Radiation Oncology Basics Practice Questions

Q: After radiotherapy, at least how long should extraction be avoided?

2 months. **Explanation:** The correct answer is **B. 2 months**. **Underlying Medical Concept:** Radiotherapy to the head and neck region causes significant vascular changes, specifically **endarteritis obliterans**. This leads to hypoxia, hypocellularity, and hypovascularity of the bone (the "3-H" principle). If a tooth is extracted immediately following radiation, the compromised blood supply prevents normal socket healing, significantly increasing the risk of **Osteoradionecrosis (ORN)**. Clinical guidelines generally recommend a waiting period of at least **2 months (8 weeks)** post-radiotherapy to allow the initial acute inflammatory response to subside and for the tissue to stabilize, though the risk of ORN remains a lifelong concern. **Analysis of Options:** * **A. 1 month:** This is insufficient time. The acute mucositis and vascular inflammation are at their peak, and the bone's regenerative capacity is severely impaired. * **C & D. 6 and 12 months:** While waiting longer is theoretically safer, the standard clinical benchmark for the *minimum* mandatory avoidance period in most protocols is 2 months. However, many clinicians prefer to avoid extractions indefinitely if possible, using endodontic therapy instead. **High-Yield Clinical Pearls for NEET-PG:** * **Golden Period:** The best time for necessary extractions is **2-3 weeks BEFORE** starting radiotherapy. * **ORN Risk:** The risk is highest in the **mandible** (due to lower vascularity compared to the maxilla) and with radiation doses exceeding **60 Gy**. * **Hyperbaric Oxygen (HBO):** Often used (Marx Protocol) before and after extractions in previously irradiated bone to stimulate angiogenesis and reduce ORN risk. * **Management:** If extraction is unavoidable post-radiation, use atraumatic techniques and avoid vasoconstrictors (like adrenaline) in local anesthesia.

Q: Reduced salivary flow following irradiation is dose dependent. At what radiation dose does salivary flow reach essentially zero?

6000 rads. **Explanation:** Salivary gland dysfunction is one of the most common and debilitating side effects of head and neck radiotherapy. The serous acini of the parotid glands are highly radiosensitive. The reduction in salivary flow is progressive and directly proportional to the cumulative radiation dose. * **Why 6000 rads (60 Gy) is correct:** While a significant reduction in flow (up to 50-60%) is seen after 2000 rads, the salivary flow reaches **essentially zero (permanent xerostomia)** when the cumulative dose reaches **6000 rads (60 Gy)**. At this threshold, the parenchymal destruction and subsequent fibrosis of the glands are so extensive that compensatory recovery is no longer possible. **Analysis of Incorrect Options:** * **4000 rads (40 Gy):** At this dose, there is a severe reduction in flow (often >75%), but some residual function may remain. * **5000 rads (50 Gy):** This is often considered the threshold for "clinically significant" permanent xerostomia, but total cessation (zero flow) is typically documented at higher doses. * **7000 rads (70 Gy):** While this dose certainly results in zero flow, it is beyond the standard threshold where flow first hits the "zero" mark. **High-Yield Clinical Pearls for NEET-PG:** * **Mean Dose Limit:** To prevent permanent xerostomia, the mean dose to at least one parotid gland should be kept **<26 Gy**. * **Early Changes:** Salivary flow can decrease by as much as 50% within the first week of treatment (after only 10 Gy). * **Clinical Consequence:** Radiation-induced xerostomia leads to "Radiation Caries" due to the loss of the buffering capacity of saliva and a shift in oral flora to acidogenic bacteria. * **Management:** Amifostine (a radioprotector) or IMRT (to spare the glands) are used to mitigate this damage.

Q: Electron beam therapy is used for which tumor?

