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Question 1Medical Dosimetry (domain list not enumerated in source)

In external-beam radiotherapy, the prescribed absorbed dose is most commonly stated in which unit?

  1. Becquerel (Bq)
  2. Gray (Gy) Correct answer.
  3. Coulomb per kilogram (C/kg)
  4. Sievert (Sv)

Explanation

Absorbed dose is energy deposited per unit mass and its SI unit is the gray, equal to one joule per kilogram. Prescriptions, dose constraints, and plan reports in radiotherapy are therefore expressed in Gy or cGy.

Question 2Medical Dosimetry (domain list not enumerated in source)

A treatment plan delivers 2 Gy per fraction to a target. Expressed in centigray, what is the daily target dose?

  1. 2000 cGy
  2. 200 cGy Correct answer.
  3. 0.2 cGy
  4. 20 cGy

Explanation

One gray equals 100 centigray, so 2 Gy corresponds to 200 cGy. Converting between Gy and cGy is a routine check when reading prescriptions and machine output values.

Question 3Medical Dosimetry (domain list not enumerated in source)

Which imaging modality is the reference standard for treatment planning dose calculation because it provides electron density information?

  1. Fluoroscopy
  2. Computed tomography Correct answer.
  3. Ultrasound
  4. Plain radiography

Explanation

CT numbers are calibrated against electron density through a Hounsfield unit to density curve, which lets the dose algorithm model tissue inhomogeneity. Ultrasound and projection radiography do not provide calibrated density data for heterogeneity correction.

Question 4Medical Dosimetry (domain list not enumerated in source)

In the ICRU reference point convention for a photon plan, the reported dose is specified at a point that lies where?

  1. On the central axis at the centre of the planning target volume Correct answer.
  2. At the maximum dose point anywhere in the patient
  3. At the skin surface on the central axis
  4. At the field edge at half the prescribed dose

Explanation

The ICRU reference point is placed on the central axis at the centre of the target volume, in a region of relatively homogeneous dose. This makes reported dose reproducible and comparable between plans and institutions.

Question 5Medical Dosimetry (domain list not enumerated in source)

Which quantity describes the variation of absorbed dose with depth along the central axis for a single photon beam at a fixed source-to-surface distance?

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

Explanation

Percentage depth dose expresses the dose at depth as a percentage of the dose at the reference depth, for a fixed field size and source-to-surface distance. It is used in manual and semi-empirical dose calculations for simple geometries.

Question 6Medical Dosimetry (domain list not enumerated in source)

What is the primary purpose of a beam's-eye-view digitally reconstructed radiograph in treatment planning?

  1. To compute the monitor units required for each field
  2. To measure the beam output constancy during treatment
  3. To verify the planned beam geometry against portal anatomy Correct answer.
  4. To determine the electron density of the target volume

Explanation

A digitally reconstructed radiograph is generated from the planning CT along the beam's-eye view and is compared with portal or cone-beam images for geometric verification. It confirms that patient position and field orientation match the approved plan.

Question 7Medical Dosimetry (domain list not enumerated in source)

Which structure receives the highest priority constraint in a plan where the target coverage conflicts with an organ-at-risk limit?

  1. The organ at risk at its tolerance dose
  2. The bolus applied to the skin
  3. The planning target volume prescription Correct answer.
  4. The normal tissue outside the target

Explanation

Target coverage to the prescribed dose is the primary clinical objective, and organ-at-risk limits are traded off against it within accepted tolerance. A plan that spares an organ at the cost of geographic miss of the target is unacceptable.

Question 8Medical Dosimetry (domain list not enumerated in source)

In a photon beam, the region beyond the maximum dose point on the central axis is known as what?

  1. Inverse square region
  2. Penumbra
  3. Dose fall-off region Correct answer.
  4. Build-up region

Explanation

Beyond dmax the absorbed dose decreases gradually with depth because attenuation outweighs the contribution from scattered photons. This descending portion of the depth-dose curve is the dose fall-off region.

Question 9Medical Dosimetry (domain list not enumerated in source)

A 6 MV photon beam has dmax of about 1.5 cm. What is the practical consequence for a tumour located at 10 cm depth when a single anterior field is used?

