I85PREP
⌘ K
CAMRT Radiation Therapy study materials
CAMRT Radiation Therapy study resources

CAMRT Radiation Therapy practice questions

One question at a time. Build your CAMRT Radiation Therapy understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Radiological Technology

Which imaging modality is most commonly used for treatment planning and daily image guidance in modern external beam radiation therapy because it provides excellent soft-tissue contrast without ionizing radiation dose to the patient?

  1. Fluoroscopy
  2. Magnetic resonance imaging Correct answer.
  3. Positron emission tomography
  4. Computed tomography

Explanation

Magnetic resonance imaging delivers no ionizing radiation and offers superior soft-tissue contrast, which makes it valuable for delineating tumors and organs at risk during planning and for MR-guided treatment systems. CT remains the standard for dose calculation because it provides electron density information.

Question 2Radiological Technology

In external beam radiotherapy, what does the term 'isocenter' describe?

  1. The reference point on the patient's skin used for setup
  2. The location of maximum dose within the patient
  3. The point in space about which the treatment unit rotates and to which the beam axes converge Correct answer.
  4. The geometric center of the planning target volume

Explanation

The isocenter is a fixed point in space defined by the intersection of the rotation axes of the treatment machine, and beams are directed to converge there. It is a machine-related reference, not a patient anatomy point, although plans are often normalized so that a target point coincides with it.

Question 3Radiological Technology

A patient is treated with a 6 MV photon beam. Which interaction process predominates for dose deposition in soft tissue at this energy?

  1. Pair production
  2. Photoelectric absorption
  3. Compton scattering Correct answer.
  4. Coherent scattering

Explanation

At megavoltage energies in soft tissue, Compton scattering dominates because the photon energy greatly exceeds the binding energies of most tissue electrons, so interactions occur with essentially free electrons. Photoelectric absorption dominates at low kilovoltage energies, and pair production requires energies above 1.022 MeV in high-Z materials.

Question 4Radiological Technology

Which statement best describes the purpose of a flattening filter in a conventional linear accelerator photon beam?

  1. It increases the dose rate at the surface of the patient
  2. It produces a relatively uniform dose profile across the treatment field at depth Correct answer.
  3. It hardens the electron beam before it reaches the target
  4. It collimates the beam to the target shape

Explanation

The flattening filter attenuates the central portion of the forward-peaked bremsstrahlung beam more than the periphery, resulting in a flatter lateral dose profile across the field. Flattening-filter-free beams intentionally omit this component to gain dose rate at the cost of a peaked profile.

Question 5Radiological Technology

In the photon build-up region, why does the absorbed dose rise from the surface to a maximum before decreasing?

  1. The inverse square law increases dose as distance from the source decreases
  2. The photon fluence increases with depth because of scatter from the collimators
  3. Beam hardening reduces attenuation at the surface
  4. Secondary electrons set in motion reach an equilibrium concentration with depth Correct answer.

Explanation

Near the surface, the number of energetic secondary electrons is lower than at greater depths because of electron range and lack of upstream scatter. At the depth of maximum dose, electronic equilibrium is achieved and the dose peaks before attenuation dominates.

Question 6Radiological Technology

A treatment plan prescribes 60 Gy in 30 fractions to a target. What is the dose per fraction?

  1. 1.5 Gy
  2. 0.5 Gy
  3. 3 Gy
  4. 2 Gy Correct answer.

Explanation

Dose per fraction is the total prescribed dose divided by the number of fractions, so 60 Gy divided by 30 fractions equals 2 Gy per fraction. This is the classic conventionally fractionated schedule used in many curative radiotherapy regimens.

Question 7Radiological Technology

According to the linear-quadratic model, what does the alpha/beta ratio of a tissue represent?

  1. The ratio of total dose to the number of fractions
  2. The relative biological effectiveness of the beam
  3. The dose at which the linear and quadratic contributions to cell killing are equal Correct answer.
  4. The maximum dose the tissue can tolerate without injury

Explanation

In the linear-quadratic model, cell survival is described by a linear term (alpha) and a quadratic term (beta) in dose. The alpha/beta ratio is the dose at which these two contributions are equal, and it characterizes the tissue's fractionation sensitivity.

Question 8Radiological Technology

Which normal tissue is generally considered to have a low alpha/beta ratio and therefore to be especially sensitive to changes in dose per fraction?

