, they are **not** the core component of the Marx Protocol. ORN is primarily a non-healing, hypoxic wound rather than a primary bacterial infection (like osteomyelitis). Therefore, high-dose antibiotics alone cannot reverse the underlying vascular damage. **High-Yield Clinical Pearls for NEET-PG:** * **Definition of ORN:** Exposed bone in a previously irradiated area that fails to heal over a period of 3–6 months. * **Most Common Site:** The **Mandible** (due to its lower vascularity compared to the maxilla). * **Marx Classification:** Stages ORN from I to III based on the response to HBO and the extent of bone destruction. * **PENTOCLO Protocol:** A newer pharmacological alternative to Marx, involving Pentoxifylline, Tocopherol (Vitamin E), and Clodronate.

Question 115: Concomitant chemoradiotherapy is indicated in all of the following malignancies except?

A. Stage III B Ca Cervix
B. T2 N0 M0 Anal Cancer
C. T2 N0 M0 Glottic Cancer (Correct Answer)
D. T1 N2 M0 Nasopharyngeal Cancer

Explanation: **Explanation** The core concept in oncology management is distinguishing between **definitive radiotherapy** (single modality) and **concomitant chemoradiotherapy (CCRT)** (multimodality). CCRT uses chemotherapy as a "radiosensitizer" to enhance the cell-killing effect of radiation in locally advanced or high-risk tumors. **Why Option C is the Correct Answer:** **T2 N0 M0 Glottic Cancer** is considered an early-stage laryngeal malignancy. The standard of care for early glottic cancer (T1-T2, N0) is **single-modality therapy**, either definitive Radiotherapy (RT) or conservative surgery (e.g., laser excision). Chemotherapy is not indicated because it adds significant toxicity without providing a survival benefit in these early stages. CCRT in the larynx is reserved for locally advanced stages (T3-T4) as part of organ-preservation protocols. **Analysis of Incorrect Options:** * **Stage IIIB Ca Cervix:** This is locally advanced cervical cancer. CCRT with Cisplatin is the global standard of care (Eversley protocol) for all stages from IB3 to IVA. * **T2 N0 M0 Anal Cancer:** Unlike other GI cancers, anal canal cancer is primarily treated with the **Nigro Protocol** (CCRT with 5-FU and Mitomycin-C), regardless of the small size, to avoid permanent colostomy. * **T1 N2 M0 Nasopharyngeal Cancer:** Nasopharyngeal carcinoma is highly radiosensitive but also has a high risk of distant metastasis. Even with a small primary (T1), the presence of nodal involvement (N2) necessitates CCRT. **High-Yield Clinical Pearls for NEET-PG:** * **Radiosensitizer of choice:** Cisplatin is the most common agent used in CCRT for head, neck, and cervical cancers. * **Nigro Protocol:** Definitive CCRT for Anal Canal Cancer (avoids surgery). * **Early Glottic Cancer:** Has an excellent prognosis with RT alone (90%+ control rates). * **Concurrent vs. Adjuvant:** Concurrent means "at the same time," whereas adjuvant means "after the primary treatment."

Question 116: In intra-operative radiotherapy, which of the following therapeutic modes is commonly employed?

A. Electron (Correct Answer)
B. Photon
C. X-ray
D. Gamma rays

Explanation: **Explanation:** **Intra-operative Radiotherapy (IORT)** involves the delivery of a single, concentrated dose of radiation directly to a tumor bed or exposed tumor during surgery. **Why Electrons are the Correct Choice:** The primary goal of IORT is to treat the target area while sparing underlying healthy tissues (e.g., heart, lungs, or major vessels). **Electrons** are the preferred modality because they have a **finite range and rapid dose fall-off**. Unlike photons, electrons penetrate only to a specific, adjustable depth based on their energy level. Once they reach that depth, the dose drops off sharply, ensuring that deep-seated critical structures receive minimal to no radiation. **Why Other Options are Incorrect:** * **B & C (Photons/X-rays):** Photons and X-rays have high penetrability and follow an exponential attenuation law. They do not have a "finite range," meaning they would continue to travel through the patient, potentially damaging healthy organs located deep to the surgical site. (Note: While "Electronic IORT" using low-energy miniature X-ray sources exists, high-energy electrons remain the traditional and most common standard in surgical suites). * **D (Gamma Rays):** Gamma rays (from sources like Cobalt-60) are highly penetrating and cannot be "turned off" or easily shielded in an operating room environment, making them impractical and unsafe for IORT. **High-Yield Clinical Pearls for NEET-PG:** * **Common Indications:** Early-stage breast cancer (TARGIT/ELIOT trials), pancreatic cancer, and recurrent colorectal cancer. * **Advantage:** It allows for direct visualization of the tumor bed and physical displacement of sensitive normal tissues (like loops of bowel) out of the radiation field. * **Equipment:** Often performed using a mobile linear accelerator (LINAC) specifically designed for the OR.

