quality in management: part 4

what are some commonly used QC tools

solid water slabs, water phantoms, diodes, phantoms, v-film, ruler, ion chambers, electrometer, levels

what does in vivo mean

in body

what are three significant safety features we should notice

deadman switch, limit switch, emergency stop

what is a deadman switch

reduces chance of accidentally activating a motion

how many different actions are required at the same time on a deadman switch

two

what is the limit switch

controls the range of motion on a particular function; stops you from rotating beyond a safe stop point

what is the emergency stop

terminates power to linac

what will the machine have to go through if the emergency stop is activated

warm up activity

what will record the number of MUs delivered prior to termination of emergency stop

backup mechanism

what are diodes

ion chambers

what are diodes used for

verification of dose received by the patient

when are readings taken by the therapist with diodes

within the first 3 days of tx

what does TLD stand for

thermoluminescence detector

what does ODL stand for

optically stimulated luminescence

what is a TLD

process of stimulating using thermal energy

what is the process of a TLD using thermal energy

emission of luminescence from a substance following the absorption of energy from an external source

what is the most common substance in a TLD

lithium fluoride

what forms does TLDs come in

capsule, discs, rods

how long does the latent time bt exposure and processing need to be with TLDs

24hrs

how many uses can you get from TLDs

1

what is OSL

process of stimulating using optical energy

what is the most common substance of a OSL

crystalline aluminum oxide with carbon

how long does the latent time need to be bt exposure and processing for OSL

10 min

how many uses can you use the OSL

multiple

which detector is less expensive

TLD

which detector felt to be more accurate

OSL

what is the 3 step process for getting measurements of a TLD and OSL

radiate, latent period, measure visible light from material

what is the most accurate method for measuring and specifying clinical beam quality

using depth dose distribution measured in a water phantom

who does the water phantom test

physicist

what is required between light field and the actual radiation field

agreement

what should we rely on as an accurate indicator of radiation field

light fieldl

what are two terms we used to describe the goal of an agreement with light field and radiation field

congruence or coincidence

what does congruence mean

in agreement

what does coincidence mean

having identical features

what are the 3 steps of checking collimator axis, light beam axis, and cross hairs

1. tape graph paper to the table and gantry vertical, set a rectangular field

2. mark the edges of the light field, intersection of the diagonals, location of cross hairs

3. rotate the collimator to 180 degree and check the coincidence of light field edges and diagonal and cross hair position

what are the steps for checking light beam with xray beam congruence

tape ready pack film on table at 100SSD, collimator at 0, set 10×10 field and mark corners, edges of light field, and CA

what do you what to add to light beam with xray beam congruence test to provide maximum electronic buildup

lucite

what is the tolerance of light beam with xray beam congruence test

+-2mm

what are 3 different wants that sources of misalignment of light and radiation field can happen

mirror is out of alignment, xray tube shifted within the housing, target misaligned or electron beam may be steered

what is the mechanical isocenter

intersection point of the axis of rotation of collimator and axis of rotation of gantry

what does sag of gantry most often cause

longitudinal displacement

what must you regularly verify of mechanical isocenter

collimator and gantry rotation around isocenter

what is the tolerance of the isocenter motion with full collimator rotation for mechanical isocenter

+-2mm

when should the frequency test of mechanical isocenter collimator be done

monthly

what is the tolerance of the isocenter motion with full gantry rotation for mechanical isocenter

+-2mm

when should the frequency test of mechanical isocenter gantry be done

monthly

what is the tolerance of radiation isocenter: coll, table, gantry

lines intersection within a 2mm diameter circle

what is the frequency test of radiation isocenter: coll, table, gantry

annually

what is another name for the lutz test

split field test

what does the lutz test detect

3 general causes of beam misalignment

what are the 3 general causes of beam misalignment of a lutz test

focal spot displacement, asymmetry of collimator jaws, displacement in the collimator or gantry rotation axis

what is the tolerance of beam alignment check

2mm

what is the frequency test run for beam alignment check

monthly

when do you test for tabletop sag

annually

how is output in linacs expressed

dose per monitor unit

what affects output

field size

increase field size = _ output

increase

why does output increase with increasing field size

increased scatter

when would dose rate errors be possible

changing magnetron/klystron or electron gun current, changes reported by monitor ion chamber, and contamination of the vacuum of ion chamber

what is the tolerance of dose rate output

3%

when should the frequency of dose rate output be done

daily

how is energy verified depth dose distribution

an ion chamber in a water phantom

what is the tolerance for energy beam performance

2%

what is the frequency test for energy beam performance

monthly

what is the acceptance criteria is usually specified in terms of depth dose variance

10×10 fs at a 100cm SSD, 10cm depth

what is a stronger indicator of machine operation than beam penetrative quality

output, flatness, and symmetry tests

what is usually used for field flatness test

film dosimetry or ion chamber

what is the most accurate test for field flatness

ion chamber

how is field flatness defined

variation of dose relative to the central axis over the central 80% of field size at 10cm depth

what does flatness of the beam reflect

flattening filter

what is the tolerance for field flatness

3%

what is the frequency of the field flatness test

monthly

what is the purpose of the flattening filter

correct for forward peak at central portion of beam

what can the flattening filter be made up of

lead, tungsten, uranium, steel, or aluminum

where do horns exist in flattening filter

superficial depths

horns cannot exceed what percent at depth of dmax

105%

what does the ion chamber in gantry check for

field symmetry and flatness

what else can the ion chamber in gantry do

monitor dose rate

what does the ion chamber not do, that you need a water phantom for

check beam energy

how is field symmetry defined

difference in dose rate between any two symmetric points within the 80% of the field size

what percent should dose not differ more than at any pair of points situated symmetrically with respect to the central ray

3%

what frequency should field symmetry be checked

daily

what is the principle objective in performing field symmetry and flatness checks

verifying dose rate consistency across the field

can a field be flat but not symmetric

yes

what are more common and accurate to check for flatness and symmetry

ion chambers

what is most likely the cause of error with lasers

vibration or bumping

what must be identified first before lasers set to proven isocenter

mechanical isocenter

when should the lasers be checked

daily or whenever a bulb is replaced or an error is suspected

what should accurately indicate exit point of the beam at central axis

backpointer

what is the tolerance of the lasers

2mm

what can laser and field alignment tools be used to verify

laser alignment, light field, jaw readout accuracy, ODI accuracy, mechanical iso

what law states that if the ODI is not accurate and the pt will result in an over/underdose

inverse square law

what kind of indicators are more accurate than electronic devices

mechanical

what is the tolerance for the distance indicator

2mm

what is the frequency of the distance indicators tests

daily

what do collimator readout errors most commonly result from

a cover being put back on improperly, bending or moving readout mechanism

what can mechanical dials for gantry and collimator lead to

parallax error

what is a parallax error

person viewing the gantry angle readout sees the angle incorrectly

what is the tolerance of gantry/coll readout accuracy

+-1 degree on a side or 2mm

what is the frequency testing of gantry/coll readout accuracy

monthly