  1. Late-responding tissues such as the spinal cord Correct answer.
  2. Acute-responding tissues such as the oral mucosa
  3. Bone marrow stem cells
  4. Rapidly proliferating skin epithelium

Explanation

Late-responding tissues such as the spinal cord and other central nervous system structures typically have low alpha/beta ratios, around 2 to 3 Gy, making them sensitive to fraction size. Acute-responding tissues generally have high alpha/beta ratios and are less affected by fraction size for a given equivalent dose.

Question 9Radiological Technology

Which of the following best describes the '4 R's' of radiobiology that influence fractionated radiotherapy outcomes?

  1. Replication, recombination, radiation, and reaction
  2. Radiosensitivity, resistance, recovery, and regression
  3. Reduction, recurrence, remission, and relapse
  4. Repair, repopulation, redistribution, and reoxygenation Correct answer.

Explanation

The classic 4 R's are repair of sublethal damage, repopulation of surviving cells, redistribution of cells through the cell cycle, and reoxygenation of hypoxic tumor regions. These processes explain how fractionation spares normal tissue while improving tumor control.

Question 10Radiological Technology

In a treatment plan, what does the planning target volume (PTV) account for?

  1. Set-up uncertainty and internal organ motion around the clinical target volume Correct answer.
  2. The dose-limiting normal structures adjacent to the target
  3. The microscopic spread of disease beyond the gross tumor
  4. Only the visible gross tumor seen on imaging

Explanation

The PTV is a geometric margin added to the clinical target volume to ensure the prescribed dose reaches the target despite setup errors and internal motion. It is a planning concept, not a biological description of disease extent.

Question 11Radiological Technology

Why is a bolus material sometimes placed on the patient's skin during megavoltage photon treatment?

  1. To reduce the dose to deeper organs at risk
  2. To improve the accuracy of the patient setup lasers
  3. To increase the beam energy reaching the tumor
  4. To bring the maximum dose closer to the skin surface in a superficial target Correct answer.

Explanation

Bolus acts as tissue-equivalent material that shifts the build-up region toward the surface, raising surface dose and bringing the depth of maximum dose closer to the skin. This is useful when the target is superficial and would otherwise lie in the build-up region.

Question 12Radiological Technology

Which collimation device allows conformal shaping of the radiation field to the target's beam's-eye-view projection in modern linear accelerators?

  1. Wedge filter
  2. Multileaf collimator Correct answer.
  3. Electron cone
  4. Compensator

Explanation

A multileaf collimator uses computer-controlled leaves to shape the field to the target projection from the beam's perspective, enabling conformal and intensity-modulated treatments. Wedges and compensators modify dose distribution but do not shape the field aperture.

Question 13Radiological Technology

What is the primary purpose of image-guided radiation therapy (IGRT)?

  1. To replace treatment planning dose calculations
  2. To reduce the number of treatment fractions required
  3. To increase the prescribed dose to the tumor
  4. To verify and correct patient position and target location immediately before or during treatment Correct answer.

Explanation

IGRT uses imaging at the treatment unit to confirm that the target and critical structures are in the intended position, allowing corrections for setup error and motion. It supports accurate delivery but does not itself change the prescribed dose or replace planning calculations.

Question 14Radiological Technology

A patient's plan uses a 6 MV photon beam with a source-to-axis distance of 100 cm. If the field size at the isocenter is 10 cm by 10 cm, what is the approximate field size at the patient's skin surface located 10 cm upstream of the isocenter?

  1. 9 cm by 9 cm Correct answer.
  2. 8 cm by 8 cm
  3. 11 cm by 11 cm
  4. 10 cm by 10 cm

Explanation

Field size scales with distance from the source by similar triangles. At 90 cm from the source compared with 100 cm, the field is 10 cm multiplied by 90/100, giving 9 cm. The field diverges as distance from the source increases.

Question 15Radiological Technology

Which of the following is the main rationale for using intensity-modulated radiation therapy (IMRT) rather than conventional conformal therapy?

  1. It allows non-uniform beam fluence to create concave dose distributions that spare nearby organs at risk Correct answer.
  2. It eliminates the need for target delineation
  3. It removes the need for patient immobilization
  4. It reduces the total number of monitor units required

Explanation

IMRT modulates fluence across each beam so that dose can be shaped around concave targets while sparing adjacent critical structures. This requires accurate target and organ-at-risk delineation and robust immobilization, and it often increases monitor units compared with simpler techniques.