  1. The tumour dose is lower than the dose at dmax Correct answer.
  2. The skin surface receives a higher dose than the tumour
  3. The beam energy is too low to reach the tumour
  4. The tumour receives the maximum dose of the beam

Explanation

Because dose falls off beyond dmax, a target at 10 cm depth receives less than the dose delivered at 1.5 cm. This depth attenuation is why multiple beams or higher energies are used for deep-seated targets.

Question 10Medical Dosimetry (domain list not enumerated in source)

Which statement correctly describes the effect of increasing field size on percentage depth dose for a megavoltage photon beam?

  1. Percentage depth dose increases only at the surface and not at depth
  2. Percentage depth dose is unchanged because scatter is negligible
  3. Percentage depth dose increases because of greater scattered dose contribution Correct answer.
  4. Percentage depth dose decreases because of increased attenuation

Explanation

A larger field produces more scattered photons that contribute dose at depth, so the percentage depth dose rises with field size. This dependence is why field size must be specified when tabulating depth-dose data.

Question 11Medical Dosimetry (domain list not enumerated in source)

In an inverse-planned intensity-modulated treatment, what does the optimizer primarily vary to satisfy the dose objectives?

  1. The gantry rotation speed during delivery
  2. The CT slice thickness used for planning
  3. The patient's immobilization index
  4. The relative weights of beamlet segments Correct answer.

Explanation

Inverse planning starts from dose objectives and adjusts beamlet or segment weights iteratively until the objective function is minimized. The resulting fluence map is then converted into deliverable multileaf collimator segments.

Question 12Medical Dosimetry (domain list not enumerated in source)

A plan shows a hot spot of 110 percent of prescription inside the planning target volume. Which interpretation is most appropriate?

  1. The plan must be rejected because any dose above prescription is unacceptable
  2. The hot spot will always cause target underdosage elsewhere
  3. The hot spot indicates a monitor unit calculation error
  4. The hot spot is acceptable if it lies within the target and organ-at-risk limits are met Correct answer.

Explanation

Modest hot spots inside the target volume are generally acceptable and even expected in some techniques, provided that organ-at-risk constraints and target minimum coverage are satisfied. The clinical judgement depends on location and magnitude relative to protocol limits.

Question 13Medical Dosimetry (domain list not enumerated in source)

Which dose-volume histogram parameter best describes the minimum dose received by 95 percent of a target volume?

  1. D5
  2. V95
  3. V5
  4. D95 Correct answer.

Explanation

A D-parameter reports the dose received by a specified percentage of a structure's volume, so D95 is the dose covering 95 percent of the volume. A V-parameter instead reports the volume receiving at least a specified dose.

Question 14Medical Dosimetry (domain list not enumerated in source)

A patient is treated with a 4-field box for a pelvic target. Doubling the number of beams to eight with the same total dose typically produces which change?

  1. Improved conformity with lower dose to each individual organ at risk Correct answer.
  2. Increased penumbra width at the target edge
  3. Reduced target coverage
  4. Higher integral dose to normal tissue

Explanation

Adding beams spreads entrance dose over more directions, reducing the dose contributed by any single beam path to each organ at risk and improving conformity. Integral dose to the patient as a whole, however, may increase slightly.

Question 15Medical Dosimetry (domain list not enumerated in source)

In brachytherapy dosimetry, which statement correctly describes the dose rate near a point source?

  1. Dose rate falls linearly with distance
  2. Dose rate falls approximately with the inverse square of distance Correct answer.
  3. Dose rate is constant with distance from the source
  4. Dose rate increases with distance because of scatter build-up

Explanation

For a small brachytherapy source the dose rate falls off approximately as the inverse square of distance, modified by tissue attenuation and scatter. This steep gradient is why source position and dwell times are critical.

Question 16Medical Dosimetry (domain list not enumerated in source)

Which factor most directly determines the penumbra width at the edge of a photon field?