Kaposi sarcoma. **Explanation:** The core principle of **Electron Beam Therapy (EBT)** is its **limited depth of penetration**. Unlike high-energy X-rays (photons) which penetrate deep into the body, electrons lose energy rapidly and have a "sharp fall-off" dose distribution. This makes EBT ideal for treating **superficial tumors** while sparing underlying deep-seated critical structures. * **Why Kaposi Sarcoma is correct:** Kaposi sarcoma typically presents as multifocal, superficial skin lesions. Since these lesions are cutaneous, EBT provides an excellent dose to the skin surface without damaging deeper tissues. Other common indications for EBT include basal cell carcinoma, squamous cell carcinoma of the skin, and Total Skin Electron Irradiation (TSEI) for Mycosis Fungoides. * **Why other options are incorrect:** * **Renal Carcinoma (A) and Hepatic Carcinoma (C):** These are deep-seated visceral organs. Electron beams would be absorbed by the body wall and would never reach the target dose at the required depth. These are treated with high-energy Photons (External Beam Radiotherapy) or SBRT. * **AML (D):** Acute Myeloid Leukemia is a systemic hematological malignancy. While radiotherapy (Total Body Irradiation) is sometimes used in conditioning for bone marrow transplants, it utilizes Photons to ensure deep, uniform penetration, not superficial electrons. **High-Yield Clinical Pearls for NEET-PG:** * **Electron Rule of Thumb:** The depth of the 80% isodose line (therapeutic depth) is approximately **Energy (MeV) / 3** in cm. * **Surface Dose:** Unlike photons (which have a "skin-sparing" effect), electrons deliver a high dose to the surface, making them the treatment of choice for **skin cancers** and **scar boosts** (e.g., post-mastectomy). * **Internal Shields:** Lead shields are often used in EBT to protect underlying structures like the eyes or oral mucosa.

Q: Stereotactic radiosurgery uses all except?

Electron. **Explanation:** Stereotactic Radiosurgery (SRS) is a specialized form of radiation therapy that delivers a single, high dose of radiation to a small, well-defined intracranial target with millimetric precision. The fundamental requirement for SRS is a **sharp dose fall-off** to protect adjacent critical structures. **Why Electron is the correct answer:** Electrons have a limited range in tissue and exhibit significant **lateral scattering**. This makes it impossible to achieve the sharp penumbra and extreme precision required for stereotactic procedures. Electrons are primarily used for treating superficial tumors (e.g., skin cancers or chest wall) rather than deep-seated intracranial targets. **Analysis of other options:** * **Gamma Knife:** The gold standard for SRS. It uses multiple cobalt-60 sources (usually 192 or 201) focused on a single point to treat intracranial lesions. * **Linear Accelerator (LINAC):** Modern LINACs (e.g., CyberKnife, Novalis) use high-energy **Photons (X-rays)** to perform SRS. They utilize specialized collimators and image guidance to deliver precise arcs of radiation. * **Proton:** Proton beam therapy is used in stereotactic settings because of the **Bragg Peak** phenomenon, which allows for superior dose localization and minimal exit dose. **High-Yield Clinical Pearls for NEET-PG:** * **Definition:** SRS is typically a **single fraction** treatment. If delivered in 2–5 fractions, it is called Stereotactic Radiotherapy (SRT). * **Indications:** Arteriovenous Malformations (AVM), Acoustic Neuroma, Meningioma, and solitary Brain Metastases. * **Gamma Knife Source:** Cobalt-60 (emits Gamma rays). * **CyberKnife:** A robotic LINAC-based system that does not require a rigid invasive head frame.

Q: Intraoperative radiotherapy for treating pancreatic carcinoma mainly uses which type of radiation?

