Dose Distribution

tumor dose (TD)

•prescribed dose

gross tumor volume (GTV)

•palpable or visible extent of tumor

Clinical Target Volume (CTV)

•GTV plus margin

Planning Target Volume (PTV)

•includes CTV with IM + setup margin

Treated Volume (TV)

•volume of tissue that receives tumor dose

irradiated volume

•volume of tissue receiving a significant dose (50% or more of the prescribed dose)

goal of treatment planning

•treat TV

•minimize dose to healthy structures

•permanent record of dose calcs and distributions

treatment planning process

•visualization

•localization

•field selection and placement

•verification

•documentation

isodose distributions factors

•energy

•source size

•SSD

•attenuation factors

contours

•can be done at CAX or others

•methods: solder, plaster of paris, ultrasound, pin system

methods to correct irregular contours

•isoshift method

•effective SSD method

•ratio TAR

isoshift factors

•

treatment techniques

•fixed or stationary

fixed treatment techniques

•single field

•parallel opposed (hourglass shaped)

•three field

•four-field box/diamond

wedge filters

•non customized

•heel / toe

•distorts iso distributions

•universal (60), dynamic, inherent

•hinge angle: 180-2(W)

wedge angle

•angle the isodose substends with a line perpendicular to CAX

•measured at depth of 10

calculation techniques

•beam on time (timer setting): Co, orthovoltage, superficial units

•monitor units: linear accelerator (no timer error)

monitor unit

=dose/dose rate

single beams

•100 SSD

•for tx of superficial structures

•supraclav, spine

parallel opposed fields

•for deeper targets

•dose highest near surface

•↑pt thickness, ↓beam energy = ↓dose uniformity

•brain, mets, lung

shrinking field

•mass has decreased in size

•field comes off critical/sensitive structures

boost fields

•small volumes treated with higher dose

vertex

•enters from superior aspect of head

•typically in conjunction with 2 lateral fields

•noncoplanar beam in relation to laterals

transmission factor

•wedge factors

•presence of wedge requires more MUs to deliver dose

•dependent on beam energy, field size

hinge angle formula

=180-2(W)

wedge angle formula

=90-1/2(H)

multiple beams forward planning

•for tx of deep seated tumors

•maximize dose to tumor and spreads out dose to normal tissues and organs

•variables: FS, blocking, # of ports, beam direction, beam weighting, beam energy, modifiers

noncoplanar beams

•enface boosts (perineal)

•stereotactic (series of arcs, tight isodose lines, rapid fall off)

•dynamic (couch and gantry motion simultaneously)

moving fields

•rotation (360): isodose lines centered on machine isocenter

•arc (<360 degrees): max occurs toward machine/gantry head, past pointing to offset, skip arc therapy

surface dose

•MV=skin sparing

•skin dose from e- contamination, backscattered e-

•dose increases rapidly in buildup region

•dependent on beam energy, SSD, FS, angle of beam incidence, tray distance from skin

bolus

•brings dmax closer to skin

•made of tissue equivalent material

•pliable/flexible to conform to contour without air gap

beam spoiler

•solid platic tray close to skin surface

•bolus may be inconvenient or difficult to highly irregular contours

tissue compensator

•avoids bolus effect

•missing tissue compensators: for sloping and contour changes

•dose compensation: accounts for missing tissues and inhomogeneities

pin and shift method

•y jaw has values

•fields are >180 degrees apart

•isocenter in breast tissue

half beam block

•y jaw at 0

•fields 180 degrees apart

•isocenter in lung

patient alignment devices

•field lifght

•protractor: measures breast and gives gantry angle

•lasers: laterals, sagittal, backpointer

•front pointer

patient positioning aids

•bite blocks

•alpha cradles

•breast boards

•vacuum bags

•timo

•thermoplastics

•accuform

therapeutic ratio

•compares total dose and probability of effects

•95% desirable, 5% acceptable risk