  1. The CT reconstruction kernel
  2. The source size and the collimator-to-surface distance Correct answer.
  3. The number of monitor units delivered
  4. The patient's body mass index

Explanation

Geometric penumbra arises from the finite focal spot size of the source and widens with increasing distance from the collimator to the surface. Scattered radiation adds a further physical penumbra component.

Question 17Medical Dosimetry (domain list not enumerated in source)

A patient's plan uses a wedge to compensate for a sloping surface. What is the dosimetric effect of the wedge on the isodose distribution?

  1. It eliminates the penumbra at the field edges
  2. It increases the beam energy to a higher megavoltage level
  3. It flattens the dose across the target by tilting the isodose lines Correct answer.
  4. It removes the need for tissue heterogeneity correction

Explanation

A wedge filter attenuates one side of the beam more than the other, tilting the isodose curves so that dose is more uniform across an uneven or curved treatment volume. Modern plans often achieve the same effect with dynamic multileaf collimator motion instead of a physical wedge.

Question 18Medical Dosimetry (domain list not enumerated in source)

Which statement about the tissue maximum ratio is correct?

  1. It is measured at a fixed source-to-axis distance and is independent of source-to-surface distance Correct answer.
  2. It depends on source-to-surface distance
  3. It is defined only for electron beams
  4. It equals the percentage depth dose divided by the field size

Explanation

The tissue maximum ratio is the ratio of dose at depth to dose at dmax for the same point and field size, and it is independent of source-to-surface distance. This property makes it useful for isocentric calculations where the source-to-axis distance is fixed.

Question 19Medical Dosimetry (domain list not enumerated in source)

A dose constraint states that the spinal cord maximum dose must be below 45 Gy. Which planning strategy best respects this while maintaining target coverage?

  1. Reduce the prescribed target dose
  2. Accept a cord dose of 50 Gy as clinically equivalent
  3. Remove the cord from the optimization objectives
  4. Increase the number of beam angles that avoid the cord Correct answer.

Explanation

Using beam directions that avoid the cord and applying dose-volume objectives to the cord allow the optimizer to reduce cord dose while preserving target coverage. Reducing prescription or ignoring the constraint would compromise safety or coverage.

Question 20Medical Dosimetry (domain list not enumerated in source)

In electron beam therapy, at what depth does the dose typically fall to approximately 50 percent of its maximum value?

  1. At the surface
  2. At the depth of maximum dose
  3. Near the practical range, approximately half the maximum range Correct answer.
  4. At twice the practical range

Explanation

Electron depth-dose curves show a rapid fall-off near the end of the electron range, with the 50 percent level occurring close to half the practical range. This steep distal fall-off is exploited to spare tissue beyond the target.

Question 21Medical Dosimetry (domain list not enumerated in source)

A treatment plan reports a conformity index of 1.0 for the prescription isodose volume relative to the target volume. What does this indicate?

  1. The prescription isodose volume exactly matches the target volume Correct answer.
  2. The plan has no hot spots
  3. The dose gradient at the target edge is zero
  4. The target received only half of the prescribed dose

Explanation

When the ratio of prescription isodose volume to target volume equals one, the two volumes coincide in size, indicating ideal conformity. Values above one indicate the high-dose volume extends beyond the target.

Question 22Medical Dosimetry (domain list not enumerated in source)

A patient with a metallic hip prosthesis requires pelvic radiotherapy. Which planning consideration is most important for accurate dose calculation?

  1. Using a lower photon energy to avoid attenuation
  2. Reducing the prescribed dose by half to compensate for backscatter
  3. Ignoring the prosthesis because it is outside the target
  4. Assigning the correct high CT number to the metal and using an algorithm that handles high-density heterogeneities Correct answer.

Explanation

Metal produces very high CT numbers and can cause streak artifacts, so the density must be assigned carefully and an algorithm capable of handling high-density heterogeneities should be used. Otherwise the calculated attenuation and backscatter near the implant will be inaccurate.

Question 23Medical Dosimetry (domain list not enumerated in source)

In intensity-modulated radiotherapy quality assurance, what does a gamma analysis compare?

