Radiation Oncology Basics — MCQs

Radiation Oncology Basics Indian Medical PG Practice Questions and MCQs

Question 81: Radiotherapy is least useful in the primary management of which of the following cancers?

A. Stomach cancer (Correct Answer)
B. Esophagus cancer
C. Cervix cancer
D. Breast cancer

Explanation: ### 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).

Question 82: All of the following isotopes are used in brachytherapy EXCEPT:

A. Iodine-125 (I125)
B. Iodine-131 (I131) (Correct Answer)
C. Cesium-137 (Cs137)
D. Iridium-192 (Ir192)

Explanation: **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.

Question 83: Which technique is useful for remote afterloading?

A. Teletherapy
B. Brachytherapy (Correct Answer)
C. Stereotactic radiotherapy
D. None of the above

Explanation: **Explanation:** **Brachytherapy** is the correct answer because "remote afterloading" is a specific safety technique used in internal radiation therapy. In brachytherapy, radioactive sources are placed directly inside or in close proximity to the tumor. To minimize radiation exposure to healthcare personnel, an applicator is first positioned in the patient; the radioactive source is then mechanically driven from a shielded safe into the applicator via a computerized remote control system (remote afterloading). This eliminates the need for manual handling of "hot" sources. **Why other options are incorrect:** * **Teletherapy (Option A):** This refers to external beam radiation therapy where the radiation source is at a distance from the body (e.g., Cobalt-60 or Linear Accelerator). Since the source is housed within the machine head and never enters the patient, the concept of "afterloading" an applicator does not apply. * **Stereotactic Radiotherapy (Option C):** This is a highly precise form of teletherapy (external beam) that delivers high doses of radiation to a small, well-defined target. It uses specialized positioning and imaging but does not involve internal source loading. **High-Yield Clinical Pearls for NEET-PG:** * **Common Isotopes:** Iridium-192 is the most common isotope used in High Dose Rate (HDR) remote afterloading. * **Types of Brachytherapy:** It can be **Interstitial** (into tissue, e.g., prostate/breast), **Intracavitary** (into a body cavity, e.g., cervix), or **Surface** (on the skin). * **Inverse Square Law:** Brachytherapy relies on this principle, where the radiation dose drops off rapidly as the distance from the source increases, sparing adjacent healthy organs (e.g., rectum/bladder in cervical cancer).

Question 84: Which of the following is a radiosensitive tumor?

A. Ewing's sarcoma (Correct Answer)
B. Osteosarcoma
C. Renal cell carcinoma
D. Pancreatic carcinoma

Explanation: ### 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 85: Complications of irradiation of the mouth may include all EXCEPT:

A. Accelerated caries activity
B. Caries in unusual sites
C. Accelerated periodontal diseases (Correct Answer)
D. Xerostomia

Explanation: **Explanation:** The correct answer is **C. Accelerated periodontal diseases**. While radiotherapy (RT) for head and neck cancers significantly impacts the oral cavity, it primarily affects the salivary glands and dental hard tissues rather than causing a rapid acceleration of pre-existing periodontal disease. **Why C is correct:** Radiation does not directly cause "accelerated" periodontal disease in the same way it causes caries. While RT can lead to decreased vascularity of the periodontium and a risk of **Osteoradionecrosis (ORN)**, the hallmark oral complication is "Radiation Caries" due to qualitative and quantitative changes in saliva, not a primary inflammatory breakdown of the periodontal ligament. **Analysis of Incorrect Options:** * **D. Xerostomia:** This is the most common side effect. Radiation causes atrophy and fibrosis of the acinar cells of the salivary glands (especially the Parotid). This loss of the buffering capacity and cleansing action of saliva is the root cause of subsequent dental issues. * **A & B. Accelerated/Unusual Caries:** Known as **Radiation Caries**, these occur rapidly (within months). Because of the lack of saliva, decay appears in "unusual sites" that are typically self-cleansing, such as the **cervical margins (gum line)**, incisal edges, and cusp tips. **High-Yield Clinical Pearls for NEET-PG:** * **Radiation Caries:** Primarily a secondary effect of xerostomia, not a direct effect of radiation on the enamel. * **Osteoradionecrosis (ORN):** The most serious complication. It is characterized by exposed bone that fails to heal for 3–6 months in a previously irradiated area. The **Mandible** is more commonly affected than the Maxilla due to its poorer blood supply. * **Management:** Patients should undergo all necessary dental extractions at least **2 weeks prior** to starting radiotherapy to prevent ORN. * **Pilocarpine:** A cholinergic agonist often used to manage radiation-induced xerostomia.

Question 86: Intraoperative radiotherapy (RT) is indicated in which of the following malignancies?

