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ABR TMP practice questions

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Question 1Treatment planning and dosimetry

For a 6 MV photon beam at a fixed depth and SSD, increasing the field size raises the percent depth dose mainly because of:

  1. increased scatter reaching the point Correct answer.
  2. increased source-to-surface distance
  3. lower effective beam energy
  4. reduced inverse-square attenuation

Explanation

A larger field irradiates more material and sends more scattered photons to the calculation point, so the dose at depth rises relative to dmax. Inverse-square attenuation depends on distance, not field size, and the beam energy is unchanged.

Question 2Treatment planning and dosimetry

Tissue maximum ratio (TMR) is the ratio of absorbed dose at a given depth in a phantom to absorbed dose at:

  1. the depth of maximum dose in that field Correct answer.
  2. the surface of the phantom
  3. the same depth in a larger reference field
  4. the same point with the phantom removed

Explanation

TMR compares dose at depth d with dose at dmax for the same field size and is independent of SSD. It is a ratio measured entirely within the phantom along the same central axis.

Question 3Treatment planning and dosimetry

A point source gives an absorbed dose rate of 200 cGy/h at 100 cm. Ignoring attenuation and scatter, what is the dose rate at 150 cm?

  1. 100 cGy/h
  2. 133 cGy/h
  3. 88.9 cGy/h Correct answer.
  4. 300 cGy/h

Explanation

Dose rate follows the inverse-square law, so the new rate is 200 × (100/150)^2 = 88.9 cGy/h. The distance ratio is squared because intensity spreads over an area proportional to distance squared.

Question 4Treatment planning and dosimetry

A tissue-equivalent bolus is placed on the skin during a photon treatment primarily to:

  1. reduce scatter to the contralateral side
  2. shorten the treatment time
  3. widen the penumbra
  4. shift dmax toward the skin surface Correct answer.

Explanation

Bolus adds tissue-equivalent material at the surface so the build-up region and dmax move closer to the skin, raising surface dose. It does not deliberately change penumbra or treatment time.

Question 5Treatment planning and dosimetry

A wedge factor is the ratio of absorbed dose on the central axis with a wedge in the beam to absorbed dose at:

  1. the surface with the wedge
  2. the same point in a larger field with the wedge
  3. the same point without the wedge Correct answer.
  4. 10 cm depth without the wedge

Explanation

The wedge factor quantifies attenuation by the wedge and compares wedged and open beams at the same point, depth, field size, and geometry. Changing any of those conditions would confound the ratio.

Question 6Treatment planning and dosimetry

In CT-based radiotherapy planning, the CT number (Hounsfield unit) of a voxel is converted to which quantity for heterogeneous dose calculation?

  1. electron density Correct answer.
  2. atomic number only
  3. prescribed dose value
  4. beam energy only

Explanation

A CT calibration curve maps Hounsfield units to electron density or mass density so the planning system can model photon and electron interactions in different tissues. Beam energy and prescription dose are independent planning inputs.

Question 7Treatment planning and dosimetry

In patient-specific IMRT QA, the gamma index evaluates agreement between measured and calculated dose by combining:

  1. dose difference and DTA Correct answer.
  2. beam energy and field size
  3. CT number and electron density
  4. monitor units and dose rate

Explanation

Gamma uses both a dose-difference criterion and a distance-to-agreement criterion, which prevents steep dose gradients from causing false failures. A point passes when the combined metric is at or below the chosen threshold.

Question 8Treatment planning and dosimetry

A line drawn on a treatment plan through points that receive the same absorbed dose is called:

  1. an isodose curve Correct answer.
  2. a beam profile
  3. a dose-volume histogram
  4. a depth-dose curve

Explanation

An isodose curve connects points of equal dose and displays the spatial dose distribution on a plan. A depth-dose curve plots dose along one axis versus depth, a DVH summarizes volume versus dose, and a beam profile shows dose across a field at one depth.

Question 9Treatment planning and dosimetry

For a 12 MeV electron beam, R90 is the depth at which the absorbed dose falls to:

  1. 50% of maximum dose
  2. 90% of maximum dose Correct answer.
  3. the surface dose
  4. the practical range

Explanation

R90 is the therapeutic range, defined at 90% of the maximum dose on the distal falloff, and it approximates the depth of clinically useful electron coverage. R50 and the practical range describe lower-dose points on the same curve.