  1. Two patient setups using couch shift values
  2. Two computed tomography scans using Hounsfield unit differences
  3. Two treatment plans using monitor unit totals only
  4. Two dose distributions using both dose difference and distance-to-agreement criteria Correct answer.

Explanation

Gamma analysis combines a dose-difference criterion and a distance-to-agreement criterion into a single index, so that points failing one criterion may still pass if they satisfy the other. It is widely used to compare measured and calculated dose distributions.

Question 24Medical Dosimetry (domain list not enumerated in source)

A tumour is treated with two opposed parallel photon fields. Compared with a single field, what is the main dosimetric advantage?

  1. More homogeneous dose through the target volume Correct answer.
  2. Complete elimination of scatter dose
  3. Reduced need for target localization
  4. Higher surface dose on both sides

Explanation

Opposed fields deliver dose from two directions so the high-dose region is spread through the target rather than concentrated near one entrance. The combined depth-dose curves produce a more uniform distribution across the target volume.

Question 25Medical Dosimetry (domain list not enumerated in source)

Which statement best explains why dose algorithms that model lateral electron transport are preferred for small fields in stereotactic treatments?

  1. Small fields eliminate the need for output factor measurements
  2. Small fields always deliver lower dose than large fields
  3. Small fields lack lateral electronic equilibrium, so simple algorithms misestimate dose Correct answer.
  4. Small fields are insensitive to detector size

Explanation

In small fields the range of secondary electrons exceeds the field dimensions, so charged-particle equilibrium is lost and dose depends strongly on lateral transport. Algorithms such as Monte Carlo or Acuros explicitly model this, whereas simpler models can over- or underestimate dose.

Question 26Medical Dosimetry (domain list not enumerated in source)

A tumour is prescribed 60 Gy in 30 fractions. What daily target dose does this fractionation correspond to?

  1. 1.5 Gy per fraction
  2. 2 Gy per fraction Correct answer.
  3. 2.5 Gy per fraction
  4. 3 Gy per fraction

Explanation

Total dose divided by the number of fractions gives the dose per fraction, so 60 Gy divided by 30 fractions equals 2 Gy per fraction. This conventional fractionation is the reference schedule against which altered fractionation schemes are compared.

Question 27Medical Dosimetry (domain list not enumerated in source)

Which imaging modality is acquired in the treatment position and forms the geometric reference for all dose calculations in a radiotherapy plan?

  1. The diagnostic MRI scan
  2. The planning CT scan Correct answer.
  3. The PET-CT scan
  4. The portal verification image

Explanation

The planning CT provides electron density information (Hounsfield units) required for heterogeneity-corrected dose calculation and defines the coordinate system for the plan. Other imaging is registered to it for target delineation but does not serve as the dose-calculation reference.

Question 28Medical Dosimetry (domain list not enumerated in source)

Which structure in the male pelvis lies immediately posterior to the prostate and is at particular risk during prostate radiotherapy?

  1. The bladder
  2. The rectum Correct answer.
  3. The femoral head
  4. The pubic symphysis

Explanation

The rectum lies directly posterior to the prostate and is separated from it only by Denonvilliers fascia, so it receives high dose in most prostate plans and is a dose-limiting organ. Bladder, pubis and femoral heads are anterior or lateral to the target.

Question 29Medical Dosimetry (domain list not enumerated in source)

What does the percentage depth dose value at 10 cm depth describe for a photon beam?

  1. The dose at 10 cm depth expressed as a percentage of the prescribed target dose
  2. The dose at 10 cm depth expressed as a percentage of the maximum dose on the central axis Correct answer.
  3. The dose at 10 cm depth expressed as a percentage of the dose at the surface
  4. The dose at the surface expressed as a percentage of the dose at 10 cm depth

Explanation

Percentage depth dose is defined relative to the maximum dose on the central axis, not to the surface or to the prescription. It therefore combines inverse-square falloff, attenuation and scatter into a single depth-dependent ratio.

Question 30Medical Dosimetry (domain list not enumerated in source)

Which quantity is used to convert dose measured in a water phantom to dose in a patient for a given beam quality?