Electron Beam. ### Explanation **Correct Answer: C. Electron Beam** **Why Electron Beam is correct:** Intraoperative Radiotherapy (IORT) involves delivering a concentrated dose of radiation directly to the tumor bed during surgery after the primary mass has been resected. **Electrons** are the preferred modality for IORT, particularly in pancreatic carcinoma, due to their unique physical property known as **limited range**. Unlike photons, electrons have a finite depth of penetration that can be precisely tuned by adjusting the energy (MeV). This allows the radiation oncologist to deliver a high dose to the superficial tumor bed while ensuring a **rapid dose fall-off**, which spares the underlying deep-seated critical structures (like the aorta or spinal cord). Mobile linear accelerators (LINACs) are used in the OR to provide these electron beams. **Why other options are incorrect:** * **Alpha Rays:** These have extremely high linear energy transfer (LET) but very low penetration (micrometers). They are used in targeted alpha therapy (e.g., Radium-223) but are not suitable for external beam IORT. * **Gamma Rays:** These are high-energy photons (e.g., from Cobalt-60). They have high penetrability and no "cutoff" point, meaning they would exit the body and damage healthy tissues deep to the surgical site. * **Proton Beam:** While protons also have a "Bragg Peak" (favorable depth-dose profile), the equipment required is massive and not portable for intraoperative use in a standard surgical suite. **High-Yield Clinical Pearls for NEET-PG:** * **Main advantage of IORT:** Allows direct visualization and physical displacement of sensitive organs (like loops of bowel) away from the radiation field. * **Common indications:** Pancreatic cancer, recurrent rectal cancer, and early-stage breast cancer (TARGIT/ELIOT trials). * **Depth Control:** Electron energy (MeV) divided by 2 gives the approximate range in cm (e.g., 12 MeV electrons travel ~6 cm).

Q: Stereotactic radiosurgery is used in which of the following conditions?

All of the above. **Explanation:** Stereotactic Radiosurgery (SRS) is a highly precise form of radiation therapy that delivers a single, high dose of ionizing radiation to a specific target within the brain or spine, while sparing the surrounding healthy tissue. Despite the name, it is a non-invasive procedure and does not involve traditional surgery. **Why "All of the Above" is Correct:** SRS is indicated for various intracranial pathologies where surgical access is difficult or high precision is required: * **Metastatic Brain Tumors (Option A):** SRS is a gold-standard treatment for brain metastases (especially if <3-4 cm in size). It offers excellent local control and is often preferred over whole-brain radiation to avoid cognitive decline. * **Arteriovenous Malformations (AVMs) (Option B):** SRS is used for small-to-medium-sized AVMs, particularly those located in eloquent areas (speech or motor cortex) where surgical resection carries high risk. It induces endothelial proliferation and eventual obliteration of the nidus over 1–3 years. * **Trigeminal Neuralgia (Option C):** It is a recognized treatment for refractory trigeminal neuralgia. The radiation is targeted at the trigeminal nerve root entry zone to disrupt pain signaling. **High-Yield Clinical Pearls for NEET-PG:** * **Modalities:** Common SRS systems include **Gamma Knife** (uses Cobalt-60 sources), **CyberKnife** (robotic linear accelerator), and modified **LINAC** systems. * **Benign Tumors:** SRS is also highly effective for **Acoustic Neuromas** (Vestibular Schwannomas) and **Meningiomas**. * **Dose Gradient:** The hallmark of SRS is a "steep dose fall-off," ensuring that the high-dose zone is strictly confined to the lesion. * **Contraindication:** It is generally not suitable for very large tumors (>3-4 cm) due to the risk of radiation-induced edema in surrounding brain tissue.

Q: Craniospinal irradiation is employed in the treatment of which of the following conditions?

Medulloblastoma. **Explanation:** **Craniospinal Irradiation (CSI)** is a specialized radiotherapy technique designed to treat the entire neuraxis (the whole brain and the entire spinal canal down to the S2-S3 level). **Why Medulloblastoma is the Correct Answer:** Medulloblastoma is a highly malignant primitive neuroectodermal tumor (PNET) arising in the posterior fossa. It has a notorious propensity for **leptomeningeal dissemination** (seeding via cerebrospinal fluid). Because the entire CNS is at risk for "drop metastases," local treatment is insufficient. CSI is the standard of care post-surgery to sterilize the CSF pathways and prevent recurrence. **Analysis of Incorrect Options:** * **A, B, & C (Oligodendroglioma, Pilocytic Astrocytoma, Mixed Oligoastrocytoma):** These are typically localized gliomas. * **Pilocytic Astrocytoma** is a Grade I benign tumor usually cured by surgical resection alone. * **Oligodendrogliomas** and **Mixed Gliomas** tend to spread locally through brain parenchyma rather than through CSF seeding. Treatment usually involves localized radiotherapy to the tumor bed with a margin, rather than the entire craniospinal axis. **High-Yield Clinical Pearls for NEET-PG:** * **Indications for CSI:** Medulloblastoma (most common), Germinoma (CNS Germ Cell Tumors), and occasionally Ependymoma (if disseminated). * **Technique:** CSI involves two lateral brain fields and one or two posterior spinal fields. The "junction" between these fields must be shifted (feathered) every 5 fractions to avoid "hot spots" or "cold spots" on the spinal cord. * **Side Effects:** Significant bone marrow suppression (as the spine contains ~40% of adult bone marrow) and growth retardation in children.