Question 10Treatment planning and dosimetry

A spread-out Bragg peak (SOBP) used in proton therapy is produced by:

  1. a single monoenergetic proton beam
  2. stacking beams of different ranges Correct answer.
  3. increasing the cyclotron dose rate
  4. a photon wedge placed in the nozzle

Explanation

Combining proton beams of different energies and ranges shifts and weights individual Bragg peaks so their sum covers the target thickness with a flat plateau. Dose rate changes output but does not create range modulation.

Question 11Treatment planning and dosimetry

For a 6 MV photon beam, replacing 5 cm of water with 5 cm of low-density lung along the beam path generally causes the dose beyond the lung to:

  1. decrease because lung absorbs more photons
  2. remain exactly the same as in water
  3. increase because lung attenuates less Correct answer.
  4. shift the maximum dose to the skin surface

Explanation

Low-density lung has fewer interactions per unit path length, so photons are attenuated less and more dose reaches distal tissues than a water-equivalent calculation would predict. This density effect is why heterogeneity correction is needed.

Question 12Treatment planning and dosimetry

In inverse IMRT planning, the optimization algorithm adjusts beamlet intensities to:

  1. minimize a dose-based objective function Correct answer.
  2. eliminate the need for dose calculation
  3. keep every segment identical
  4. maximize the number of treatment beams

Explanation

Inverse planning begins with desired dose-volume objectives and iteratively varies beamlet weights to minimize the resulting objective function. A dose calculation is still performed during optimization to evaluate each iteration.

Question 13Treatment planning and dosimetry

Compared with fixed-field step-and-shoot IMRT, VMAT delivery typically:

  1. does not require inverse planning
  2. uses only static gantry positions
  3. cannot be delivered on a standard linear accelerator
  4. uses continuous gantry rotation with dynamic MLC Correct answer.

Explanation

VMAT modulates dose while the gantry rotates continuously, using variable dose rate, gantry speed, and dynamic MLC positions. It is an inverse-planned technique delivered on modern linear accelerators.

Question 14Treatment planning and dosimetry

On a cumulative DVH, D95 for a target is the dose received by:

  1. 5% of the target volume
  2. 95% of the prescribed dose
  3. the hottest 95% of voxels
  4. 95% of the target volume Correct answer.

Explanation

D95 is the dose covering 95% of the target volume and is read from the cumulative DVH at the 0.95 volume fraction. It is a dose value tied to a volume percentage, not a percentage of the prescription.

Question 15Treatment planning and dosimetry

The CTV-to-PTV margin in radiotherapy planning is intended mainly to account for:

  1. the beam penumbra alone
  2. setup error and organ motion Correct answer.
  3. the finite dose-calculation grid
  4. microscopic tumor spread beyond the visible GTV

Explanation

The PTV adds geometric uncertainty from patient setup, immobilization, and internal organ motion to the CTV. Microscopic spread is addressed by the CTV rather than by the PTV.

Question 16Treatment planning and dosimetry

The penumbra of a megavoltage photon beam is produced primarily by:

  1. the inverse-square law alone
  2. finite source size and scatter Correct answer.
  3. the CT calibration curve
  4. the wedge factor

Explanation

Penumbra arises from the finite focal spot, collimator transmission and scatter, and lateral scatter of secondary electrons in the patient. These effects blur the geometric field edge and create a dose gradient.

Question 17Treatment planning and dosimetry

At a fixed SSD, the total scatter output factor for a photon beam generally rises with field size because:

  1. more scattered photons reach the central axis Correct answer.
  2. larger fields always receive more monitor units
  3. the source-to-axis distance decreases
  4. the beam energy increases

Explanation

Larger fields irradiate more material, increasing collimator and phantom scatter that reaches the central axis, so output per MU rises. Monitor units are a delivery setting, and source distance and energy are held fixed.

Question 18Treatment planning and dosimetry

For a fixed field size at the surface, increasing SSD from 80 cm to 100 cm generally causes the percent depth dose at a given depth to:

  1. increase Correct answer.
  2. decrease
  3. remain unchanged
  4. fall to zero

Explanation

At a larger SSD, the inverse-square falloff between dmax and the point of interest is less steep relative to dmax, so PDD increases. The field size in air must be held fixed for this comparison.

Question 19Treatment planning and dosimetry

A tissue compensator in photon therapy is used mainly to:

  1. change the beam quality
  2. increase surface dose
  3. even out dose across irregular tissue Correct answer.
  4. reduce the need for patient imaging

Explanation

Compensators add or remove attenuation across the field so the dose distribution becomes more uniform when tissue thickness or surface contour varies. They do not change beam quality or replace imaging.