Question 16Radiological Technology

What is the purpose of a dose-volume histogram (DVH) in treatment plan evaluation?

  1. It measures the actual dose delivered during each fraction
  2. It verifies the output calibration of the linear accelerator
  3. It summarizes the distribution of dose within a structure as a function of volume Correct answer.
  4. It records the patient's cumulative dose from prior treatments

Explanation

A DVH plots the volume of a structure receiving at least a given dose, allowing quantitative assessment of target coverage and organ-at-risk dose. It is a planning evaluation tool derived from the calculated dose distribution, not a delivery measurement.

Question 17Radiological Technology

In external beam radiotherapy, which quantity describes the ratio of absorbed dose at a given depth to the dose at a reference depth for a fixed source-to-surface distance?

  1. Percentage depth dose Correct answer.
  2. Inverse square factor
  3. Backscatter factor
  4. Tissue-air ratio

Explanation

Percentage depth dose expresses the dose at depth as a percentage of the dose at a reference depth along the central axis, measured at a fixed source-to-surface distance. It depends on beam energy, field size, and depth.

Question 18Radiological Technology

Which quality assurance test is performed to verify that the linear accelerator's radiation output is within tolerance?

  1. Weekly measurement of the patient's skin dose
  2. Annual replacement of the target material
  3. Monthly recalculation of all patient treatment plans
  4. Daily constancy check of output with an ionization chamber or reference detector Correct answer.

Explanation

Output constancy is checked regularly, typically daily, using a stable reference detector such as an ionization chamber to confirm that machine output remains within tolerance. This ensures that the prescribed dose corresponds to the delivered dose.

Question 19Radiological Technology

During CT simulation for radiotherapy planning, why is patient positioning equipment matched to the treatment unit?

  1. To improve the contrast resolution of the CT images
  2. To reduce the CT radiation dose to the patient
  3. To reproduce the treatment position so that planning images correspond to the delivery geometry Correct answer.
  4. To shorten the CT scan acquisition time

Explanation

Consistent positioning devices and reference marks ensure that the anatomy captured at simulation matches the geometry used for planning and treatment. This reproducibility is essential for accurate setup and dose delivery.

Question 20Radiological Technology

Which statement correctly describes the difference between electron and photon beams in terms of depth-dose characteristics?

  1. Electron beams produce a higher surface dose than photons at all energies
  2. Photon beams have a sharper distal fall-off than electron beams
  3. Electron beams penetrate deeper than photon beams of the same nominal energy
  4. Electron beams have a finite range with a rapid dose fall-off beyond it, whereas photon beams continue to deposit dose beyond the target Correct answer.

Explanation

Electron beams deposit dose over a finite range determined by their energy and then fall off steeply, making them suited to superficial targets. Photon beams attenuate exponentially and continue to deposit dose at greater depths.

Question 21Radiological Technology

Which of the following describes the purpose of a wedge in photon radiotherapy?

  1. To collimate the beam to an irregular target shape
  2. To increase the dose rate by removing the flattening filter
  3. To create a deliberate dose gradient across the field to compensate for missing tissue or sloping surfaces Correct answer.
  4. To measure the beam energy during treatment

Explanation

A wedge introduces differential attenuation across the field, producing a tilt in the dose distribution that can compensate for sloping surfaces or improve homogeneity in certain geometries. It does not shape the aperture or change the nominal energy.

Question 22Radiological Technology

A treatment plan delivers 2 Gy per fraction to a tumor with an alpha/beta ratio of 10 Gy. If the fraction size is increased to 3 Gy while the total dose is adjusted to keep the same biologically effective dose, what is the main radiobiological concern?

  1. The total number of monitor units will decrease proportionally
  2. The beam energy will effectively increase
  3. Late-responding normal tissues with low alpha/beta ratios may receive a higher biologically effective dose Correct answer.
  4. The tumor will become completely radioresistant

Explanation

Larger fraction sizes increase the biologically effective dose more for tissues with low alpha/beta ratios, such as late-responding normal tissues. Even if tumor effect is maintained, the risk of late toxicity rises unless constraints are respected.