A. Carcinoma of the cervix
B. Carcinoma of the breast
C. Carcinoma of the pancreas (Correct Answer)
D. Carcinoma of the thyroid

Explanation: **Explanation:** **Intraoperative Radiotherapy (IORT)** is a specialized technique where a concentrated dose of radiation is delivered directly to the tumor bed or residual tumor during surgery, while the abdomen or chest is open. This allows for the displacement of radiosensitive normal structures (like small bowel loops) away from the radiation field. **Why Carcinoma of the Pancreas is Correct:** Pancreatic adenocarcinoma often presents with close or positive surgical margins due to its proximity to major vascular structures (SMA, portal vein). IORT is particularly indicated here because it allows for a high dose of radiation to be delivered to the retroperitoneal space—an area prone to local recurrence—while sparing the sensitive duodenum and small intestines that would otherwise be in the path of external beam radiation (EBRT). **Analysis of Incorrect Options:** * **Carcinoma of the Cervix:** Standard treatment involves a combination of EBRT and **Brachytherapy** (intracavitary). IORT is rarely used and is not a standard indication. * **Carcinoma of the Breast:** While IORT (e.g., TARGIT trial) is an emerging option for very early-stage breast cancer as a form of accelerated partial breast irradiation, it is not the "classic" or primary indication compared to the established role in difficult-to-resect abdominal malignancies like pancreatic or colorectal cancers. * **Carcinoma of the Thyroid:** Primary treatment is surgical (Total Thyroidectomy) followed by **Radioactive Iodine (I-131) ablation**. RT is reserved only for palliative or unresectable cases, and IORT is not used. **High-Yield Clinical Pearls for NEET-PG:** * **Common Indications for IORT:** Pancreatic cancer, locally advanced rectal cancer, retroperitoneal sarcomas, and occasionally early-stage breast cancer. * **Main Advantage:** Maximizes the **Therapeutic Index** by increasing the dose to the tumor while minimizing the dose to "Organs at Risk" (OARs). * **Type of Radiation:** Usually delivered via **electrons** (IOERT) or low-energy X-rays.

Question 87: What is the optimal timing for oxygen administration to be effective during radiotherapy?

A. Just before starting (Correct Answer)
B. During and within microseconds of starting
C. After 5 minutes
D. After 10 minutes

Explanation: **Explanation:** The effectiveness of radiotherapy relies heavily on the **Oxygen Enhancement Ratio (OER)**. Oxygen acts as a potent radiosensitizer by reacting with free radicals produced by ionizing radiation. This reaction "fixes" the damage to the DNA (the **Oxygen Fixation Hypothesis**), making it permanent and lethal to the cancer cell. **1. Why "Just before starting" is correct:** For oxygen to sensitize cells, it must be present at the exact moment of irradiation. Administering oxygen just before starting ensures that the tumor microenvironment is sufficiently oxygenated and that the gas has diffused into the hypoxic core of the tumor before the first beam is delivered. **2. Analysis of incorrect options:** * **During and within microseconds:** While oxygen must be present during the radiation pulse, waiting until the treatment has started to administer it is too late. The chemical reactions (free radical formation) occur in fractions of a second ($10^{-5}$ seconds); if oxygen isn't already present in the tissue, the damage remains repairable. * **After 5 or 10 minutes:** Administering oxygen after the radiation dose is useless. Once the radiation beam stops, the free radicals have already either been repaired by intracellular thiols or have caused non-fixable damage. Oxygen has no "rescue" or delayed sensitizing effect. **Clinical Pearls for NEET-PG:** * **OER Value:** The OER for X-rays and gamma rays is typically **2.5 to 3.0**. * **Hypoxic Cells:** These are **3 times more resistant** to radiation than oxygenated cells. * **Hyperbaric Oxygen:** Historically used to overcome tumor hypoxia, though now largely replaced by hypoxic cell sensitizers (e.g., Nimorazole) or advanced fractionation. * **LET Relationship:** The oxygen effect is maximal with **Low-LET radiation** (X-rays) and minimal/absent with **High-LET radiation** (Alpha particles, Neutrons).

Question 88: Which of the following tumors is considered radioresistant?

A. Ewing's sarcoma
B. Retinoblastoma
C. Osteosarcoma (Correct Answer)
D. Neuroblastoma

Explanation: ### Explanation The radiosensitivity of a tumor is primarily determined by its cell of origin, growth fraction, and inherent DNA repair mechanisms. **Why Osteosarcoma is the Correct Answer:** **Osteosarcoma** is a primary bone-forming malignancy characterized by the production of osteoid. It is classically considered **radioresistant**. This resistance is attributed to the tumor's slow doubling time in certain areas, its ability to repair sublethal radiation damage effectively, and the dense, mineralized matrix it produces. Because it does not respond predictably to standard radiotherapy doses, the primary treatment modality remains surgical resection with neoadjuvant and adjuvant chemotherapy. **Analysis of Incorrect Options:** * **A. Ewing’s Sarcoma:** Unlike Osteosarcoma, Ewing’s is a small round blue cell tumor. These tumors typically have a high growth fraction and are highly **radiosensitive**. Radiotherapy is a standard component of management, especially for local control. * **B. Retinoblastoma:** This is a highly **radiosensitive** embryonal tumor. While laser and cryotherapy are used for small lesions, radiotherapy (external beam or brachytherapy) is an effective treatment for larger tumors. * **C. Neuroblastoma:** Another member of the small round blue cell tumor family, it is generally **radiosensitive**. Radiation is frequently used in high-risk cases to treat the primary site or metastatic deposits. **NEET-PG High-Yield Pearls:** * **Most Radiosensitive Tumors:** Seminoma, Dysgerminoma, Lymphoma, and Small round blue cell tumors (Ewing’s, Wilms’, Neuroblastoma). * **Most Radioresistant Tumors:** Osteosarcoma, Chondrosarcoma, Malignant Melanoma, and Renal Cell Carcinoma (RCC). * **Bergonie-Tribondeau Law:** Cells are more radiosensitive if they have a high mitotic rate, a long mitotic future, and are undifferentiated.