Question 20Treatment planning and dosimetry

For an electron beam, a custom cutout placed near the patient surface is used to:

  1. increase the bremsstrahlung tail substantially
  2. shape the field and spare nearby tissue Correct answer.
  3. reduce only the dose rate
  4. shift the Bragg peak

Explanation

Electron cutouts collimate the beam close to the skin, shaping the field and reducing dose outside the target. They do not shift a Bragg peak, which is a proton feature, or intentionally alter beam quality.

Question 21Treatment planning and dosimetry

In proton therapy planning, a 3.5% range uncertainty is most appropriately managed by:

  1. placing a photon wedge in the beam
  2. increasing dose rate to shorten treatment
  3. reducing the plan to a single field
  4. a distal margin or robust optimization Correct answer.

Explanation

Range uncertainty from CT conversion, stopping-power ratios, and setup is handled by distal margins or robust optimization that ensures target coverage under uncertainty scenarios. Dose rate, wedges, and field count do not directly address proton range error.

Question 22Treatment planning and dosimetry

Halving the dose-calculation grid spacing in a heterogeneous region generally:

  1. removes the need for heterogeneity correction
  2. increases computation time and may alter local dose Correct answer.
  3. reduces computation time and substantially improves accuracy
  4. has no effect on either local dose or computation time

Explanation

Finer grids sample steep gradients and tissue interfaces better but require more voxels and longer calculation; local dose can change near heterogeneities. They do not remove the need for heterogeneity modeling.

Question 23Treatment planning and dosimetry

The Paddick conformity index differs from the RTOG conformity index mainly because it includes:

  1. the beam energy and number of treatment fields
  2. coverage and dose outside the target Correct answer.
  3. a target dose-homogeneity term only
  4. the total monitor units and treatment time

Explanation

The Paddick index combines target coverage with a term penalizing prescription dose outside the target, capturing both undercoverage and normal-tissue irradiation. RTOG conformity focuses on the ratio of prescription isodose volume to target volume.

Question 24Treatment planning and dosimetry

In TG-51 reference dosimetry for a photon beam, the beam quality specifier %dd(10)x is used to:

  1. determine the source-to-surface distance
  2. calculate the wedge factor
  3. set the MLC leaf positions
  4. select the kQ factor Correct answer.

Explanation

TG-51 uses %dd(10)x to characterize photon beam quality and select kQ, which converts the calibrated ion-chamber response to absorbed dose in the user beam. SSD, MLC position, and wedge factor are separate measurements.

Question 25Treatment planning and dosimetry

In radiobiological plan evaluation, tumor control probability (TCP) generally increases with:

  1. larger dose-calculation grid spacing
  2. poorer dose homogeneity in the target
  3. higher normal-tissue dose
  4. higher target dose and better coverage Correct answer.

Explanation

TCP rises as the dose and coverage of clonogenic tumor cells increase, assuming normal-tissue limits are respected. Normal-tissue dose drives NTCP, while homogeneity and grid spacing are not direct TCP gains.

Question 26Treatment planning and dosimetry

On a cumulative dose-volume histogram for a lung structure, V20 = 30% means that:

  1. 20% of the structure volume receives at least 30 Gy.
  2. 30% of the structure volume receives exactly 20 Gy.
  3. The structure's maximum dose is 20 Gy in 30% of patients.
  4. 30% of the structure volume receives 20 Gy or more. Correct answer.

Explanation

A cumulative DVH gives, for each dose, the volume receiving that dose or more; therefore V20 is the percentage or absolute volume at or above 20 Gy.

Question 27Treatment planning and dosimetry

A tissue-equivalent bolus placed on the skin in a megavoltage photon plan is used mainly to:

  1. compensate for missing tissue at the opposite skin surface.
  2. reduce scatter dose beyond the treatment volume.
  3. raise the surface dose by shifting the build-up region. Correct answer.
  4. reduce the surface dose by increasing skin sparing.

Explanation

Bolus has attenuation and build-up properties similar to tissue, so it shifts the maximum dose toward the surface and raises the skin or surface dose.

Question 28Treatment planning and dosimetry

The isocenter of a linear accelerator is best described as the point:

  1. at the geometric center of the planning target volume.
  2. where the beam output is calibrated under reference conditions.
  3. where the three rotational axes intersect. Correct answer.
  4. on the skin where the central axis enters the patient.