Question 23Radiological Technology

Which of the following is the most appropriate action if daily image guidance shows a systematic setup shift of several millimeters over multiple fractions?

  1. Reduce the number of image guidance checks
  2. Ignore the shift because random errors are expected
  3. Review and correct the immobilization or planning margins and consider a plan adaptation Correct answer.
  4. Increase the prescribed dose to compensate

Explanation

A consistent directional shift across fractions indicates a systematic error, which should be investigated and corrected through immobilization, setup technique, or margin review. Ignoring it or increasing dose does not address the underlying cause.

Question 24Radiological Technology

In inverse treatment planning for IMRT, what is the role of the objective function?

  1. It quantifies how far a candidate dose distribution deviates from the clinical goals during optimization Correct answer.
  2. It measures the actual dose delivered to the patient
  3. It calculates the monitor units directly from the prescription
  4. It defines the mechanical limits of the multileaf collimator

Explanation

The objective function assigns penalties to deviations from desired dose or dose-volume goals for targets and organs at risk, and the optimizer minimizes these penalties iteratively. It is a mathematical tool for plan optimization, not a delivery measurement.

Question 25Radiological Technology

Which statement best explains why stereotactic body radiotherapy (SBRT) uses few, large fractions?

  1. Large fractions eliminate the need for image guidance
  2. SBRT is used because linear accelerators cannot deliver many small fractions
  3. SBRT requires no immobilization because fractions are few
  4. Large fraction sizes exploit the low alpha/beta ratio of some tumors and improve tumor control while limiting late effects through precise targeting Correct answer.

Explanation

SBRT delivers ablative doses in few fractions, and the large fraction size can be advantageous for tumors with low alpha/beta ratios. Precise immobilization and image guidance are essential because steep dose gradients surround the target.

Question 26Radiological Technology

Which imaging modality is most commonly used for treatment planning because it provides electron density information needed for inhomogeneity correction?

  1. Magnetic resonance imaging
  2. Computed tomography Correct answer.
  3. Ultrasound
  4. Positron emission tomography

Explanation

Computed tomography (CT) is the standard planning modality because Hounsfield units can be converted into electron density, which is required for accurate dose calculation in heterogeneous tissue. MRI and PET provide excellent soft-tissue or functional information but do not directly give electron density, and ultrasound is not used for primary radiotherapy planning.

Question 27Radiological Technology

What is the primary function of a multileaf collimator (MLC) on a medical linear accelerator?

  1. To monitor the delivered dose during treatment
  2. To flatten the photon beam across the treatment field
  3. To shape the radiation field by independently positioning tungsten leaves Correct answer.
  4. To increase the beam energy by accelerating electrons

Explanation

A multileaf collimator consists of independently movable tungsten leaves that project into the beam path to create conformal field shapes. It replaces or supplements conventional jaws and enables techniques such as IMRT and VMAT. Beam flattening, dose monitoring, and acceleration are performed by other components.

Question 28Radiological Technology

In the context of radiation protection, what does the ALARA principle require?

  1. Keeping radiation doses as low as reasonably achievable while considering economic and social factors Correct answer.
  2. Avoiding all use of ionising radiation in medicine
  3. Administering a fixed annual dose limit to every radiation worker
  4. Using the highest possible beam energy to reduce treatment time

Explanation

ALARA stands for 'as low as reasonably achievable' and requires that all exposures be kept below regulatory limits and further reduced where practical, taking into account economic and social considerations. It applies to occupational, public, and patient exposures and is a cornerstone of radiation protection philosophy. It does not mandate a fixed dose or prohibit beneficial medical exposures.

Question 29Radiological Technology

Which structure is typically delineated as the clinical target volume (CTV)?

  1. The visible gross tumour plus a margin for subclinical microscopic spread Correct answer.
  2. The patient's entire anatomy within the treatment couch
  3. The region encompassing setup and motion uncertainties
  4. The gross tumour volume only, as seen on imaging

Explanation

The clinical target volume (CTV) contains the gross tumour volume (GTV) plus a margin for suspected subclinical microscopic disease. The GTV is the visible tumour, while the PTV adds geometric margins for setup and motion. The CTV therefore represents the volume that must receive a therapeutic dose to control microscopic spread.

Question 30Radiological Technology

What is the main advantage of using a flattening filter free (FFF) photon beam in radiotherapy?

