physics semester 2 (ALL EXAMS)

T/F the isocenter of a given treatment unit is not a fixed point and therefore can be modified as needed

false

T/F the isocenter is a point in space at a specified distance from the source of radiation around which the gantry rotates

true

T/F the isocenter is a point of intersection of the gantry, collimator and table axes of rotation

true

T/F the treatment room lasers are aligned to the isocenter

true

what are the factors influencing the dose delivered to a patient

beam energy, tissue density, calculation depth, field size, distance

determine the fraction dose for a prescription which calls for a total dose of 45 Gy delivered in 25 fractions

180 cGy

a conventional fractionation schedule delivers a fraction dose of

180-200 cGy

the division of the body into equal left and right segments and can also be referred to as midsagittal

midline

defined as the measure of energy transferred from radiation photons to charged particles (electrons) in matter

KERMA

the distance beneath the skin surface where the prescribed dose is to be delivered

depth

A patient has lost over 7 pounds during the course of treatment. a therapist is asked to acquire an AP/PA separation measurement while the patient is on the treatment table, but calipers are not available in the department. what is another effective approach to obtaining the patient's separation measurement

use the ODI to obtain SSD readings and use the readings to determine separation

the anterior field of an isocentric setup has an SSD reading of 89.5 cm, based on this information, what is the depth to isocenter for this field

10.5 cm

corresponds to the field defined by the light localizer

geometric field size

defined as the area included within the 50% isodose curve at the depth of maximum dose

physical field size

solve for the equivalent square field size of a rectangular field measuring 12cm x 22cm

15.5cm x 15.5cm

based on a standard configuration of field dimensions as shown in the image, which axis reflects field length

Y2-Y1

T/F the radiation therapy beam is comprised of primary radiation and scatter radiation

true

radiation that is scattered back toward the surface after entering tissue is termed

backscatter radiation

as beam energy increases, the depth of Dmax

increases

the dose at Dmax, also called given dose, is always ___ ____ the prescribed dose for single fields

greater than

the effect whereby megavoltage photon beams reach 100% dose at a depth greater than zero thus reducing dose to superficial tissue best defines

skin sparing

the amount of radiation exposure produced by a treatment machine at a specified reference field size and distance best defines

output

as field size decreases, output or dose rate

decreases

as distance increases, output or dose rate

decreases

as attenuating medium thickness increases, output or dose rate

decreases

the mathematical relationship between radiation intensity and beam divergence whereby the intensity us inversely proportional to the square of the distance from the source of radiation best defines

inverse square law

the relationship between field size and distance is described as

direct

a lung cancer patient treated on a 100cm SAD unit has a AP/PA separation measuring 25 cm. if the isocenter is to be placed at midplane, what is the depth of isocenter

12.5cm

the intensity of a beam is 5mR/h at a distance of 10 cm. what is the intensity of the beam at 5 cm

20mR/hr

if the dose rate of a coblat-60 unit is 80 cGy/minut at 100cm SSD, what is the dose rate at 80cm SSD

125cGy/minute

a patient is treated with an isocentric technique on a 100cm SAD unit. the patient's separation is 23cm. if the isocenter is placed at a depth of 11cm from the anterior surface, at what depth is the isocenter from the posterior surface

12cm

if the collimator setting is 10cm x 10cm for a 100cm isocentric setup in which of the isocenter is placed at a depth of 8cm, what is the field size on the patient's skin surface

9.2cm x 9.2cm

T/F beam energy is an influencing factor for output

false

Dmax: 10 mV

2.5cm

Dmax 6 MV

1.5cm

Dmax 18MV

3.5cm

Dmax 4MV

1cm

T/F the depth of Dmax is influence primarily by beam quality

true

T/F Dmax is a point in space around which the linear accelerator rotates

false

T/F Dmax is the depth at which electron equilibrium occurs for photon beams

true

T/F Dmax is a point where the percent depth dose is normalized to 50%

true

units of measurements for absorbed dose

J/kg, cGy, Gy, rad

calculate the exit calculation point for a patient with a separation of 20cm being treated with a 6MV beam

18.5

Cerrobend blocks must achieve less than 5% transmission and therefore typically consist of ____ half value layers

5

the rotational axis of the gantry, collimator and treatment table

isocenter

pertaining to or involving right angles or perpendiculars

orthogonal

involves the use of reference points at a more stable location for alignment purposes and shifts are made to arrive at the planned isocenter

pinning

concept in which the isocenter is located relative to three setup coordinates

triangulation

indicates that points or beams lie within the same two-dimensional geometric plane

coplanar

what may be employed to achieve a uniform dose distribution among adjacent photon fields

half beam block, geometrically calculated skin gap, beam splitter

if a field measures 15 cm in length at 95cm SSD what will it measure at 120 cm SSD