Explanation

The isocenter is the spatial point where the gantry rotation axis, collimator axis, and couch rotation axis intersect, and it is used as the reference for isocentric planning.

Question 29Treatment planning and dosimetry

For an ICRU-style reference dose specification in a simple opposed-field plan, the reference point is typically placed:

  1. at the global maximum dose point anywhere in the patient.
  2. at the skin entrance point of the first treatment beam.
  3. at the most lateral edge of the planning target volume.
  4. on the central axis within the central PTV region. Correct answer.

Explanation

For simple plans the ICRU reference point is chosen on the central axis in the central part of the PTV, where the dose is representative and can be calculated reliably.

Question 30Treatment planning and dosimetry

For a physical wedge, the nominal wedge angle is defined as the angle between:

  1. the wedge filter face and the patient's skin surface.
  2. the beam central axis and the collimator rotation axis.
  3. the 80% and 20% isodose lines at the skin surface.
  4. the 10-cm isodose line and a normal to the central axis. Correct answer.

Explanation

The nominal wedge angle is specified at 10 cm depth as the angle between the tilted isodose curve and a plane perpendicular to the central axis; it describes the tilt of the isodose distribution.

Question 31Treatment planning and dosimetry

A photon field is calculated at 100 cm SSD. If the same field is delivered at 90 cm SSD, the inverse-square factor relative to the 100-cm calculation is approximately:

  1. 0.81
  2. 1.11
  3. 1.52
  4. 1.23 Correct answer.

Explanation

Photon fluence changes with the inverse square of distance, so the factor is (100/90)^2 = 1.23; the dose at a point closer to the source is therefore about 23% higher before other corrections.

Question 32Treatment planning and dosimetry

In forward-planned field-in-field breast radiotherapy, additional subfields are added mainly to:

  1. shorten treatment time by reducing monitor units.
  2. reduce hot spots and improve dose homogeneity. Correct answer.
  3. increase the dose to the axillary lymph nodes.
  4. create a deliberate dose gradient across the target.

Explanation

Field-in-field planning adds shaped subfields that preferentially reduce dose in hot regions, smoothing the composite dose distribution without changing the intended target coverage.

Question 33Treatment planning and dosimetry

Compared with a simple pencil-beam algorithm, a collapsed-cone convolution algorithm in a lung tumor plan generally:

  1. ignores differences in tissue density outside the target.
  2. eliminates the need for CT-based electron density information.
  3. forces the dose grid to match the CT slice thickness.
  4. models lateral scatter more accurately in low-density tissue. Correct answer.

Explanation

Collapsed-cone convolution transports dose through a finite number of solid-angle cones and accounts for lateral scatter and density inhomogeneity better than simple pencil-beam methods, especially in lung.

Question 34Treatment planning and dosimetry

In static MLC conformal planning, the leaf positions for each beam are typically derived from:

  1. the isocenter position projected onto the planning CT.
  2. the beam's-eye-view projection of the skin surface alone.
  3. the beam's-eye-view PTV projection plus a margin. Correct answer.
  4. the DVH of the organ at risk for that beam.

Explanation

For each gantry angle, the MLC leaves are set to the beam's-eye-view outline of the PTV, usually expanded by a margin to account for penumbra and geometric uncertainties.

Question 35Treatment planning and dosimetry

The internal target volume (ITV) in respiratory-gated or motion-inclusive planning represents:

  1. the GTV plus a fixed 5 mm margin in all directions.
  2. the CTV plus setup error and machine tolerances.
  3. the CTV envelope over the breathing cycle. Correct answer.
  4. the PTV minus the planning organ-at-risk margin.

Explanation

The ITV is the union of CTV positions observed over the respiratory cycle, so it accounts for target motion rather than setup error or machine tolerances.

Question 36Treatment planning and dosimetry

A superficial target extends from the skin surface to 1.5 cm depth. Which electron beam energy is most appropriate to cover the target while limiting dose to deeper tissue?

  1. 12 MeV
  2. 6 MeV Correct answer.
  3. 16 MeV
  4. 20 MeV

Explanation

A 6 MeV electron beam has a practical range of about 3 cm and therapeutic depth sufficient for a 1.5-cm target while sparing deeper tissue; higher energies would penetrate unnecessarily.

