Radiation Oncology Basics Practice Questions

Q: Which isotope of iodine is used in brachytherapy for prostate carcinoma?

I-125. **Explanation:** **Iodine-125 (I-125)** is the isotope of choice for permanent interstitial brachytherapy in low-risk prostate carcinoma. The underlying medical concept relies on its physical properties: it has a **long half-life (approx. 60 days)** and emits **low-energy gamma radiation (35 keV)**. This allows for a continuous, low-dose rate of radiation to be delivered directly to the tumor over several months, ensuring high local control while minimizing radiation exposure to adjacent critical structures like the rectum and bladder. **Analysis of Incorrect Options:** * **I-127:** This is the only stable, naturally occurring isotope of iodine. It is non-radioactive and therefore has no therapeutic application in oncology. * **I-131:** While used therapeutically, it is primarily used for **thyroid ablation** and treating thyroid cancer. It emits high-energy beta particles and gamma rays with a shorter half-life (8 days), making it unsuitable for the permanent seed implantation required in prostate brachytherapy. * **I-124:** This is a positron emitter used primarily as a radiopharmaceutical for **PET imaging**, not for brachytherapy. **High-Yield Clinical Pearls for NEET-PG:** * **Brachytherapy Isotopes:** Apart from I-125, **Palladium-103 (Pd-103)** is also commonly used for prostate brachytherapy (shorter half-life of 17 days, used for more aggressive tumors). * **I-123:** Used for diagnostic thyroid scans (pure gamma emitter, 13-hour half-life). * **I-131 MIBG:** Used for imaging and treating neuroblastoma and pheochromocytoma. * **Rule of Thumb:** Permanent implants (seeds) use isotopes with low energy and longer half-lives; temporary high-dose-rate (HDR) brachytherapy typically uses **Iridium-192**.

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 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: 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."

Q: Which isotope is replacing radium?

Iridium. **Explanation:** In modern brachytherapy, **Iridium-192** has largely replaced Radium-226. Radium was the first isotope used in radiotherapy, but it fell out of favor due to significant safety concerns: it decays into **Radon gas** (a leakage hazard), has a very long half-life (1,600 years) making disposal difficult, and emits high-energy gamma rays that require heavy shielding. **Why Iridium-192 is preferred:** * **Physical Properties:** It has a high specific activity, allowing for the production of very small "seeds" or wires, which are ideal for interstitial implants. * **Safety:** It does not produce gaseous daughter products. Its lower average photon energy (0.38 MeV) compared to Radium makes radiation protection easier for staff. * **Versatility:** It is the isotope of choice for **High Dose Rate (HDR)** remote afterloading systems. **Analysis of Incorrect Options:** * **Cesium-137:** While it replaced Radium for intracavitary applications (like cervical cancer) due to its longer half-life (30 years) and better safety profile, it has been largely superseded by Iridium-192 in modern HDR units. * **Gold-198:** Used primarily for permanent interstitial implants (e.g., prostate), but its short half-life (2.7 days) makes it unsuitable as a general replacement for Radium. * **Californium-252:** A neutron emitter used experimentally for bulky tumors; it is not a routine replacement for Radium. **High-Yield Clinical Pearls for NEET-PG:** * **Half-life of Ir-192:** ~74 days. * **Half-life of Co-60:** 5.26 years (used in Teletherapy). * **Half-life of Cs-137:** 30 years. * **Brachytherapy Principle:** Follows the **Inverse Square Law**, delivering a high dose to the tumor with a rapid dose fall-off to protect surrounding healthy tissue.

Q: Taste loss due to radiation therapy is recovered in approximately what timeframe?