  1. Lower skin dose for the same field size
  2. Improved dose homogeneity across the entire field
  3. Elimination of the need for image guidance
  4. Higher dose rate, which can shorten treatment delivery time Correct answer.

Explanation

Removing the flattening filter increases the dose rate at the isocentre, allowing faster delivery, which is beneficial for hypofractionated treatments and reduces intrafraction motion risk. However, the beam profile is less homogeneous and skin dose may be higher, so FFF beams require careful planning. Image guidance is still necessary.

Question 31Radiological Technology

Which of the following best describes the purpose of a beam's-eye-view (BEV) display in treatment planning?

  1. To display the dose distribution in a transverse CT slice
  2. To show the target and organs at risk as projected along the beam axis Correct answer.
  3. To evaluate the cumulative dose-volume histogram
  4. To calculate the monitor units required for a treatment field

Explanation

A beam's-eye-view projects anatomy along the central axis of a treatment beam, allowing the planner to shape the field aperture to the target while avoiding organs at risk. It is used for field design and MLC leaf positioning. Dose distributions, monitor unit calculations, and DVHs are displayed or computed separately.

Question 32Radiological Technology

What is the primary role of a radiation therapist in the treatment delivery process?

  1. To prescribe the total radiation dose and fractionation schedule
  2. To interpret diagnostic imaging and decide on the treatment intent
  3. To position the patient accurately and deliver the prescribed radiation dose safely Correct answer.
  4. To design the treatment plan and calculate dose distributions

Explanation

Radiation therapists are responsible for accurate patient setup, immobilisation, image verification, and operation of the treatment unit to deliver the prescribed dose. Prescribing and planning are performed by radiation oncologists and medical physicists, respectively. The therapist's role is central to ensuring that the planned dose is delivered reproducibly and safely.

Question 33Radiological Technology

Which device is used to verify that a patient's treatment field matches the planned field before delivery?

  1. Electronic portal imaging device Correct answer.
  2. Water phantom
  3. Ionisation chamber
  4. Electrometer

Explanation

An electronic portal imaging device (EPID) captures megavoltage images of the patient in the treatment position, allowing comparison with reference images to verify field placement. Ionisation chambers and electrometers are used for dosimetry measurements, and water phantoms are used for beam commissioning and quality assurance.

Question 34Radiological Technology

A patient is treated with a 6 MV photon beam at 100 cm source-to-axis distance. If the field size at the isocentre is doubled while keeping the same beam energy and depth, what happens to the percentage depth dose?

  1. It increases because of greater scattered dose Correct answer.
  2. It remains exactly the same
  3. It becomes zero at the surface
  4. It decreases because of greater attenuation

Explanation

For megavoltage photon beams, increasing the field size increases the amount of scattered radiation reaching the central axis, which raises the percentage depth dose at depth. The effect is most pronounced at larger depths and field sizes. Attenuation changes are negligible for the same energy and depth.

Question 35Radiological Technology

In a VMAT plan, what is the main advantage of rotating the gantry continuously during delivery?

  1. It removes the requirement for image guidance
  2. It eliminates the need for a multileaf collimator
  3. It allows many beam angles to be used, improving conformality while reducing delivery time Correct answer.
  4. It guarantees a lower integral dose than any fixed-field plan

Explanation

VMAT delivers radiation while the gantry rotates and the MLC and dose rate are modulated, effectively creating many beam angles in a single arc. This improves target conformality and reduces treatment time compared with fixed-field IMRT. It still requires an MLC and image guidance, and integral dose may not always be lower.

Question 36Radiological Technology

A patient requires treatment to a superficial skin lesion. Which beam characteristic makes electrons preferable to megavoltage photons for this case?

  1. A steep distal fall-off that spares deeper tissues Correct answer.
  2. A larger penumbra at depth
  3. A lower surface dose than photons
  4. A higher penetration depth than photons

Explanation

Electron beams deposit most of their dose within a limited range determined by their energy and then fall off steeply, which allows treatment of superficial lesions while sparing underlying structures. Photon beams penetrate more deeply and have a gradual attenuation. Electron surface dose is relatively high, and penumbra is not the reason for their use.

Question 37Radiological Technology

During a course of radiotherapy, a patient's weight loss causes a noticeable change in body contour. What is the most appropriate action?

