Radiation Oncology Basics Practice Questions

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

Mycosis Fungoides. **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).

Q: High brachytherapy dose rate is:

12 Gy/hour. **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).

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

Ewing's sarcoma. **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.

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

Seminoma. **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.

Q: For teletherapy setup, all are used except?

Iridium 127. **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.

Q: Radiotherapy is least useful in the primary management of which of the following cancers?

Stomach cancer. ### Explanation The effectiveness of radiotherapy (RT) in primary management depends on the **radiosensitivity** of the tumor and the **tolerance** of surrounding normal tissues. **Why Stomach Cancer is the Correct Answer:** Stomach cancer is primarily managed with **surgery** (Total or Subtotal Gastrectomy). Radiotherapy is **least useful as a primary modality** because: 1. **Radio-resistance:** Adenocarcinomas of the stomach are relatively resistant to radiation. 2. **Anatomical Constraints:** The stomach is a mobile organ surrounded by highly radiosensitive structures (liver, kidneys, and small bowel). Delivering a tumoricidal dose (usually >45-50 Gy) would cause significant toxicity to these adjacent organs. RT is typically reserved for adjuvant (post-op) or palliative settings rather than primary curative intent. **Analysis of Incorrect Options:** * **Esophagus Cancer:** RT (often combined with chemotherapy) is a standard primary treatment, especially for squamous cell carcinoma, or as neoadjuvant therapy to downstage tumors before surgery. * **Cervix Cancer:** Radiotherapy (External Beam + Brachytherapy) is the **treatment of choice** for locally advanced cervical cancer (Stage IIB to IVA) and is as effective as surgery in early stages. * **Breast Cancer:** RT is a cornerstone of primary management in **Breast Conserving Therapy (BCT)** to reduce local recurrence and is also used post-mastectomy in high-risk patients. **High-Yield Clinical Pearls for NEET-PG:** * **Most Radiosensitive Tumor:** Seminoma (Testis) and Dysgerminoma (Ovary). * **Most Radioresistant Tumor:** Malignant Melanoma, Osteosarcoma, and Pancreatic Cancer. * **Law of Bergonie and Tribondeau:** Cells are more radiosensitive if they have a high mitotic rate, a long mitotic future, and are undifferentiated. * **Radiotherapy in Stomach:** The most common role is the **MacDonald Regimen** (Adjuvant Chemoradiotherapy).

Q: All of the following isotopes are used in brachytherapy EXCEPT:

Iodine-131 (I131). **Explanation:** The core concept in this question is distinguishing between **Brachytherapy** (sealed source radiotherapy) and **Systemic Radionuclide Therapy** (unsealed source). **Why Iodine-131 is the correct answer:** Iodine-131 is an **unsealed source** primarily used for systemic therapy. It is administered orally or intravenously for the treatment of hyperthyroidism and differentiated thyroid cancer. Because it is distributed throughout the body via the bloodstream to target specific tissues, it is not classified as brachytherapy, which requires the physical placement of a solid, encapsulated source near or inside a tumor. **Analysis of Incorrect Options (Isotopes used in Brachytherapy):** * **Iridium-192 (Ir192):** The most commonly used isotope in modern High Dose Rate (HDR) brachytherapy. It is used in "afterloading" machines for various cancers (e.g., cervix, breast, head, and neck). * **Cesium-137 (Cs137):** Historically the gold standard for Low Dose Rate (LDR) brachytherapy, particularly for cervical cancer (Manchester system). It has a long half-life (~30 years). * **Iodine-125 (I125):** Used as "permanent seeds" for interstitial brachytherapy, most notably in the treatment of low-risk prostate cancer and ocular melanomas. **High-Yield NEET-PG Pearls:** * **Cobalt-60:** Used in Teletherapy (External Beam Radiation), not typically brachytherapy. * **Gold-198 & Palladium-103:** Other common isotopes used for permanent interstitial implants. * **Half-life Check:** Ir-192 (74 days), I-125 (60 days), Cs-137 (30 years), I-131 (8 days). * **Rule of Thumb:** If the isotope is "swallowed or injected" to circulate, it’s systemic; if it’s "placed or implanted" as a solid, it’s brachytherapy.

