Procedures: Prescription and Dose Calculation (10.5%)

ARRT-T · 31 questions

A pelvic target is planned with opposed anterior and posterior fields to a prescription point at midplane, with both fields the same size and the same machine settings. The plan gives 26 Gy from the anterior beam and 24 Gy from the posterior beam at that point over the full course, and the two must still sum to that course total. How should the two fields be weighted so each one contributes the same dose at midplane?
For an isocentric treatment delivered at a 100 cm source-to-axis distance, the target lies 12 cm below the skin on the central axis, and the beam shaping gives a 15 × 15 cm field at the isocenter of a 6 MV beam. Which quantity should supply the depth dose factor used in this calculation?
On a 6 MV unit, the skin surface sits 15 cm farther from the source than the nominal setup distance, the target lies 4 cm off the central axis at a depth of 12 cm, and the dose is prescribed to that point. Which set of corrections must be applied to this field's monitor units?
A post-mastectomy chest wall plan uses a 6 MV beam with a physical wedge, a cerrobend block mounted on the tray, and daily bolus over the scar. The open-field output factor was measured in water for the same field size, and the tray/block and bolus effects are accounted for separately. Which correction accounts for adding the wedge when determining the wedged-field central-axis output?
While reviewing a simulation for a lung primary you see a GTV on the visible tumor, a CTV adding microscopic spread, an ITV growing and shrinking with breathing, and a PTV adding setup margin. Which prescription volume must receive the prescribed dose at every fraction?
Before the first fraction, a colleague asks you to verify the monitor units for a new 6 MV plan. The chart lists the prescribed dose at the target, the target depth, the field size and the distance from the source for the setup. Which recalculation confirms that the monitor units match the prescription?
A 6 MV beam treats a mediastinal target through a physical wedge, a tray-mounted block, and chest wall bolus, with the wedge and tray output factors already applied. An ion chamber beyond a low-density lung region, where primary-beam transmission dominates, reads higher than the homogeneous water calculation predicts. Which dose calculation factor explains the higher reading?
The linac is calibrated for a 100 cm source-to-skin distance, but a thick immobilization vest places this patient's skin at 118 cm for a target that sits on the central axis at 6 cm depth. Monitor units were copied unchanged from a nominal-distance plan. What should the physicist do before treatment?
A patient is treated isocentrically at SAD 100 cm with a whole-brain field matched to a posterior spine field. Both fields have a half-beam blocked edge at the junction, and the physicist marked the projected junction line on the skin. The therapist checks the light field before the first fraction. Which check confirms that the projected field edges align at the skin mark?
During a chart review you find a four-field pelvic plan prescribed to 60 Gy at the isocenter, with beam weights listed as AP 0.4, PA 0.4, right lateral 0.1 and left lateral 0.1. The physicist notes that every weight is referenced to the isocenter dose. Which statement correctly describes what these weights control in the plan?
On the planning CT, the team contoured the visible mass, expanded it 8 mm for possible subclinical disease, added the full breathing motion envelope, and then added setup uncertainty. The prescription delivers 50 Gy in 25 fractions to this final contour. Which target volume does the final contour represent?
A 6 MV plan delivers the prescription to a midline target at 10 cm depth with a 10 x 10 cm open field and 180 monitor units. The physician then widens the treatment field to 20 x 20 cm at the same energy and depth, and the record-and-verify entry still shows 180 monitor units. Which beam parameter must be recalculated before the plan is approved for treatment?
During the final check of an isocentric 6 MV plan, the isocenter sits at the standard 100 cm source-to-axis distance and 6.0 cm deep in the target, the prescription is 200 cGy at that point, the beam is open with no wedge, and the MLC shapes the aperture. The machine output for this aperture is 1 cGy per monitor unit at the calibration depth of maximum dose. Which calculation best verifies the monitor units for this beam?
A pelvis case is planned on a 6 MV linear accelerator with the target sitting on the rotation axis at the machine's stated distance, and the gantry travels an arc around that fixed point. The monitor units were written for the axis depth rather than for the skin entry point. Which depth-dose quantity should verify these monitor units for this isocentric setup?
A long treatment volume must be covered by two adjacent photon fields matched over the thoracic spine. Simulation shows beam divergence shifts the two edges past each other at the target depth, so the physicist recommends half-beam blocking at the junction line. What function does the half-beam block serve for the abutting fields?
A patient is treated on a setup where the entry surface slopes, so tissue thickness varies across the treatment area. The physicist wants the dose at the prescription depth to be uniform across the field while the beam arrangement and field size stay unchanged. Which beam modifier best accomplishes this?