  1. Continue the original plan without change because dose calculation is unaffected
  2. Repeat the planning CT and assess whether the plan should be adapted Correct answer.
  3. Increase the prescribed dose to compensate for the smaller body
  4. Reduce the number of fractions to finish treatment sooner

Explanation

Changes in body contour alter tissue thickness and electron density, which can change dose distribution and target coverage. Re-imaging and possibly replanning are needed to ensure the delivered dose remains accurate. Continuing the original plan may result in underdosing the target or overdosing organs at risk, and prescribing decisions remain with the oncologist.

Question 38Radiological Technology

What is the purpose of a half-value layer (HVL) measurement for a photon beam?

  1. To calibrate the electron energy
  2. To determine the output factor of the accelerator
  3. To characterise the beam's penetrating quality Correct answer.
  4. To measure the leakage radiation from the head

Explanation

The half-value layer is the thickness of a material that reduces the beam intensity to half its original value and is used to describe beam quality or penetrating ability. Output factors, leakage, and electron energy calibration are separate measurements. HVL is commonly measured for kilovoltage beams and for shielding calculations.

Question 39Radiological Technology

In brachytherapy, what does the term 'dwell position' refer to?

  1. The total activity of the source
  2. A location where the radioactive source stops to deliver dose Correct answer.
  3. The time the source remains outside the patient
  4. The distance from the source to the first organ at risk

Explanation

In remote afterloading brachytherapy, the source moves through a series of programmed positions called dwell positions and pauses at each for a specified dwell time. The combination of dwell positions and times determines the dose distribution. Dwell position does not refer to source activity or external time.

Question 40Radiological Technology

Which of the following is a key function of the treatment planning system's dose calculation algorithm?

  1. To model radiation transport and compute dose in heterogeneous tissue Correct answer.
  2. To schedule patients for follow-up appointments
  3. To verify patient identity before each fraction
  4. To control the linear accelerator during treatment delivery

Explanation

Dose calculation algorithms model photon and electron transport through tissues of varying density to predict the dose distribution. They do not control the accelerator, verify patient identity, or manage appointments. Accurate modelling of inhomogeneities is essential for safe and effective treatment planning.

Question 41Radiological Technology

A treatment plan is designed with a 1 cm planning target volume margin. If the systematic setup error is found to be 5 mm in one direction, what is the most appropriate initial response?

  1. Ignore the error because it is within the PTV margin
  2. Increase the PTV margin to 2 cm without further investigation
  3. Apply a systematic shift to the treatment couch position for subsequent fractions Correct answer.
  4. Reduce the prescribed dose to compensate

Explanation

A consistent systematic error should be corrected by adjusting the treatment couch position or by replanning so that the target receives the intended dose. Simply ignoring the error may compromise coverage or spare normal tissue. Dose reduction or arbitrary margin changes are not appropriate without clinical and dosimetric review.

Question 42Radiological Technology

What is the main reason for using a thermoplastic mask during head and neck radiotherapy?

  1. To increase the radiation dose at the skin surface
  2. To immobilise the patient and improve setup reproducibility Correct answer.
  3. To shield the salivary glands from radiation
  4. To reduce the need for a planning CT

Explanation

Thermoplastic masks are moulded to the patient's head and neck and fixed to the treatment couch, limiting movement and improving reproducibility between fractions. They do not selectively shield organs or increase dose, and a planning CT is still required. Immobilisation is essential for accurate delivery, especially with steep dose gradients.

Question 43Radiological Technology

Which statement correctly describes the difference between photon and proton depth-dose curves?

  1. Protons deposit most of their dose at a defined depth and then stop, while photons continue to deposit dose beyond the target Correct answer.
  2. Photons can be stopped at a precise depth by adjusting their energy
  3. Protons deliver a higher entrance dose than photons for the same target dose
  4. Photons have a Bragg peak, while protons have an exponential attenuation curve

Explanation

Protons exhibit a Bragg peak, depositing maximum dose at a depth determined by their energy and then falling to near zero, which allows sparing of distal tissues. Photons attenuate exponentially and continue to deposit dose beyond the target. Proton entrance dose is generally lower than the peak dose.

Question 44Radiological Technology

In a total body irradiation (TBI) technique, why is tissue compensation often used?