Question 37Treatment planning and dosimetry

In plan evaluation, which statement best reflects the radiobiological behavior of a serial organ such as the spinal cord?

  1. Complication risk is negligible whenever the organ's V5 is below 50%.
  2. It is driven mainly by maximum dose or small high-dose volumes. Correct answer.
  3. Complication risk depends mainly on the mean dose to the whole organ.
  4. Complication risk is proportional to the total volume irradiated above 2 Gy.

Explanation

Serial organs have a threshold-like behavior: exceeding tolerance in even a small high-dose region can cause a severe complication, so maximum dose and small high-dose volumes dominate the risk.

Question 38Treatment planning and dosimetry

In TG-43 brachytherapy dosimetry, the dose rate around a sealed source is characterized using the air kerma strength, geometry factor, radial dose function, and:

  1. wedge transmission factor.
  2. tissue maximum ratio.
  3. percentage depth dose.
  4. anisotropy function. Correct answer.

Explanation

The TG-43 formalism expresses dose rate as air kerma strength times a geometry factor, radial dose function, anisotropy function, and conversion factor; the anisotropy function accounts for angular attenuation and scatter in the source and encapsulation.

Question 39Treatment planning and dosimetry

When a photon field is treated at an extended SSD, the percent depth dose at depth increases. Which factor is commonly used to convert the PDD from the standard SSD to the extended SSD?

  1. Scp field factor
  2. Mayneord F factor Correct answer.
  3. Batchelor wedge factor
  4. TMR tissue factor

Explanation

The Mayneord F factor scales PDD measured at a standard SSD to an extended SSD using the ratio of distances and the inverse-square law; it is an approximation that ignores changes in scatter.

Question 40Treatment planning and dosimetry

For a 1 × 1 cm photon field used in stereotactic radiosurgery, output factor measurement is difficult mainly because:

  1. the beam energy increases sharply at small field sizes.
  2. scatter from the collimator becomes negligible at small fields.
  3. loss of lateral electronic equilibrium and detector size effects occur. Correct answer.
  4. the inverse-square law no longer applies at small distances.

Explanation

In small fields, the lateral range of secondary electrons may exceed the field width, so charged-particle equilibrium is lost and detectors with finite volume or high-Z components can over- or under-respond.

Question 41Treatment delivery, machine calibration, and quality assurance

Under TG-51, the reference calibration of a 6 MV photon beam is performed at which depth in water?

  1. 5 cm depth in water
  2. 20 cm depth in water
  3. 10 cm depth in water Correct answer.
  4. The depth of maximum dose

Explanation

TG-51 defines the reference condition for photon calibration as 10 cm depth in water at 100 cm SSD with a 10 x 10 cm2 field. At this depth the dose gradient is modest, so small chamber-positioning errors have little effect on the result. The same depth is used to define the beam-quality specifier, which keeps calibration and quality specification consistent.

Question 42Treatment delivery, machine calibration, and quality assurance

In TG-51, the beam-quality specifier for a photon beam is best described as which of the following?

  1. %dd(10)x, the photon-component percent depth dose at 10 cm Correct answer.
  2. The nominal accelerating potential in MV
  3. TPR20,10 measured in water
  4. The half-value layer measured in aluminum

Explanation

TG-51 replaced TPR20,10 with %dd(10)x, the percent depth dose at 10 cm due to photons alone, derived from a measurement made with a lead foil in place to remove electron contamination. This quantity is directly related to the stopping-power ratio used for the calibration. It is a measured index, not the machine's nominal energy label.

Question 43Treatment delivery, machine calibration, and quality assurance

An ion chamber was calibrated by an accredited laboratory at 22 C and 101.33 kPa. On a treatment day the pressure is 98.0 kPa and the water temperature is 21 C. The temperature-pressure correction factor P_TP is closest to which value?

  1. 1.030 Correct answer.
  2. 0.970
  3. 1.065
  4. 0.997

Explanation

P_TP equals (273.2 + T)/(273.2 + 22) multiplied by 101.33/P, with T in degrees Celsius. Substituting gives (294.2/295.2) x (101.33/98.0) = 0.9966 x 1.0340, which is about 1.030. The reading must be multiplied by this factor because the chamber holds less air mass at lower pressure and therefore produces a smaller signal per unit dose.

Question 44Treatment delivery, machine calibration, and quality assurance

TG-142 recommends that the daily output constancy check for a photon beam agree with the baseline within which tolerance?

