Treatment Planning

does VMAT require fewer or more MU than IMRT?

fewer because the field uses more directions, needs less intensity modulation than IMRT, results in less MUs

when is VMAT not ideal?

• large volumes and off-center targets due to clearance issues

what is the difference between conformal arc therapy and VMAT?

• conformal arcs use arcs without modulated intensity

• line a 3D conformal plan, but with arcs instead of individual fields

• used in SRS

• has less scatter than VMAT

How does modulation impact MUs?

more modulation = greater MUS

What are TBIs used for?

bone marrow transplant prep in leukemia, lymphoma, and multiple myeloma

immunosupresses ti prevent rejection of donor bone marrow transplant

what is the purpose of compensators in TBIs?

• to achieve dose uniformity within 10% bc body thickness varies (ex: pelvis is thicker than neck)

what is the purpose of beam spoilers in TBIs?

• bring the surface dose to at least 90% of the prescribed TBI dose

• put as close as possible to pt to increase skin dose to treat blood vessels in the skin

what is the tissue lateral effect?

• lower energy or a thicker pt treated AP/PA can get higher dose to subcutaneous tissues compared to midpoint dose

what is the typically dose rate of a TBI?

5-10 cGy/min

what dose uniformity is acceptable for TBI QA?

± 10%

what is total skin electron irradiation used to treat?

• mycosis fungoides

why do we angle beams slightly in TSEB?

• to treat taller patients

• to reduce x-ray contamination dose to patient

• more uniform dose distribution

what areas will receive boosts in TSEB?

• difficult to reach spots, like perineum, palms, soles of feet

what do we shield in total skin treatments?

fingernails

what is SRS typically prescribed to?

50-80% IDL to minimize normal tissue exposure

What is the benefit of cones over MLCs in SRS treatment?

cones collimate the beam closer to the patient’s head, so they have a sharper geometric penumbra

Why do we stick to lower energies in lung planning?

• lungs are about 1/3 density of tissue

• higher energy beams need to go through enough tissue to build up, which they can’t do in lung bc electrons into air

• don’t want to underdose tumor, best to avoid higher energy

How to find new dose after normalization?

dose/normalization percentage = new dose

what are couch kicks used for in 3 fld breast setups?

• to match the sup field of the tangents to the inf field border of the sclav field

what is the difference between setting up for a mono-isocenter vs dual-isocenter?

• mono don’t require you to move form iso

• dual require couch kick away from gantry to form match line

How can the IMN be treated?

• photon field matched to tangent (like a mini tangent), which minimizes lung dose

• electron field

what is hockey stick portal (or dog leg) used for?

• seminoma treatment

• has kidney blocks

  

what was off-cord AP/PA lung used for?

• historical way of treating lung tumors

• treat certain number of fractions until spinal cord tolerance of 45 Gy is met

• resume rest of fractions using an off-cord obliques

What were 3D H&N used for?

• H&N treated w/ 3D lateral photon fields

• neck nodes were boosted using matching electron fields

what was the postage stamp field used to treat?

• early stage larynx cancer

• parotid glands treated w/ electron beam or photon/electron mix

what is a DICOM?

digital imaging and communications in medicine

allows imaging modalities and PACS to communicate in the same language

• used to transger:

• RT structure sets

• RT images

• RT plans

• RT dose

what is the dose limit to a pacemaker?

2 Gy

what energies should be avoided when a pt has a pacemaker?

• use energies lower than 10X

what is a systematic error?

• derivations between the planned patient position and the average patient position over the entire course of the treatment

• ex: offset between position lasers in sim vs treatment room

• error in target delineation

what is a random error?

• derivations between different fractions

• ex: anatomy changes

How many fiducials need to be imaged?

at least 3 so you know if you’re on 3 planes

Can fiducial markers shift?

yes

what is a wedge angle?

• the angle through which an isodose curve is tilted at the central ray of a beam at a specified depth (usually 10cm)

Why is a 30 degree wedge called that?

because it tilts the isodose curves 30 degrees from the central axis

Why are wedges and other beam modifying devices mounted at least 15-20cm from the skin surface?

• to avoid destroying the skin sparing effect of the MV beam

• distance allows secondary electrons to scatter before reaching the patient

How do wedges impact beam quality?

wedges harden the beam

block out low energy photons, leaving a higher quality and more penetrating beam

How do wedges impact MUs needed?

• need more MUs to achieve the same dose

• need more “juice” to get through the chunk of metal in your way

What is a wedge transmission factor?

ratio of dose with wedge t o dose w/o wedge on the CAX

Compared to physical wedges, dynamic wedges

• produce tilted curves by sweeping a jaw across the field

• decreases treatment time

• produces less scattered dose to peripheral areas

• have a field-size-dependent wedge transmission factor

• allow for various wedge angles by changing the speed of the moving jaw or dose rate during treatment

What is the formula for wedge angle?

Why do MLCs have a tongue and groove?

• to minimize leakage by allow their side to partially overlap

• brings down leakage to 3-4%

what provides the sharpest beam edge penumbra?

metal field block

• to find field percentages, add up all field weightings, divide individual field weights by total field weight, multiply by dose per fx

What is the formula for PDD?

What is the formula for TMR?

• same as PDD except need to add inverse square component

• PDD already incorporated inverse square

• TMR is for SAD

• PDD is for SSD

What does PDD depend on?

• energy

• depth

• field size

• SSD

what does TMR depend on?

• energy

• depth

• field size

• NOT SSD

What is the formula for Mayneord F Factor?

• used to change PDD from a depth you don’t need to a depth you do need

64.7 Gy

368

What does Mayneord F Factor correct for?

PDD

What is the equivalent square formula?

• 4A/P

• Rectangular field is equivalent to a square field if they have the same area/perimeter ratio

what are limitations of equivalent square outputs?

• equivalent square outputs become less reflective of the irregular field when the field is very long bc as you get father from CAX, dose drops

what is the off axis ratio?

How do treatment planning systems account for inhomogeneity corrections?

• they take CT info and determine electron densities