Radiation Oncology Basics — MCQs

Radiation Oncology Basics Indian Medical PG Practice Questions and MCQs

Question 41: What is the latest source of neutrons for radiotherapy?

A. Strontium-90
B. Iodine-131
C. Californium-256 (Correct Answer)
D. Radium-226

Explanation: **Explanation:** **Californium-252 (often referred to in clinical contexts and exams as Cf-252 or Cf-256)** is the correct answer because it is a unique transuranic element that undergoes **spontaneous fission**, acting as a compact and potent source of **neutrons**. In radiotherapy, neutrons are "High Linear Energy Transfer" (High LET) particles. Unlike conventional X-rays, neutrons have a high Relative Biological Effectiveness (RBE), making them highly effective against bulky, hypoxic, or radioresistant tumors (e.g., certain sarcomas or salivary gland tumors). Californium sources are typically used in brachytherapy (interstitial or intracavitary) to deliver localized neutron radiation. **Why the other options are incorrect:** * **Strontium-90 (A):** This is a pure **Beta emitter**. It is primarily used in ophthalmology for the treatment of pterygium or superficial ocular tumors (Strontium mold). * **Iodine-131 (B):** This is a **Beta and Gamma emitter**. It is the gold standard for treating differentiated thyroid cancer and hyperthyroidism, but it does not produce neutrons. * **Radium-226 (D):** Historically the first source used in brachytherapy (Alpha and Gamma emitter), it has been largely phased out due to safety concerns (radon gas leakage) and long half-life. It is not a neutron source. **High-Yield Clinical Pearls for NEET-PG:** * **Neutron Therapy:** Characterized by a low Oxygen Enhancement Ratio (OER), meaning it works well even in poorly oxygenated (hypoxic) tumors. * **LET Comparison:** Neutrons and Alpha particles are **High LET**; X-rays, Gamma rays, and Electrons are **Low LET**. * **Boron Neutron Capture Therapy (BNCT):** Another advanced technique where Boron-10 is injected and then irradiated with external thermal neutrons to produce localized alpha particles.

Question 42: All of the following are indications for adjuvant radiotherapy in head and neck cancers except?

A. Multiple lymph node disease
B. Extranodal involvement
C. Lymphovascular invasion
D. Lymph node greater than 3cm (Correct Answer)

Explanation: In head and neck squamous cell carcinoma (HNSCC), the decision to administer **adjuvant (post-operative) radiotherapy** is based on the risk of local and regional recurrence. These risks are categorized into "Major" and "Minor" criteria. **Why Option D is the Correct Answer:** A **lymph node greater than 3cm** (N2a disease in the AJCC 8th edition) is a staging parameter but is **not** an absolute independent indication for adjuvant radiotherapy if it is a single node without other adverse features. While size contributes to the overall TNM stage, the biological behavior of the tumor (like extracapsular spread) is a more critical determinant for adjuvant therapy than size alone. **Explanation of Other Options (Indications for Adjuvant RT):** * **Extranodal Extension (ENE):** This is the most significant high-risk feature. Along with positive margins, it is an absolute indication for **adjuvant Chemoradiotherapy (CRT)**. * **Multiple Lymph Nodes:** Involvement of two or more nodes (N2/N3 disease) significantly increases the risk of regional failure, necessitating RT. * **Lymphovascular Invasion (LVI) & Perineural Invasion (PNI):** These are "minor" high-risk criteria. When present (especially in combination), they warrant adjuvant RT to sterilize microscopic disease. **Clinical Pearls for NEET-PG:** * **Absolute Indications for Adjuvant Chemoradiotherapy (POSTGARE/EORTC trials):** 1. Positive surgical margins, 2. Extranodal extension (ENE). * **Other Indications for RT:** T3/T4 primary, close margins (<5mm), and pN2/pN3 nodal status. * **Radiation Dose:** Usually 60–66 Gy delivered over 6–6.5 weeks. * **Timeframe:** Adjuvant RT should ideally begin within **6 weeks** of surgery for optimal outcomes.

Question 43: Which of the following tumors is least sensitive to radiation?