Question 117: Mantel radiation therapy is indicated for which of the following conditions?

A. Hodgkin's lymphoma (Correct Answer)
B. Non-Hodgkin's lymphoma
C. Burkitt's lymphoma
D. Mycosis fungoides

Explanation: **Explanation:** **Mantel field radiation** is a classic radiotherapy technique historically used as the standard of care for **Hodgkin’s Lymphoma (HL)**. It is designed to treat all lymph node stations above the diaphragm while shielding the lungs, heart, and spinal cord. 1. **Why Hodgkin’s Lymphoma is Correct:** HL typically spreads in a predictable, contiguous fashion through lymphatic chains. The Mantel field covers the submandibular, cervical, supraclavicular, infraclavicular, axillary, and mediastinal nodes in a single portal. While modern practice has shifted toward "Involved Site Radiation Therapy" (ISRT) to reduce toxicity, Mantel radiation remains the definitive historical association for HL in exams. 2. **Why Other Options are Incorrect:** * **Non-Hodgkin’s Lymphoma (NHL):** Unlike HL, NHL is often widespread and non-contiguous at presentation, making localized "field" radiation like the Mantel field less effective as a primary strategy. * **Burkitt’s Lymphoma:** This is a highly aggressive B-cell NHL characterized by rapid systemic spread. Treatment is primarily intensive systemic chemotherapy; radiation plays a very limited role. * **Mycosis Fungoides:** This is a cutaneous T-cell lymphoma. The standard radiation treatment is **Total Electron Beam Skin Irradiation (TSET)**, not nodal field radiation. **High-Yield Clinical Pearls for NEET-PG:** * **Inverted Y Field:** Used to treat nodes below the diaphragm (iliac, inguinal, and para-aortic nodes). * **Total Nodal Irradiation (TNI):** A combination of the Mantel field and the Inverted Y field. * **Complications of Mantel Field:** Increased risk of secondary malignancies (Breast cancer, Papillary Thyroid cancer), premature coronary artery disease, and hypothyroidism. * **Lhermitte’s Sign:** An electric-shock sensation down the spine upon neck flexion, sometimes seen after Mantel radiation due to transient myelopathy.

Question 118: After radiotherapy, at least how long should extraction be avoided?

A. 1 month
B. 2 months (Correct Answer)
C. 6 months
D. 12 months

Explanation: **Explanation:** The correct answer is **B. 2 months**. **Underlying Medical Concept:** Radiotherapy to the head and neck region causes significant vascular changes, specifically **endarteritis obliterans**. This leads to hypoxia, hypocellularity, and hypovascularity of the bone (the "3-H" principle). If a tooth is extracted immediately following radiation, the compromised blood supply prevents normal socket healing, significantly increasing the risk of **Osteoradionecrosis (ORN)**. Clinical guidelines generally recommend a waiting period of at least **2 months (8 weeks)** post-radiotherapy to allow the initial acute inflammatory response to subside and for the tissue to stabilize, though the risk of ORN remains a lifelong concern. **Analysis of Options:** * **A. 1 month:** This is insufficient time. The acute mucositis and vascular inflammation are at their peak, and the bone's regenerative capacity is severely impaired. * **C & D. 6 and 12 months:** While waiting longer is theoretically safer, the standard clinical benchmark for the *minimum* mandatory avoidance period in most protocols is 2 months. However, many clinicians prefer to avoid extractions indefinitely if possible, using endodontic therapy instead. **High-Yield Clinical Pearls for NEET-PG:** * **Golden Period:** The best time for necessary extractions is **2-3 weeks BEFORE** starting radiotherapy. * **ORN Risk:** The risk is highest in the **mandible** (due to lower vascularity compared to the maxilla) and with radiation doses exceeding **60 Gy**. * **Hyperbaric Oxygen (HBO):** Often used (Marx Protocol) before and after extractions in previously irradiated bone to stimulate angiogenesis and reduce ORN risk. * **Management:** If extraction is unavoidable post-radiation, use atraumatic techniques and avoid vasoconstrictors (like adrenaline) in local anesthesia.