Q: A patient requires tooth extractions from an area that has been subjected to radiation therapy. Which of the following represents the greatest danger to this patient?

Osteoradionecrosis. ### Explanation **Correct Option: B. Osteoradionecrosis (ORN)** The most severe complication following dental extractions in a pre-irradiated field is **Osteoradionecrosis**. Radiation therapy causes permanent damage to the bone's microvasculature, leading to a state of **hypovascularity, hypocellularity, and hypoxia** (Marx’s 3H theory). This impairs the bone's ability to heal and mount an immune response. When a tooth is extracted, the resulting trauma creates a portal for infection in bone that cannot repair itself, leading to non-healing, exposed necrotic bone that may persist for months. **Analysis of Incorrect Options:** * **A. Alveolar osteitis:** Also known as "dry socket," this is a common localized complication of extraction due to premature clot loss. While painful, it is not the "greatest danger" compared to the extensive bone destruction seen in ORN. * **C. Prolonged healing:** While radiation does cause delayed wound healing, this is a general symptom. ORN is a specific, pathological disease process that represents a much more significant clinical threat than mere delay. * **D. Fracture of the mandible:** While a pathological fracture can occur as a *sequela* of advanced ORN, the primary underlying danger and the disease entity itself is Osteoradionecrosis. **NEET-PG High-Yield Pearls:** * **Marx’s Theory:** The classic triad of Hypovascular-Hypocellular-Hypoxic tissue. * **Critical Dose:** The risk of ORN increases significantly with radiation doses above **60 Gy**. * **Prevention:** Ideally, all necessary extractions should be performed **at least 2–3 weeks before** starting radiation therapy. * **Management:** If extraction is mandatory post-radiation, **Hyperbaric Oxygen (HBO) therapy** is often used to stimulate angiogenesis and improve tissue oxygenation before and after the procedure.

Q: For shield (mould) brachytherapy in eye tumors, which of the following isotopes is preferred?

Phosphorus-32. **Explanation:** **Phosphorus-32 (P-32)** is the preferred isotope for shield/mould brachytherapy in superficial eye tumors (such as conjunctival melanomas or squamous cell carcinomas) because it is a **pure beta emitter**. Beta particles have a very short range in tissue (limited penetration). This property allows for the delivery of a high dose of radiation to the superficial lesion while ensuring a rapid dose fall-off, thereby sparing the deeper, sensitive intraocular structures like the lens, retina, and optic nerve from radiation damage. **Analysis of Incorrect Options:** * **Iodine-131 (A):** While used in thyroid pathologies, it emits both beta and high-energy gamma rays. The gamma component would penetrate too deeply, causing significant collateral damage to the internal structures of the eye. * **Gold-198 (B):** This is a gamma emitter used primarily for permanent interstitial implants (e.g., prostate or head and neck). Its high energy makes it unsuitable for superficial ocular shield therapy. * **Strontium-90 (C):** Although a beta emitter used for very superficial "pterygium" treatment, P-32 is often preferred for specific mould applications due to its physical properties and half-life (14.3 days) in specific clinical protocols. **Clinical Pearls for NEET-PG:** * **Pure Beta Emitters:** Remember the mnemonic **"P-S-Y"** (Phosphorus-32, Strontium-90, Yttrium-90). * **Ruthenium-106 & Iodine-125:** These are also commonly used for **Uveal Melanoma** (Plaque Brachytherapy). * **P-32 Half-life:** 14.3 days. * **Treatment Choice:** For deep-seated intraocular tumors (like Retinoblastoma), external beam radiotherapy or plaque brachytherapy with gamma/X-ray emitters is used; for superficial lesions, beta-emitting moulds are ideal.