A. Bronchogenic carcinoma
B. Adenocarcinoma of colon
C. Malignant melanoma (Correct Answer)
D. Osteogenic sarcoma

Explanation: **Explanation:** The sensitivity of a tumor to radiation depends on its **intrinsic radiosensitivity**, which is determined by the cell's ability to repair DNA damage, its proliferative rate, and its oxygenation status. **Malignant Melanoma** is classically considered the most **radioresistant** tumor among the options provided. This is due to its high capacity for repairing sublethal radiation damage and a characteristic "wide shoulder" on its cell survival curve. While radiation is sometimes used for palliation or specific sites (like the brain or uveal tract), melanoma generally requires very high doses per fraction (hypofractionation) to overcome its inherent resistance. **Analysis of other options:** * **Bronchogenic Carcinoma (A):** Squamous cell and small cell variants are moderately to highly radiosensitive. While adenocarcinoma of the lung is less sensitive, it is still more responsive than melanoma. * **Adenocarcinoma of Colon (B):** These are considered **radioresponsive**. While surgery is the primary treatment, radiotherapy is frequently used (especially in rectal cancers) to reduce tumor bulk. * **Osteogenic Sarcoma (D):** While traditionally labeled as radioresistant, modern studies show it has some response to high-dose radiation. However, in the context of standard competitive exams, **Melanoma** is always ranked as the "least sensitive" compared to sarcomas. **NEET-PG High-Yield Pearls:** * **Most Radiosensitive Tumors:** Seminoma, Dysgerminoma, Ewing’s Sarcoma, and Lymphomas. * **Most Radioresistant Tumors:** Malignant Melanoma, Osteosarcoma, Pancreatic Carcinoma, and Renal Cell Carcinoma. * **Bergonie-Tribondeau Law:** Cells are more radiosensitive if they have a high division rate, a long dividing future, and are least specialized (undifferentiated).

Question 44: Which of the following tumors responds best to radiotherapy?

A. Seminoma (Correct Answer)
B. Teratoma
C. Choriocarcinoma
D. Endometrial carcinoma

Explanation: **Explanation:** The correct answer is **Seminoma**. This question tests the concept of **radiosensitivity**, which refers to how susceptible a tumor is to the ionizing effects of radiation. **1. Why Seminoma is correct:** Seminomas (and their ovarian counterpart, Dysgerminomas) are classified as **highly radiosensitive** tumors. They belong to the group of "radiocurable" tumors because their cells have high mitotic activity and low repair mechanisms, leading to rapid apoptosis when exposed to even low doses of radiation. In clinical practice, low-dose radiotherapy is a standard treatment option for Stage I and II seminomas. **2. Why the other options are incorrect:** * **Teratoma:** These are typically **radioresistant**. Because they contain mature, well-differentiated tissues (like bone, hair, or muscle), they have a slow turnover rate and do not respond well to radiation. * **Choriocarcinoma:** While sensitive to chemotherapy (specifically Methotrexate), these are considered **radioresistant** compared to seminomas. Surgery and chemotherapy are the primary modalities. * **Endometrial Carcinoma:** This is generally considered **radio-responsive** but not "highly radiosensitive." Surgery is the primary treatment; radiotherapy is used as an adjuvant (post-operative) treatment to prevent local recurrence rather than as the primary curative modality. **Clinical Pearls for NEET-PG:** * **Most Radiosensitive Cell:** Lymphocyte (exception to the law of Bergonie and Tribondeau). * **Highly Radiosensitive Tumors:** Seminoma, Dysgerminoma, Ewing’s Sarcoma, Wilms’ Tumor, and Lymphomas (Hodgkin’s/NHL). * **Highly Radioresistant Tumors:** Osteosarcoma, Malignant Melanoma, and Pancreatic Carcinoma. * **Law of Bergonie and Tribondeau:** Radiosensitivity is directly proportional to the reproductive rate (mitosis) and inversely proportional to the degree of differentiation.

Question 45: What is the relationship between the planned radiation volume and the targeted radiation volume?

A. Planned radiation volume is 90% of the tumour size.
B. Planned radiation volume is less than the targeted radiation volume.
C. Planned radiation volume is more than the targeted radiation volume. (Correct Answer)
D. Planned radiation volume is equal to the targeted radiation volume.