  1. To account for varying body thickness and achieve a uniform dose throughout the body Correct answer.
  2. To eliminate the need for lung shielding
  3. To reduce the risk of secondary malignancies
  4. To increase the dose rate to shorten treatment time

Explanation

TBI requires a relatively uniform dose to the whole body, but body thickness varies considerably. Tissue compensators or bolus are used to even out the dose distribution. They do not primarily increase dose rate, reduce secondary cancer risk, or replace lung shielding, which may still be necessary.

Question 45Radiological Technology

What is the role of a radio-opaque marker placed on the patient's skin during CT simulation?

  1. To increase the CT image contrast of the tumour
  2. To correlate the skin surface with internal anatomy and assist in treatment setup Correct answer.
  3. To shield the skin from radiation
  4. To measure the radiation dose at the skin surface

Explanation

Radio-opaque markers are placed on the skin to identify reference points that can be seen on the planning CT and used to align the patient during treatment. They do not measure dose, shield the skin, or enhance tumour contrast. They are important for reproducible setup and isocentre localisation.

Question 46Radiological Technology

A patient with a pacemaker requires radiotherapy. What is the most important initial consideration before treatment planning?

  1. Assuming the pacemaker is unaffected and proceeding with the standard plan
  2. Increasing the dose rate to reduce treatment time regardless of risk
  3. Determining the pacemaker's radiation tolerance and consulting with cardiology Correct answer.
  4. Removing the pacemaker before simulation without medical advice

Explanation

Pacemakers and implantable cardioverter-defibrillators may be damaged by radiation, and dose limits depend on the device model and beam energy. The device's tolerance must be established and cardiology consulted, with dose to the device estimated and minimised. Proceeding blindly or removing the device without medical advice is unsafe.

Question 47Radiological Technology

Which of the following best explains why a small field size requires careful dosimetric consideration in stereotactic radiotherapy?

  1. Small fields eliminate the need for inhomogeneity correction
  2. Lateral electronic disequilibrium reduces dose at the field edges and requires small-field dosimetry Correct answer.
  3. Small fields always have a higher output factor than large fields
  4. Small fields produce a larger penumbra than large fields

Explanation

When the field size is smaller than the lateral range of secondary electrons, electronic equilibrium is lost, causing dose at the central axis and edges to be affected by detector size and material. Accurate small-field dosimetry and appropriate detectors are therefore essential. Small fields do not eliminate inhomogeneity correction or necessarily increase output.

Question 48Radiological Technology

What is the main purpose of quality assurance of a treatment planning system?

  1. To check that the accelerator's door interlock functions
  2. To verify that the treatment couch can support the patient's weight
  3. To ensure that calculated dose distributions are accurate and consistent with measurements Correct answer.
  4. To confirm that the waiting room is comfortable for patients

Explanation

Treatment planning system QA verifies that dose calculations agree with measured data across representative clinical cases and beam configurations. It ensures that plans are clinically reliable. Couch weight capacity, waiting room comfort, and door interlocks are unrelated to planning system accuracy.

Question 49Radiological Technology

In a plan using intensity-modulated radiation therapy, what is the effect of increasing the number of intensity levels (segments) on the dose distribution?

  1. Decreased integral dose in all cases
  2. Reduced target coverage because of fewer degrees of freedom
  3. Elimination of the need for inverse planning
  4. Improved target conformality and homogeneity, often at the cost of increased delivery time Correct answer.

Explanation

More intensity levels allow the fluence map to be modulated more finely, improving conformality and homogeneity around the target. However, additional segments increase treatment delivery time and may increase low-dose spread. Inverse planning is still required to optimise the fluence map.

Question 50Radiological Technology

Which statement best describes the radiobiological rationale for using hyperfractionation in radiotherapy?

  1. Smaller doses per fraction allow higher total doses with less late normal tissue damage Correct answer.
  2. Larger doses per fraction improve tumour control without affecting normal tissues
  3. Giving all dose in a single fraction maximises the therapeutic ratio
  4. Hyperfractionation eliminates the need for a planning target volume margin

Explanation

Hyperfractionation uses smaller doses per fraction, often given more than once daily, to exploit the greater repair capacity of late-responding normal tissues relative to tumours. This can allow a higher total dose with acceptable late toxicity. It does not involve larger fractions or eliminate the need for margins.