Question 119: Reduced salivary flow following irradiation is dose dependent. At what radiation dose does salivary flow reach essentially zero?

A. 4000 rads
B. 5000 rads
C. 6000 rads (Correct Answer)
D. 7000 rads

Explanation: **Explanation:** Salivary gland dysfunction is one of the most common and debilitating side effects of head and neck radiotherapy. The serous acini of the parotid glands are highly radiosensitive. The reduction in salivary flow is progressive and directly proportional to the cumulative radiation dose. * **Why 6000 rads (60 Gy) is correct:** While a significant reduction in flow (up to 50-60%) is seen after 2000 rads, the salivary flow reaches **essentially zero (permanent xerostomia)** when the cumulative dose reaches **6000 rads (60 Gy)**. At this threshold, the parenchymal destruction and subsequent fibrosis of the glands are so extensive that compensatory recovery is no longer possible. **Analysis of Incorrect Options:** * **4000 rads (40 Gy):** At this dose, there is a severe reduction in flow (often >75%), but some residual function may remain. * **5000 rads (50 Gy):** This is often considered the threshold for "clinically significant" permanent xerostomia, but total cessation (zero flow) is typically documented at higher doses. * **7000 rads (70 Gy):** While this dose certainly results in zero flow, it is beyond the standard threshold where flow first hits the "zero" mark. **High-Yield Clinical Pearls for NEET-PG:** * **Mean Dose Limit:** To prevent permanent xerostomia, the mean dose to at least one parotid gland should be kept **<26 Gy**. * **Early Changes:** Salivary flow can decrease by as much as 50% within the first week of treatment (after only 10 Gy). * **Clinical Consequence:** Radiation-induced xerostomia leads to "Radiation Caries" due to the loss of the buffering capacity of saliva and a shift in oral flora to acidogenic bacteria. * **Management:** Amifostine (a radioprotector) or IMRT (to spare the glands) are used to mitigate this damage.

Question 120: Electron beam therapy is used for which tumor?

A. Renal carcinoma
B. Kaposi sarcoma (Correct Answer)
C. Hepatic carcinoma
D. AML

Explanation: **Explanation:** The core principle of **Electron Beam Therapy (EBT)** is its **limited depth of penetration**. Unlike high-energy X-rays (photons) which penetrate deep into the body, electrons lose energy rapidly and have a "sharp fall-off" dose distribution. This makes EBT ideal for treating **superficial tumors** while sparing underlying deep-seated critical structures. * **Why Kaposi Sarcoma is correct:** Kaposi sarcoma typically presents as multifocal, superficial skin lesions. Since these lesions are cutaneous, EBT provides an excellent dose to the skin surface without damaging deeper tissues. Other common indications for EBT include basal cell carcinoma, squamous cell carcinoma of the skin, and Total Skin Electron Irradiation (TSEI) for Mycosis Fungoides. * **Why other options are incorrect:** * **Renal Carcinoma (A) and Hepatic Carcinoma (C):** These are deep-seated visceral organs. Electron beams would be absorbed by the body wall and would never reach the target dose at the required depth. These are treated with high-energy Photons (External Beam Radiotherapy) or SBRT. * **AML (D):** Acute Myeloid Leukemia is a systemic hematological malignancy. While radiotherapy (Total Body Irradiation) is sometimes used in conditioning for bone marrow transplants, it utilizes Photons to ensure deep, uniform penetration, not superficial electrons. **High-Yield Clinical Pearls for NEET-PG:** * **Electron Rule of Thumb:** The depth of the 80% isodose line (therapeutic depth) is approximately **Energy (MeV) / 3** in cm. * **Surface Dose:** Unlike photons (which have a "skin-sparing" effect), electrons deliver a high dose to the surface, making them the treatment of choice for **skin cancers** and **scar boosts** (e.g., post-mastectomy). * **Internal Shields:** Lead shields are often used in EBT to protect underlying structures like the eyes or oral mucosa.

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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