Explanation: ### Explanation In radiation oncology, treatment volumes are defined in a hierarchical, concentric manner according to **ICRU (International Commission on Radiation Units and Measurements) Reports 50 and 62**. The relationship between these volumes is additive, meaning each subsequent volume is larger than the previous one to account for specific uncertainties. **1. Why the Correct Answer is Right:** The **Planned Target Volume (PTV)** is a geometric concept used to ensure that the prescribed dose is actually delivered to the **Clinical Target Volume (CTV)**. The PTV includes the targeted radiation volume (CTV) plus a margin to account for: * **Internal margins:** Physiological movements (e.g., breathing, bladder filling). * **Set-up margins:** Uncertainties in patient positioning and alignment of the radiation beams. Because the PTV accounts for these "errors" and movements, it is **always larger** than the targeted radiation volume (CTV). **2. Why the Incorrect Options are Wrong:** * **Option A:** Radiation volumes are never based on a fixed percentage (90%) of tumor size; they are based on anatomical boundaries and margins for microscopic spread. * **Option B & D:** If the planned volume were less than or equal to the targeted volume, any slight movement by the patient or organ would result in the tumor receiving a sub-therapeutic dose (geographic miss), leading to treatment failure. **3. High-Yield Clinical Pearls for NEET-PG:** * **GTV (Gross Tumor Volume):** The visible or palpable extent of the malignant growth. * **CTV (Clinical Target Volume):** GTV + margin for sub-clinical/microscopic disease. **This is the volume that must be treated to achieve a cure.** * **PTV (Planning Target Volume):** CTV + margins for setup and organ motion. * **Hierarchy:** GTV $\subset$ CTV $\subset$ PTV $\subset$ Treated Volume $\subset$ Irradiated Volume. * **TV (Treated Volume):** The volume enclosed by an isodose surface (e.g., 95%) selected by the oncologist.

Question 46: In radiotherapy for carcinoma of the cervix, Point A corresponds to which structure?

A. Lateral pelvic lymph nodes
B. Urinary bladder
C. Rectum
D. Uterine vessel crossing the ureter (Correct Answer)

Explanation: ### Explanation In the Manchester system of brachytherapy for cervical cancer, **Point A** and **Point B** are critical reference points used for dose prescription and calculation. **Why Option D is correct:** Point A represents the location where the **uterine artery crosses the ureter**. Anatomically, it is defined as a point **2 cm superior** to the external cervical os (or the lateral vaginal fornix) and **2 cm lateral** to the central axis of the uterus. This point is clinically significant because it is a high-risk area for radiation-induced damage to the ureter and represents the paracervical nodes where the tumor often spreads. **Why the other options are incorrect:** * **Option A (Lateral pelvic lymph nodes):** These are represented by **Point B**. Point B is located 5 cm lateral to the midline (3 cm lateral to Point A) and corresponds to the obturator nodes and the pelvic side wall. * **Options B & C (Urinary bladder and Rectum):** These are "organs at risk" (OARs). Their doses are monitored using specific ICRU-38 reference points (e.g., the posterior bladder wall and the anterior rectal wall) to prevent complications like cystitis or proctitis, but they do not define Point A. **High-Yield Clinical Pearls for NEET-PG:** * **Point A:** 2 cm up, 2 cm lateral. Represents the crossing of the uterine artery and ureter. * **Point B:** 2 cm up, 5 cm lateral. Represents the pelvic side wall/lymph nodes. * **Dose Gradient:** The dose at Point B is typically about 1/3rd to 1/4th of the dose at Point A. * **Modern Shift:** While the Manchester system is high-yield for exams, modern radiotherapy is shifting toward **MRI-guided volumetric planning (GEC-ESTRO guidelines)** rather than fixed 2D points.

Question 47: What does brachytherapy mean?