19cm

refers to changing the location of the match line or the skin gap on a scheduled basis so that possible points of over or under dose do not occur at one spot throughout the entire course of treatment

feathering

two fields are planned to abut at a depth of 9cm. calculate the skin separation required if the field lengths are 40cm and 32cm respectively. Both fields are treated at 100cm SSD

3.2cm

what is the magnification factor if the actual collimator readout is 12cm x 12cm and the field size on the image measures 16cm x 16cm

1.33

photon blocks must be a minimum of ____cm from the patient's skin surface to reduce scatter from reaching the patient and contributing a non-therapeutic dose

15-20

the open collimator equivalent square field size is used for the inverse square correction factor

false

the open collimator equivalent square field size is used for the collimator scatter factor

true

the open collimator equivalent square field size is used for the phantom scatter factor

false

the open collimator equivalent square field size is used for the patient attenuation factor

false

as beam energy decreases, the PDD will

decrease

as depth decreases, the PDD will

increase

as field size decreases, the tissue maximum ratio will

decrease

as beam energy increases, the tissue phantom ratio will

increase

back scatter factor (BSF) becomes negligible around ____ MeV

8

a calculated value that identifies how long the machine is "on" to deliver a specific dose under specified conditions beset defines

MU

the relationship between dose and PDD is described as being

direct

the relationship between dose and MU is described as being

direct

a wedge factor of .831 indicates that _% of the dose is being transmitted through the wedge

83.1

radiation that is scattered 180 degrees from its original path is termed

backscatter

when comparing two arrangements in which everything remains equal except the use of a wedge, which field will have the higher monitor unit setting

wedge field

for a non-isocentric treatment setup, _ is the patient attenuation factor used in MU calculations to demonstrate the central axis at a given depth

PDD

a lung patient has a chest separation measuring 24cm. if the isocenter is to be placed at midplane, what is the depth of isocenter

12cm

a special application of the inverse square law used in special cases of non-isocentric MU calculations, for the purpose of converting the PDD at a reference distance to the PDD at a non-reference distance is the

mayneord's factor

when comparing two arrangements in which all elements remain equal except field size, will a smaller or larger field size have a lower MU setting

larger

tissue air ratio is dependent on distance

false

tissue maximum ratio is dependent on distance

false

tissue phantom ratio is dependent on distance

false

PDD is dependent on distance

true

as distance increases, the PDD will

increase

as field size increases beyond the established reference field size for output, the beam output will

increase

the ratio of absorbed dose at a given depth in phantom (tissue) to the absorbed dose at the same point in free space (air) best defines

tissue air ratio

the ratio of absorbed dose at depth to absorbed dose at Dmax is termed

PDD

represents the ratio of dose at a given depth in phantom (tissue) to the dose in phantom (tissue) at a depth of Dmax

tissue maximum ratio

the angle between the slanted isodose line and a line perpendicular to the central axis of the beam best defines

wedge angle

treatment planning volume encompasses the palpable, visible, and/or demonstrable extent of malignant disease and areas of possible microscopic or subclinical disease

CTV

to reduce surface dose resulting from scatter, compensators and wedges must be a minimum of _Cm from the patient's surface

15-20

treatment planning volumes encompass the CTV plus a margin to account for physiologic movement (such as heartbeat, respiration, and peristalsis) and changes in the size, shape, or position of the CTV

ITV

treatment techniques refers to an intensity-modulated radiation therapy (IMRT) treatment characterized by the gantry rotating while the treatment beam is on, along with multiple MLC positions and changing dose rate?

intensity modulated arc therapy

The treatment planning volume that encompasses the internal target volume plus a margin for geometric uncertainties such as patient motion, beam penumbra, and treatment setup differences is the

PTV

the ratio of normal tissue tolerance dose to tumor lethal dose is referred to as the

therapeutic ratio

generally, the dose at any depth is on the central axis of the beam and gradually toward the beam edges

highest, decreases

a beam modification device that modifies the distribution of dose by progressively decreasing beam intensity across the irradiated field

wedge

a _ field arrangement may be used to treat small, unilateral tumors to depths up to 7cm

wedge pair

what is the wedge angle needed to achieve improved dose homogeneity for two beams which have a hinge angle of 90 degrees

45 degrees

wedge fields commonly demonstrate a hot spot under the ____ end of the wedge

toe

small positional errors may affect the treatment outcome of IMRT more so than when conventional or 3D-CRT is used

true

the difference between the max and min intensity of the central 80% of the beam profile or the evenness of dose across a beam profile is termed

flatness

beam compensators such as wedges have been used in radiation therapy to attenuate the beam in an effort to produce a more dose distribution within the target volume

homogeneous

the volume of tissue receiving a significant percentage of the specified target dose (in relation to normal tissue tolerance values) is termed the

irradiated volume

an extremely objective, premliminary treatment plan evaluation tool which plots target or normal structure volume as a function of dose is termed

dose volume histogram