Question 89: Point A and Point B in Manchester localization are used in the radiotherapy treatment of which cancer?

A. Kidney
B. Uterus
C. Cervix (Correct Answer)
D. Vagina

Explanation: **Explanation:** The **Manchester System** is a classic dosimetry system used in **Brachytherapy** for the treatment of **Carcinoma Cervix**. It utilizes specific anatomical reference points to ensure adequate dosage to the tumor while sparing surrounding critical structures. * **Point A:** Located 2 cm superior to the external cervical os and 2 cm lateral to the midline (uterine canal). It represents the location where the uterine artery crosses the ureter. It is the primary point for dose prescription, as it corresponds to the paracervical triangle. * **Point B:** Located 3 cm lateral to Point A (5 cm from the midline). It represents the pelvic side wall and the location of the obturator lymph nodes. It is used to assess the dose to the lateral pelvic structures. **Why other options are incorrect:** * **Kidney:** Radiotherapy for renal tumors (like Wilms tumor) typically involves External Beam Radiation Therapy (EBRT), not intracavitary brachytherapy using the Manchester system. * **Uterus:** While endometrial cancer may use brachytherapy, the Manchester system is specific to the anatomy of the cervix and the paracervical tissues. * **Vagina:** Primary vaginal cancers use different dosimetry systems often based on the depth of the lesion or the surface of the vaginal cylinder, rather than Points A and B. **High-Yield Clinical Pearls for NEET-PG:** * **Point A** is the point of **prescription**; **Point B** is the point of **monitoring** (pelvic wall dose). * The dose at Point B is typically **1/3rd to 1/4th** of the dose at Point A. * Modern radiotherapy is shifting from Point A-based planning to **Image-Guided Brachytherapy (IGBT)** using MRI to define the High-Risk Clinical Target Volume (HR-CTV).

Question 90: Which of the following tumors is most radioresistant?

A. Ewing sarcoma
B. Osteosarcoma (Correct Answer)
C. Cervical carcinoma
D. Lymphoma

Explanation: **Explanation:** The radiosensitivity of a tumor is primarily determined by its cell of origin, growth fraction, and inherent DNA repair mechanisms. In clinical radiotherapy, tumors are classified on a spectrum from **radiosensitive** (easily destroyed by low doses) to **radioresistant** (requiring extremely high, often toxic, doses for control). **1. Why Osteosarcoma is the Correct Answer:** Osteosarcoma is a bone-forming malignant tumor characterized by the production of osteoid. It is classically considered **radioresistant**. The tumor cells have highly efficient DNA repair mechanisms and often exist in a dense, hypoxic matrix, making them less susceptible to the free radical damage induced by ionizing radiation. Therefore, the primary treatment for Osteosarcoma is surgical resection and chemotherapy, rather than primary radiotherapy. **2. Analysis of Incorrect Options:** * **Lymphoma (Option D):** These are **highly radiosensitive**. Lymphocytes are among the most sensitive cells in the human body (Bergonie-Tribondeau law), and low doses of radiation can induce rapid apoptosis. * **Ewing Sarcoma (Option A):** Unlike Osteosarcoma, Ewing sarcoma is **radiosensitive**. While surgery is often preferred, radiotherapy is a standard component of management for local control. * **Cervical Carcinoma (Option C):** This is **radiosensitive/radiocurable**. Radiotherapy (External Beam + Brachytherapy) is a definitive treatment modality for advanced stages. **Clinical Pearls for NEET-PG:** * **Most Radiosensitive Tumor:** Seminoma (Testis) and Dysgerminoma (Ovary). * **Most Radiosensitive Cell in the Body:** Small Lymphocyte. * **Most Radioresistant Phase of Cell Cycle:** Late S-phase. * **Most Radiosensitive Phase of Cell Cycle:** M-phase (followed by G2). * **Mnemonic for Radioresistant Tumors:** "MOP" — **M**elanoma, **O**steosarcoma, **P**ancreatic Cancer/Renal Cell Carcinoma.

Practice by Chapter

Principles of Radiation Therapy

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Radiation Therapy Equipment

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Treatment Planning Process

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External Beam Radiation Therapy

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Brachytherapy

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3D Conformal Radiation Therapy

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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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