  1. The beam quality conversion factor combined with the absorbed dose to water calibration Correct answer.
  2. The inverse square correction alone
  3. The tissue phantom ratio
  4. The backscatter factor

Explanation

Reference dosimetry is traceable to an absorbed dose to water calibration, and a beam quality conversion factor accounts for the difference between the calibration quality and the user's beam. Phantom ratios and inverse square corrections describe relative dose behaviour, not absolute calibration.

Question 31Medical Dosimetry (domain list not enumerated in source)

A patient is set up with a 3-point laser system and tattoos. What is the main purpose of daily imaging before treatment?

  1. To decide whether to change the prescription
  2. To recalibrate the linear accelerator output
  3. To measure the delivered dose inside the patient
  4. To verify that the internal anatomy matches the planned position within tolerance Correct answer.

Explanation

Daily imaging detects interfraction setup error and anatomical change so that corrections can be applied before dose is delivered. It does not measure absolute dose in tissue or substitute for machine calibration.

Question 32Medical Dosimetry (domain list not enumerated in source)

In a step-and-shoot IMRT delivery, what distinguishes it from sliding-window delivery?

  1. The gantry rotates continuously during delivery
  2. The leaves move continuously while the beam is on
  3. The beam is switched off while leaves move between fixed aperture segments Correct answer.
  4. Only the jaw positions change during delivery

Explanation

Step-and-shoot delivery uses a sequence of static multileaf collimator apertures, with the beam interrupted while leaves reposition. Sliding-window delivery instead moves leaves continuously during irradiation to modulate fluence.

Question 33Medical Dosimetry (domain list not enumerated in source)

Which dose specification convention is standard for reporting dose in external beam radiotherapy?

  1. Mean dose to the whole body
  2. Dose at the skin surface
  3. Dose at the isocentre measured in air
  4. Dose to the ICRU reference point with a stated heterogeneity correction Correct answer.

Explanation

ICRU reporting conventions specify dose at a defined reference point within the target, with the calculation algorithm and heterogeneity handling stated. Dose in air or at the skin is not representative of target dose.

Question 34Medical Dosimetry (domain list not enumerated in source)

A target is prescribed 50 Gy at the 95 percent isodose line. What absolute dose corresponds to that isodose line?

  1. 50 Gy
  2. 47.5 Gy Correct answer.
  3. 55 Gy
  4. 52.6 Gy

Explanation

The isodose value is a percentage of the prescribed dose, so 95 percent of 50 Gy equals 47.5 Gy. The prescription point is therefore covered by an isodose line slightly below the maximum.

Question 35Medical Dosimetry (domain list not enumerated in source)

A plan delivers 70 Gy to the prostate while the rectal wall receives a maximum of 72 Gy. What is the most appropriate dosimetric interpretation?

  1. The plan is acceptable because the target dose is high
  2. The rectal dose is irrelevant if the patient is asymptomatic
  3. The plan is acceptable because the rectum is outside the field
  4. The rectal maximum exceeds the target dose and the constraint should be reviewed Correct answer.

Explanation

A rectal maximum above the prescribed target dose indicates that the organ at risk constraint is not being met and the plan needs re-optimisation. Rectal tolerance is dose-limiting and is not waived by absence of symptoms at planning.

Question 36Medical Dosimetry (domain list not enumerated in source)

Which plan evaluation metric reports the volume of a structure receiving at least a specified dose?

  1. The homogeneity index
  2. The conformity index
  3. The differential dose-volume histogram
  4. The cumulative dose-volume histogram Correct answer.

Explanation

The cumulative DVH plots volume receiving at least a given dose, so a value such as V20 reads directly from it. The differential DVH shows volume per dose bin instead, and the conformity and homogeneity indices are single summary numbers.

Question 37Medical Dosimetry (domain list not enumerated in source)

A small lung lesion is treated with a 6 MV beam using a field of 3 by 3 cm. Which dosimetric issue is most important to address?

  1. Overestimation of dose from the inverse square law
  2. Reduced penumbra caused by the collimator
  3. Loss of charged-particle equilibrium and lateral scatter requiring a small-field-corrected algorithm Correct answer.
  4. Excessive build-up dose at the skin surface

Explanation

Small fields lack lateral charged-particle equilibrium because the range of secondary electrons exceeds the field width, so detector response and algorithm accuracy both degrade. Algorithms that model lateral electron transport and small-field-corrected detectors are needed.