During plan review at a regional cancer center, a therapist reads the written prescription for a lung primary: a radical course of 6 MV photons given in daily fractions to a named target volume, while the plan summary lists the dose to a different volume. Which volume must be verified to confirm coverage of microscopic disease plus setup and breathing margins?
Before the first fraction of a chest wall plan you check the printout and find four modifiers: a physical wedge, an accessory tray, a bolus sheet over the scar, and lung in the beam path. Each one should have been handled in the prescribed dose calculation. Which statement correctly describes how these four modifiers affect the dose at the treatment site?
During peer review of a lung plan, the physicist notes that the evaluation structure was built from the microscopic disease volume and then expanded for setup uncertainty and respiratory motion. The physician asks which prescription volume the plan should be judged against. Which target volume is being described?
Two adjacent fields abut along a junction marked on the upper chest of a patient who is set up for treatment. The plan uses half-beam blocking so each field edge sits at its own central axis, and the therapist must confirm the match before treating. Which measurement verifies that the two field edges abut without overlap or gap?
A patient with a large anterior-posterior separation needs a photon field set at SSD 118 cm rather than the usual 100 cm, keeping the same field size at the skin. For the geometric inverse-square correction to the primary fluence at 7 cm depth, which factor gives the dose per monitor unit at the extended SSD relative to the usual SSD?
A 6 MV anterior field treats a pelvic target at 100 cm SSD with a wedge in place. The prescription is 200 cGy at a depth where the wedged beam's percent depth dose is 80.0%, the 15×15 cm open field has an output factor of 1.050, and the measured wedge factor is 0.700. The unit delivers 1.00 cGy per MU at reference conditions. Which monitor unit value delivers the prescription dose at that depth?
A 6 MV beam is planned for an isocentric pelvic treatment at 100 cm SAD, with the target centered 9 cm deep at the isocenter, and the skin surface sits at a different distance for each patient. The physicist needs a depth ratio that holds no matter where that surface sits. Which dose ratio should express the depth effect for this isocentric geometry?
A palliative plan for a sternum lesion is prescribed as 2 Gy per fraction at the midplane, and the plan record explicitly defines the anterior field weight of 1.5 and posterior field weight of 0.5 as normalized dose contributions to that prescription. During the check a therapist asks how those weighting numbers translate into the daily dose. Which dose split between the two fields does that notation describe?
A course is prescribed as 54 Gy in 30 fractions with a midline normalization and a 3:1 weighting favoring the right lateral field. The two lateral fields enter at different depths and use different MLC aperture sizes, so their beam settings are not comparable. Which statement correctly interprets the 3:1 field weighting in this prescription?
A 6 MV isocentric pelvis plan puts the isocenter at 10 cm depth, and the physicist wants one beam-data quantity that stays valid as patient thickness changes the SSD from fraction to fraction, with all beam data collected while the detector stays in the water phantom. Which depth-dose quantity should you use to verify dose at the isocenter?
A patient who cannot be raised to isocenter is treated at an extended SSD of 115 cm for a parallel-opposed abdominal field; the chart lists an 18 x 20 cm field and a midplane depth of 14 cm. At what distance from the source should the light field be measured to verify the stated size?
A therapist verifies the light field during setup for an isocenter-based treatment with a 100 cm source-to-isocenter distance. The chart lists a nominal 10 x 10 cm field at the isocenter, the central-axis target depth is 10 cm, and a ruler on the patient's skin shows about 9 cm between the light-field edges. Which statement correctly explains the skin measurement?
A head and neck patient is treated on a 6 MV linac, and today's setup places the skin at an extended 115 cm SSD because the shoulder immobilizer raises the treatment level. The target depth, the field size at the skin, and the beam energy all match the plan calculated at the nominal 100 cm SSD, no wedge or tray is in the beam, and the target sits on the beam axis. Which correction must be applied to the prescribed monitor units for this extended-distance setup?
A lung prescription reads 60 Gy in 30 fractions with 6 MV photons, and the plan defines GTV, CTV, ITV, and PTV structures. The physician wants the structure that already accounts for breathing motion but excludes the setup margin. Which target volume matches that description?
A plan for a lung target was calculated on a water-equivalent phantom. On review you note that the beam crosses a low-density lung segment, uses a physical wedge with its tray mounted, and passes through bolus over the entry skin. Which statement correctly describes how each element changes the dose relative to that calculation?