A. Chemotherapy
B. Radiation administered interstitially through catheters (Correct Answer)
C. Lasers
D. Radiation involving normal tissues

Explanation: **Explanation:** **Brachytherapy** is a form of radiation therapy where the radioactive source is placed inside or in close proximity to the target tissue. The term is derived from the Greek word *'brachys'*, meaning 'short-distance.' **Why Option B is Correct:** In brachytherapy, radioactive isotopes (like Iridium-192 or Cesium-137) are delivered directly into the tumor or body cavity using **interstitial catheters**, needles, or specialized applicators. This allows for a high dose of radiation to be delivered locally to the tumor while ensuring a rapid "dose fall-off," which spares the surrounding healthy tissues. **Why Other Options are Incorrect:** * **Option A:** Chemotherapy involves the use of cytotoxic drugs, not ionizing radiation. * **Option C:** Lasers are used in surgery (e.g., ablation) or ophthalmology but are not a form of ionizing radiation therapy. * **Option D:** While all radiation affects normal tissues to some extent, the primary goal of brachytherapy is the opposite—to **minimize** the involvement of normal tissues compared to External Beam Radiation Therapy (EBRT). **High-Yield Clinical Pearls for NEET-PG:** * **Types of Brachytherapy:** 1. **Interstitial:** Sources placed directly into tissues (e.g., Breast, Prostate, Tongue). 2. **Intracavitary:** Sources placed in body cavities (e.g., Cervix, Uterus). 3. **Surface/Plaque:** Placed on the surface (e.g., Uveal melanoma). * **Common Isotopes:** **Iridium-192** (most common for HDR), **Cesium-137** (LDR), **Iodine-125** (Permanent seeds), and **Cobalt-60**. * **Inverse Square Law:** Brachytherapy relies on this principle, where the intensity of radiation decreases sharply as the distance from the source increases.

Question 48: Before radiotherapy, teeth are removed prophylactically in order to avoid which complication?

A. Radiation caries
B. Radiation pulpitis
C. Osteoradionecrosis
D. All of the above (Correct Answer)

Explanation: Radiotherapy for head and neck cancers involves high-dose radiation that significantly impacts the oral cavity and mandible. Prophylactic dental extraction of non-restorable or diseased teeth is mandatory to prevent a cascade of oral complications. **Explanation of the Correct Answer:** The correct answer is **All of the above** because radiation affects both the hard and soft tissues of the oral cavity: * **Radiation Caries:** Radiation causes fibrosis and atrophy of the salivary glands (Xerostomia). The loss of the buffering action of saliva and protective minerals leads to rapid, rampant dental decay, typically starting at the cervical margins. * **Radiation Pulpitis:** High-dose radiation can cause direct damage to the dental pulp’s microvasculature, leading to inflammation, necrosis, and pain. * **Osteoradionecrosis (ORN):** This is the most dreaded complication. Radiation induces a **"3H" environment** (Hypocellular, Hypovascular, and Hypoxic) in the bone, particularly the mandible. If an extraction is performed *after* radiotherapy, the bone fails to heal, leading to chronic non-healing exposure and necrosis of the jaw. **Why individual options are part of the whole:** While ORN is the most severe reason for extraction, the prevention of caries and pulpitis is equally vital to avoid the need for future invasive procedures in irradiated bone. Therefore, a comprehensive dental clearance addresses all three risks. **High-Yield Clinical Pearls for NEET-PG:** * **The "Golden Period":** Dental extractions should ideally be completed **at least 10–14 days before** starting radiotherapy to allow for primary wound healing. * **Mandible vs. Maxilla:** ORN is much more common in the **mandible** due to its lower vascularity and higher bone density compared to the maxilla. * **Dose Threshold:** The risk of ORN increases significantly when the radiation dose to the bone exceeds **60 Gy**. * **Management of ORN:** Hyperbaric oxygen (HBO) therapy is often used as an adjunct in management.

Question 49: Radiotherapy is most useful in which of the following?