Question 38Medical Dosimetry (domain list not enumerated in source)

What is the main dosimetric consequence of using a flattening-filter-free photon beam?

  1. A higher dose rate with a non-flat, peaked cross-plane profile Correct answer.
  2. A lower dose rate with a perfectly flat profile
  3. A narrower penumbra at all depths
  4. A shallower depth of maximum dose

Explanation

Removing the flattening filter raises output substantially, allowing faster delivery, but the unflattened beam retains a peaked profile that must be accounted for in planning. Depth of maximum dose and penumbra are not the defining changes.

Question 39Medical Dosimetry (domain list not enumerated in source)

A patient requires craniospinal irradiation. Which planning consideration most strongly drives the technique choice?

  1. Eliminating the need for image guidance
  2. Matching of adjacent fields to avoid hot and cold bands along the neuraxis Correct answer.
  3. Avoiding the use of immobilisation
  4. Minimising the number of monitor units

Explanation

Craniospinal irradiation treats a long, curved target, so field junctions must be feathered or the plan delivered with a continuous technique to prevent overdose or underdose at the match line. This junction problem dominates technique selection.

Question 40Medical Dosimetry (domain list not enumerated in source)

In a VMAT plan, what does the dose rate modulation during arc delivery primarily allow?

  1. Removal of the multileaf collimator from the beam path
  2. Elimination of the need for inverse planning
  3. Constant dose rate throughout the arc
  4. Continuous variation of fluence while the gantry rotates Correct answer.

Explanation

VMAT combines gantry rotation with dynamic multileaf collimation and variable dose rate to shape fluence continuously around the patient. It still relies on inverse planning and uses the collimator actively.

Question 41Medical Dosimetry (domain list not enumerated in source)

A patient's plan shows a target D98 of 93 percent of prescription while the constraint requires at least 95 percent. What is the best next step?

  1. Re-optimise, for example by adjusting objectives or beam arrangement, to improve coverage Correct answer.
  2. Increase the prescription dose to compensate
  3. Remove the target volume from the optimisation
  4. Accept the plan because D98 is close to the target

Explanation

D98 below the stated constraint means the plan fails the coverage requirement and must be re-optimised, typically by re-weighting objectives or revising beam geometry. Accepting a known constraint violation or altering the prescription is not appropriate.

Question 42Medical Dosimetry (domain list not enumerated in source)

Which source of uncertainty is quantified by the planning target volume margin derived from random and systematic error analysis?

  1. Uncertainty in the dose calculation algorithm
  2. Uncertainty in the linear accelerator output calibration
  3. Setup and organ motion uncertainty around the clinical target volume Correct answer.
  4. Uncertainty in the prescribed fraction number

Explanation

The CTV-to-PTV margin accounts for geometric uncertainties such as setup variation and internal organ motion, using recipes that combine systematic and random error components. Algorithm and output uncertainties are addressed by other quality-assurance processes.

Question 43Medical Dosimetry (domain list not enumerated in source)

What is the purpose of the gamma index in patient-specific IMRT quality assurance?

  1. To calculate the monitor units for each segment
  2. To measure the absolute output of the linear accelerator
  3. To combine dose difference and distance-to-agreement into a single pass or fail criterion Correct answer.
  4. To determine the target volume from the planning CT

Explanation

The gamma index evaluates measured against calculated dose by considering both dose difference and spatial distance, so a small positional shift does not automatically fail the comparison. It is a comparison metric, not a calibration or planning tool.

Question 44Medical Dosimetry (domain list not enumerated in source)

A patient is treated with a single anterior 6 MV field to a target at 8 cm depth. Compared with a parallel-opposed pair, what is the main disadvantage?