60 - 120 days. **Explanation:** Radiation-induced dysgeusia (taste loss) is a common side effect in patients receiving radiotherapy for head and neck cancers. The underlying mechanism involves damage to the rapidly dividing basal cells of the taste buds and the microvilli of gustatory cells, which have a high turnover rate. **Why 60 - 120 days is correct:** Taste buds are highly radiosensitive. While taste impairment typically begins within the first 1-2 weeks of treatment (at doses around 10-20 Gy), the recovery process begins after the completion of radiotherapy. It takes approximately **2 to 4 months (60 - 120 days)** for the taste bud cells to regenerate and for the neural pathways to stabilize, leading to a significant restoration of taste function in most patients. **Analysis of Incorrect Options:** * **A. 2 weeks:** This is too early. While acute mucosal inflammation (mucositis) may begin to subside, the regeneration of specialized gustatory cells takes longer. * **B. 60 - 120 weeks:** This represents a timeframe of over 1-2 years. While some patients experience long-term changes, the majority of clinical recovery occurs much sooner. * **D. Not reversible:** Radiation-induced taste loss is generally reversible. Permanent loss (ageusia) is rare unless very high doses are delivered directly to the tongue or if there is severe, permanent xerostomia (dry mouth), as saliva is essential for tasting. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Taste Loss:** Sensitivity to **bitter and acid** flavors is usually lost first, while **salty and sweet** are lost later. * **Xerostomia Connection:** Damage to the salivary glands (especially the parotid) exacerbates taste loss because saliva acts as a solvent for food particles to reach taste receptors. * **Zinc Supplementation:** Some studies suggest that Zinc sulfate may help in the early recovery of taste post-radiation.

Q: Intraoperative radiation therapy is typically indicated for which of the following organs?

Pancreas. **Explanation:** **Intraoperative Radiation Therapy (IORT)** involves the delivery of a single, concentrated dose of ionizing radiation directly to the tumor bed during surgery, immediately after the tumor is resected. **Why Pancreas is the Correct Answer:** The primary advantage of IORT is the ability to physically displace or shield radiosensitive dose-limiting structures (like the small intestine, stomach, and liver) away from the radiation field. In **pancreatic adenocarcinoma**, achieving negative surgical margins is difficult due to proximity to major vessels. IORT is typically indicated here to sterilize microscopic residual disease in the retroperitoneum, where conventional external beam radiation (EBRT) would be limited by bowel toxicity. **Analysis of Incorrect Options:** * **Breast:** While IORT is an emerging option for early-stage breast cancer (e.g., TARGIT trials), it is considered an *alternative* to whole-breast irradiation in highly selected cases, rather than a "typical" or standard indication compared to its established role in deep-seated abdominal malignancies. * **Cervix:** Cervical cancer is primarily managed with EBRT and **Brachytherapy** (intracavitary). IORT is rarely used unless there is a specific pelvic sidewall recurrence. * **Thyroid:** Differentiated thyroid cancer is managed with surgery and **Radioactive Iodine (I-131) therapy**. Radiation is rarely used, and IORT has no standard role here. **High-Yield Clinical Pearls for NEET-PG:** * **Common IORT Indications:** Pancreatic cancer, locally advanced rectal cancer, and retroperitoneal sarcomas. * **Key Advantage:** Maximizes the "Therapeutic Ratio" by allowing a high dose to the target while sparing OARs (Organs at Risk). * **Radiation Type:** Usually delivered via mobile linear accelerators (electrons) or low-energy X-rays.

Question 1

Which isotope of iodine is used in brachytherapy for prostate carcinoma?

Accepted Answer

I-125

Answer

I-127

Answer

I-131

Answer

I-124

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 responds best to radiotherapy?

Accepted Answer

Seminoma

Answer

Teratoma

Answer

Choriocarcinoma

Answer

Endometrial carcinoma

Question 4

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 5

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 6

Radiotherapy is most useful in which of the following?

Accepted Answer

Seminomas

Answer

Choriocarcinoma

Answer

Osteosarcoma

Answer

Renal cell carcinoma

Question 7

All of the following are true about accelerated partial breast irradiation (APBI) except?

Accepted Answer

Suitable patients for APBI include those with multifocal disease.

Answer

APBI is delivered in an abbreviated fashion and a lower total dose than a standard course of whole breast radiation.

Answer

Suitable patients for APBI include women older than or equal to 60 years.

Answer

Suitable patients for APBI include patients whose tumor margins are negative by at least 2 mm.

Question 8

Which isotope is replacing radium?

Accepted Answer

Iridium

Answer

Cesium

Answer

Gold

Answer

Californium

Question 9

Taste loss due to radiation therapy is recovered in approximately what timeframe?

Accepted Answer

60 - 120 days

Answer

2 weeks

Answer

60 - 120 weeks

Answer

Not reversible

Question 10

Intraoperative radiation therapy is typically indicated for which of the following organs?

Accepted Answer

Pancreas

Answer

Breast

Answer

Cervix

Answer

Thyroid

Radiation Oncology Basics — MCQs

Radiation Oncology Basics — MCQs

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