Radiation Oncology Basics Practice Questions

Q: What is the latest source of neutrons for radiotherapy?

Californium-256. **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.

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

Lymph node greater than 3cm. 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.

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

Malignant melanoma. **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).

Q: Which of the following tumors responds best to radiotherapy?

Seminoma. **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.

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

Uterine vessel crossing the ureter. ### 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.

Q: What does brachytherapy mean?

Radiation administered interstitially through catheters. **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.

Q: Radiotherapy is most useful in which of the following?

Seminomas. **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 1

What is the latest source of neutrons for radiotherapy?

Accepted Answer

Californium-256

Answer

Strontium-90

Answer

Iodine-131

Answer

Radium-226

Question 2

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

Accepted Answer

Lymph node greater than 3cm

Answer

Multiple lymph node disease

Answer

Extranodal involvement

Answer

Lymphovascular invasion

Question 3

Which of the following tumors is least sensitive to radiation?

Accepted Answer

Malignant melanoma

Answer

Bronchogenic carcinoma

Answer

Adenocarcinoma of colon

Answer

Osteogenic sarcoma

Question 4

Which of the following tumors responds best to radiotherapy?

Accepted Answer

Seminoma

Answer

Teratoma

Answer

Choriocarcinoma

Answer

Endometrial carcinoma

Question 5

What is the relationship between the planned radiation volume and the targeted radiation volume?

Accepted Answer

Planned radiation volume is more than the targeted radiation volume.

Answer

Planned radiation volume is 90% of the tumour size.

Answer

Planned radiation volume is less than the targeted radiation volume.

Answer

Planned radiation volume is equal to the targeted radiation volume.

Question 6

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

Accepted Answer

Uterine vessel crossing the ureter

Answer

Lateral pelvic lymph nodes

Answer

Urinary bladder

Answer

Rectum

Question 7

What does brachytherapy mean?

Accepted Answer

Radiation administered interstitially through catheters

Answer

Chemotherapy

Answer

Lasers

Answer

Radiation involving normal tissues

Question 8

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

Accepted Answer

All of the above

Answer

Radiation caries

Answer

Radiation pulpitis

Answer

Osteoradionecrosis

Question 9

Radiotherapy is most useful in which of the following?

Accepted Answer

Seminomas

Answer

Choriocarcinoma

Answer

Osteosarcoma

Answer

Renal cell carcinoma

Question 10

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

Accepted Answer

Internal iliac lymph node

Answer

External iliac lymph node

Answer

Common iliac lymph node

Answer

Sacral lymph node

Radiation Oncology Basics — MCQs

Radiation Oncology Basics — MCQs

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