  1. A shallower depth of maximum dose
  2. A larger penumbra at the field edges
  3. A higher entrance dose on the treated side and a rapid dose falloff beyond the target Correct answer.
  4. A lower target dose for the same monitor units

Explanation

A single field deposits a high entrance dose along its path while the exit side receives little dose, producing poor dose uniformity across the target. Opposed fields share the entrance dose between two sides and improve homogeneity.

Question 45Medical Dosimetry (domain list not enumerated in source)

Which quantity must be measured to characterise the output factor of a multileaf collimator-defined field?

  1. Dose at the surface relative to the maximum dose
  2. Dose at the isocentre measured without any collimation
  3. Dose per monitor unit relative to a reference square field at the same depth Correct answer.
  4. Dose in air relative to dose in water

Explanation

The output factor is a ratio of dose per monitor unit for the treatment field to that of a reference field at the same depth and setup. It captures collimator scatter and phantom scatter changes with field size.

Question 46Medical Dosimetry (domain list not enumerated in source)

A head and neck plan uses three coplanar fields with a 6 MV beam. Which arrangement most improves target homogeneity when the target wraps around the spinal cord?

  1. Reducing the number of fields to two opposed laterals
  2. Removing the cord from the optimisation objectives
  3. Increasing the field size to cover the whole neck
  4. Using non-coplanar or arc-based delivery with careful cord sparing objectives Correct answer.

Explanation

When the target surrounds a serial organ at risk, additional degrees of freedom such as non-coplanar beams or arc delivery allow dose to be shaped around the cord while maintaining coverage. Simply enlarging fields or removing the cord constraint would violate tolerance.

Question 47Medical Dosimetry (domain list not enumerated in source)

A plan is calculated with a simple pencil-beam algorithm and then recomputed with a Monte Carlo algorithm for a lung target. What difference is most expected?

  1. The prescribed dose will change automatically
  2. The monitor units will be independent of the algorithm
  3. The Monte Carlo dose in the low-density lung and at tissue interfaces will differ from the pencil-beam result Correct answer.
  4. The Monte Carlo dose will be identical everywhere

Explanation

Simple pencil-beam algorithms handle lateral electron transport poorly in low-density lung and near interfaces, so Monte Carlo typically predicts different dose in those regions. The prescription is a clinical decision and monitor units depend on the calculated dose distribution.

Question 48Medical Dosimetry (domain list not enumerated in source)

A patient with a cardiac pacemaker requires breast radiotherapy. Which planning precaution is most relevant?

  1. Removing all bolus to reduce scatter
  2. Estimating the dose to the device and keeping it within the manufacturer's recommended limit Correct answer.
  3. Ignoring the device because it lies outside the target volume
  4. Increasing the dose rate to shorten the exposure

Explanation

The dose to an implanted electronic device must be estimated from the plan and kept below the manufacturer's limit, with cardiology input where relevant. Devices outside the target can still receive scatter or exit dose and are not automatically safe.

Question 49Medical Dosimetry (domain list not enumerated in source)

A plan achieves excellent target coverage but the parotid mean dose is 30 Gy bilaterally. Which statement best reflects the clinical dosimetric concern?

  1. The parotid dose is irrelevant because the glands are outside the high-dose region
  2. The parotid dose should be increased to improve homogeneity
  3. The parotid dose exceeds the usual sparing goal and raises the risk of xerostomia Correct answer.
  4. The parotid dose is acceptable because coverage is excellent

Explanation

Mean parotid doses above roughly 26 Gy are associated with a substantially increased risk of xerostomia, so a value of 30 Gy bilaterally signals inadequate sparing. Good target coverage does not compensate for exceeding an organ-at-risk tolerance.

Question 50Medical Dosimetry (domain list not enumerated in source)

During adaptive radiotherapy for a shrinking tumour, which dosimetric change is most likely if the plan is not adapted?

  1. Organs at risk adjacent to the target may receive higher dose than originally planned Correct answer.
  2. The target dose will automatically decrease
  3. The monitor units will become invalid
  4. The beam energy will change

Explanation

As a tumour shrinks, adjacent organs at risk can move into the high-dose region that was previously occupied by tumour, increasing their dose. Re-planning restores the intended sparing, and beam energy and monitor unit calculation remain defined by the machine and plan.