Q: Which of the following is a radiosensitive tumor?

Ewing's sarcoma. ### Explanation **Correct Option: A. Ewing’s Sarcoma** Radiosensitivity refers to the relative susceptibility of cells, tissues, or tumors to the effects of ionizing radiation. In clinical oncology, tumors are categorized based on their response to radiotherapy. **Ewing’s sarcoma** is a highly radiosensitive "small round blue cell tumor." While surgery is often the primary treatment, radiotherapy plays a critical role in local control, especially in unresectable cases or as adjuvant therapy, because these cells undergo rapid apoptosis when exposed to radiation. **Incorrect Options:** * **B. Osteosarcoma:** Unlike Ewing’s, osteosarcoma is considered **radioresistant**. It is a bone-forming tumor that requires very high doses of radiation to achieve cell kill, which often exceeds the tolerance of surrounding normal tissues. Surgery is the mainstay of treatment. * **C. Renal Cell Carcinoma (RCC):** RCC is classically described as **radioresistant**. While stereotactic body radiotherapy (SBRT) is increasingly used for palliation or small lesions, standard fractionated radiotherapy is generally ineffective for primary curative intent. * **D. Pancreatic Carcinoma:** This is a highly aggressive, **radioresistant** adenocarcinoma. The hypoxic microenvironment of pancreatic tumors often contributes to their poor response to ionizing radiation. **High-Yield Clinical Pearls for NEET-PG:** * **Highly Radiosensitive Tumors:** Seminoma (most sensitive), Dysgerminoma, Ewing’s sarcoma, Lymphomas, and Wilms’ tumor. * **Radio-responsive Tumors:** Squamous cell carcinoma (e.g., Cervix, Head & Neck). * **Radioresistant Tumors:** Osteosarcoma, Malignant Melanoma, Renal Cell Carcinoma, and Pancreatic Cancer. * **Bergonie-Tribondeau Law:** Cells are more radiosensitive if they have a high mitotic rate, a long mitotic future, and are undifferentiated.

Question 1

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

Accepted Answer

Mycosis Fungoides

Answer

Deep Seated Brain Tumors

Answer

Skull Base Chordomas

Answer

Lung Cancer

Question 2

High brachytherapy dose rate is:

Accepted Answer

12 Gy/hour

Answer

0.4-2 Gy/hour

Answer

2-12 Gy/hour

Answer

0.01-0.3 Gy/hour

Question 3

Radiotherapy is the treatment of choice in which of the following malignancies?

Accepted Answer

Ewing's sarcoma

Answer

Osteosarcoma

Answer

Osteoclastoma

Answer

Synovial sarcoma

Question 4

For which malignancy is dog leg field radiotherapy primarily indicated?

Accepted Answer

Seminoma

Answer

Lymphoma

Answer

Bronchogenic cancer

Answer

Cancer cervix

Question 5

For teletherapy setup, all are used except?

Accepted Answer

Iridium 127

Answer

Cobalt 60

Answer

Simulator

Answer

Computer

Question 6

Radiotherapy is least useful in the primary management of which of the following cancers?

Accepted Answer

Stomach cancer

Answer

Esophagus cancer

Answer

Cervix cancer

Answer

Breast cancer

Question 7

All of the following isotopes are used in brachytherapy EXCEPT:

Accepted Answer

Iodine-131 (I131)

Answer

Iodine-125 (I125)

Answer

Cesium-137 (Cs137)

Answer

Iridium-192 (Ir192)

Question 8

Which of the following is a radiosensitive tumor?

Accepted Answer

Ewing's sarcoma

Answer

Osteosarcoma

Answer

Renal cell carcinoma

Answer

Pancreatic carcinoma

Question 9

What is the ideal timing for radiotherapy for Wilms' tumor after surgery?

Accepted Answer

Within 10 days

Answer

Within 2 weeks

Answer

Within 2 months

Answer

Any time after surgery

Question 10

Complications of irradiation of the mouth may include all EXCEPT:

Accepted Answer

Accelerated periodontal diseases

Answer

Accelerated caries activity

Answer

Caries in unusual sites

Answer

Xerostomia

Radiation Oncology Basics — MCQs

Radiation Oncology Basics — MCQs

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