Q: For teletherapy, which isotope is commonly used?

Co-60. **Explanation:** **Cobalt-60 (Co-60)** is the correct answer because it is the standard radioisotope used in external beam radiation therapy (Teletherapy). It undergoes beta decay followed by the emission of two high-energy gamma photons (1.17 MeV and 1.33 MeV), with an average energy of **1.25 MeV**. This energy is sufficient to treat deep-seated tumors. Co-60 has a half-life of **5.27 years**, making it practical for clinical use as the source only needs replacement every 5–7 years. **Analysis of Incorrect Options:** * **I-123:** This is a diagnostic isotope used primarily in nuclear medicine for thyroid scans and uptake studies due to its pure gamma emission and short half-life (13 hours). It is not used for therapy. * **Cs-137:** While used in radiation oncology, Cesium-137 is primarily used for **Brachytherapy** (manual afterloading) rather than teletherapy. Its lower energy (0.66 MeV) makes it less ideal for external beams compared to Cobalt. * **Tc-99m:** Technetium-99m is the most common diagnostic isotope used in **Scintigraphy** (Gamma camera imaging). Its short half-life (6 hours) and low energy (140 keV) make it unsuitable for cancer treatment. **High-Yield Clinical Pearls for NEET-PG:** * **Penumbra:** Cobalt-60 machines have a larger geometric penumbra compared to Linear Accelerators (LINAC) because the source has a finite diameter (1-2 cm). * **Dmax:** For Co-60, the maximum dose is reached at a depth of **0.5 cm** below the skin (skin-sparing effect). * **Source Decay:** The output of a Cobalt unit decreases by about **1% per month**, requiring regular adjustment of treatment times.

Question 1

After radiotherapy, at least how long should extraction be avoided?

Accepted Answer

2 months

Answer

1 month

Answer

6 months

Answer

12 months

Question 2

Reduced salivary flow following irradiation is dose dependent. At what radiation dose does salivary flow reach essentially zero?

Accepted Answer

6000 rads

Answer

4000 rads

Answer

5000 rads

Answer

7000 rads

Question 3

Electron beam therapy is used for which tumor?

Accepted Answer

Kaposi sarcoma

Answer

Renal carcinoma

Answer

Hepatic carcinoma

Answer

AML

Question 4

Stereotactic radiosurgery uses all except?

Accepted Answer

Electron

Answer

Proton

Answer

Linear accelerator

Answer

Gamma knife

Question 5

Intraoperative radiotherapy for treating pancreatic carcinoma mainly uses which type of radiation?

Accepted Answer

Electron Beam

Answer

Alpha Rays

Answer

Gamma Rays

Answer

Proton beam

Question 6

Stereotactic radiosurgery is used in which of the following conditions?

Accepted Answer

All of the above

Answer

Metastatic brain tumors

Answer

Arteriovenous malformations

Answer

Trigeminal neuralgia

Question 7

Craniospinal irradiation is employed in the treatment of which of the following conditions?

Accepted Answer

Medulloblastoma

Answer

Oligodendroglioma

Answer

Pilocytic astrocytoma

Answer

Mixed oligoastrocytoma

Question 8

A patient requires tooth extractions from an area that has been subjected to radiation therapy. Which of the following represents the greatest danger to this patient?

Accepted Answer

Osteoradionecrosis

Answer

Alveolar osteitis

Answer

Prolonged healing

Answer

Fracture of the mandible

Question 9

For shield (mould) brachytherapy in eye tumors, which of the following isotopes is preferred?

Accepted Answer

Phosphorus-32

Answer

Iodine-131

Answer

Gold-198

Answer

Strontium-90

Question 10

For teletherapy, which isotope is commonly used?

Accepted Answer

Co-60

Answer

I-123

Answer

Cs-137

Answer

Tc-99

Radiation Oncology Basics — MCQs

Radiation Oncology Basics — MCQs

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