  1. Plus or minus 3 percent Correct answer.
  2. Plus or minus 10 percent
  3. Plus or minus 5 percent
  4. Plus or minus 1 percent

Explanation

TG-142 sets the daily photon and electron output constancy tolerance at 3 percent. This level is tight enough to detect meaningful drift while remaining achievable given the reproducibility of routine constancy devices. Larger deviations require repeat measurement and, if confirmed, investigation before further treatment.

Question 45Treatment delivery, machine calibration, and quality assurance

For a multileaf collimator used for step-and-shoot IMRT, TG-142 recommends a routine (monthly) leaf position accuracy tolerance of approximately which value?

  1. 5 mm
  2. 3 mm
  3. 2 mm
  4. 1 mm Correct answer.

Explanation

TG-142 specifies a routine leaf position accuracy of 1 mm for MLCs used in IMRT delivery, with the same or tighter values applied at annual testing. Leaf position errors translate directly into field-edge and segment-weight errors, which is why the tolerance is tighter than for jaw-defined fields. Testing is typically performed with a picket-fence or similar pattern imaged on film or an electronic portal imager.

Question 46Treatment delivery, machine calibration, and quality assurance

The TG-142 tolerance for gantry angle accuracy relative to the digital readout is which of the following?

  1. Plus or minus 1 degree Correct answer.
  2. Plus or minus 5 degrees
  3. Plus or minus 2 degrees
  4. Plus or minus 0.1 degree

Explanation

TG-142 sets gantry, collimator, and couch angle accuracy at plus or minus 1 degree for routine testing. A one-degree error changes a beam's entry direction modestly but is well within the margin added for setup and motion. Tighter values are generally reserved for special procedures rather than routine gantry readout checks.

Question 47Treatment delivery, machine calibration, and quality assurance

For a non-stereotactic linac, the TG-142 tolerance for coincidence of the light field and the radiation field is which of the following?

  1. 1 mm on a side
  2. 5 mm on a side
  3. 0.5 mm on a side
  4. 2 mm on a side Correct answer.

Explanation

TG-142 allows 2 mm per side for light field versus radiation field coincidence on conventional machines, tightened to 1 mm for stereotactic treatments. The test is usually performed with a radio-opaque marker at each field edge on an image. The looser conventional tolerance reflects the field sizes and margins used in routine fractionated treatment.

Question 48Treatment delivery, machine calibration, and quality assurance

What is the function of the secondary (backup) monitor chamber or timer on a medical linear accelerator?

  1. To display the instantaneous dose rate on the console
  2. To drive the multileaf collimator during dynamic delivery
  3. To terminate the beam if the primary monitor delivers more than a preset amount Correct answer.
  4. To verify gantry angle during arc therapy

Explanation

The backup monitor is an independent dosimetric channel that limits the total delivered monitor units if the primary channel fails or over-runs. It is intentionally set to terminate slightly above the intended MU so that it stays silent during normal operation. This redundancy protects the patient from a single-point failure in the primary dosimetry chain.

Question 49Treatment delivery, machine calibration, and quality assurance

Which photon dose quantity is by definition independent of source-to-surface distance?

  1. Depth of maximum dose in the patient
  2. Tissue-maximum ratio (TMR) Correct answer.
  3. Percent depth dose (PDD)
  4. Percent depth ionization (PDI)

Explanation

TMR is defined as the ratio of dose at depth to dose at the same point at dmax in the same phantom with the source fixed, so no inverse-square term is included. PDD, by contrast, falls with increasing SSD because it incorporates inverse-square attenuation and scatter changes. This property makes TMR the natural quantity for isocentric calculations.

Question 50Treatment delivery, machine calibration, and quality assurance

For a 6 MV beam, collimator scatter factor Sc(20x20)/Sc(10x10) is 1.05 and phantom scatter factor Sp(20x20)/Sp(10x10) is 1.02. The total scatter factor ratio for the 20 x 20 cm2 field relative to 10 x 10 cm2 is which of the following?

  1. 1.020
  2. 0.971
  3. 1.071 Correct answer.
  4. 1.050

Explanation

The total scatter factor is the product of collimator scatter and phantom scatter factors, so 1.05 x 1.02 = 1.071. Sc describes head scatter reaching the point of interest and Sp describes scatter generated within the phantom. Multiplying them gives the relative output change that must be applied to the 10 x 10 calibration when treating the larger field.