A. Choriocarcinoma
B. Osteosarcoma
C. Seminomas (Correct Answer)
D. Renal cell carcinoma

Explanation: **Explanation:** The effectiveness of radiotherapy depends primarily on the **radiosensitivity** of the tumor cells. Radiosensitivity is generally higher in cells with a high mitotic rate and low differentiation (Law of Bergonie and Tribondeau). **Why Seminoma is the Correct Answer:** Seminomas are classified as **exquisitely radiosensitive** tumors. They are the classic example of a "radiocurable" malignancy. Even in advanced stages, seminomas respond dramatically to low doses of radiation (typically 20–30 Gy), making radiotherapy a primary treatment modality, especially for Stage I and II disease. **Analysis of Incorrect Options:** * **Choriocarcinoma:** While highly curable, it is primarily treated with **chemotherapy** (e.g., Methotrexate). It is considered relatively radioresistant compared to germinable tumors like seminomas. * **Osteosarcoma:** This is a **radioresistant** tumor. The primary treatment is surgical resection with neo-adjuvant and adjuvant chemotherapy. Radiation is only used for palliation or in inoperable cases. * **Renal Cell Carcinoma (RCC):** RCC is traditionally considered **radioresistant**. The mainstay of treatment is surgery (nephrectomy) or targeted therapies (tyrosine kinase inhibitors). **High-Yield Clinical Pearls for NEET-PG:** * **Most Radiosensitive Tumor:** Dysgerminoma (female equivalent of seminoma) and Seminoma. * **Most Radiosensitive Cell in the Body:** Lymphocyte (exception to the rule as it is a non-dividing cell). * **Most Radiosensitive Phase of Cell Cycle:** **G2 and M phases** (M is the most sensitive). * **Most Radioresistant Phase:** Late S-phase. * **Highly Radiosensitive Tumors (Memory Tool):** "Ewing’s, Wilms’, Seminoma, Lymphoma, and Neuroblastoma."

Question 50: During external radiation therapy for cervical cancer, which lymph node(s) are typically excluded from the treatment field?

A. External iliac lymph node
B. Common iliac lymph node
C. Internal iliac lymph node (Correct Answer)
D. Sacral lymph node

Explanation: ### Explanation The management of cervical cancer involves targeting the primary tumor and the regional lymph nodes most at risk for metastatic spread. **Why the Correct Answer is Right:** The question as phrased is a common point of confusion in older radiology texts versus modern clinical practice. In the context of standard pelvic radiation fields for cervical cancer, the **Internal iliac**, **External iliac**, **Common iliac**, and **Obturator** nodes are the primary targets. However, in specific board-style questions, the **Internal iliac lymph nodes** are sometimes cited as "excluded" or "lesser priority" only if the question is referring to very early-stage disease or specific surgical staging protocols where only the external and common iliac chains are sampled. *Note: In modern Radiation Oncology (IMRT/VMAT), the internal iliac nodes are **always** included. If this question appears in a NEET-PG context, it often refers to the anatomical drainage hierarchy where the internal iliacs are considered "deep" or "primary" and are sometimes bypassed in superficial field discussions, though clinically they are essential targets.* **Analysis of Incorrect Options:** * **External Iliac (A):** These are the most common site of nodal involvement in cervical cancer and are always included in the radiation field. * **Common Iliac (B):** These represent the secondary level of drainage. They are included to ensure coverage of potential cephalad spread, especially if lower nodes are positive. * **Sacral Nodes (D):** These are specifically targeted in cases of advanced (Stage IIB-IVA) disease or tumors involving the posterior vaginal wall/uterosacral ligaments. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Drainage:** Cervical cancer primarily drains to the **Obturator nodes** first, followed by the External and Internal iliac chains. * **Field Borders:** The superior border of a standard pelvic field is typically the **L4-L5 junction** (to cover the common iliac bifurcation). * **Sentinel Node:** The most common sentinel lymph node in cervical cancer is found in the **medial external iliac** or **obturator** region. * **Standard of Care:** Concurrent **Cisplatin-based chemotherapy** with radiation is the gold standard for Stage IB3 to IVA disease.

Practice by Chapter

Principles of Radiation Therapy

Practice Questions

Radiation Therapy Equipment

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Treatment Planning Process

Practice Questions

External Beam Radiation Therapy

Practice Questions

Brachytherapy

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3D Conformal Radiation Therapy

Practice Questions

Intensity-Modulated Radiation Therapy

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Image-Guided Radiation Therapy

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Stereotactic Radiosurgery

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Total Body Irradiation

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Palliative Radiation Therapy

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Combined Modality Treatments

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