TMI/TBI Flashcards by Clara Fallone

what is TBI used for?

conditioning regime:
-bone marrow ablation
-kill malignant cells (destroy patient’s leukemic bone marrow)
-immune system suppression to decrease risk of graft vs host disease (especially for transplants from matched unrelated donors i.e., from histocompatible donor; allogeneic transplant)
-deliver dose to sanctuary sites (brain, spinal cord, testes) where chemotherapy may be ineffective

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what is histocompatible

having same/similar alleles of a set of genes called human leukocyte antigens

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what is autologous

transplant from self

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diseases that require bone marrow or peripheral stem cell transplant

leukemia (blood cancers beginning in bone marrow)
lymphoma (blood cancer developing from lymphocytes, type of white blood cell)
multiple myeloma (cancer of plasma cells, a type of white blood cell)
aplastic anemia (autoimmunie disorder in which body fails to produce blood cells in sufficient number)

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treatment time for TBI

~ 1 hour

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treatment options for TBI

swept beam
translating couch or translating beam
extended SSD (stationary beam)

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describe swept beam

treatment head rotates
treat patient prone and supine

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describe translating couch or translating beam

 Treat patient prone and supine (treat head first then feet first to max time between lung irradiations; allow time for normal tissue sublethal damage repair of lung)
 Couch speed can be varied to account for variation in patient thickness

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describe extended SSD

 Standing: treat ant/post with horizontal beam
 Laying down: treat right/left with horizontal beam; patient lying prone/supine
 Laying down: treat ant/post with vertical beams (if beam wide enough); patient lying prone/supine
• To make wide beam on Co-60 machine, remove collimator
 Laying down: treat ant/post with horizontal beams; patient lying on their side
 Extended SSD = 3-5 m
 Use POP technique with collimator 45 degrees to cover patient
 May require knees bent to fit patient in field
 Laying down and treating right/left is more comfortable than standing and treating ant/post, but results in more variation in patient thickness  more dose non-uniformity.
 Can alternatively treat patient sitting in semifetal position with legs semicollapsed. Arms positioned to shadow lungs. Beams coming in laterally (“bilateral TBI”).
 Lying on side is more comfortable for patient (and makes possibility of patient vomiting easier to deal with) and results in less variation in patient thickness. However, reproducibility is an issue.

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How can uniformity be improved?

compensator or bolus

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something to keep in mind when designing compensators for TBI

-size at patient has to be magnified to size at iso
-flattened beams will be softer near field periphery-Effective linear attenuation coefficients may vary by ~10%
-• High density compensator material is good because is less bulky, but downside is that small machining errors would amount to larger dose errors

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what films are used to design lung compensators?

port films (CV Sim)

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reuired thickness of a compensator that gives same dose at point of interest as would a bolus of thickness equal to the tissue deficit

thickness ratio tau
Tau is approx. 0.7 for TBI across all beam energies and scenarios. I.e., required compensator thickness < bolus thickness to give same dose at some point of interest because compensator results in more scatter in air.

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thickness of compensator

tissue deficit * tau/density of compensator

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why are beam spoilers used?

increase surface dose to at least 90% of Rx
-increase surface dose without changing dose profile with depth (ie. PDD or TMR will have different surfacer dose than beam without spoilers, but with depth, the 2 PDDs or TMRs look identical)
-provide scatter
-1-2 cm of acrylic as close to patient as possible

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fractionations

12 Gy/6, BID (6 hours between treatments, high dose TBI
-Rx point is mid-separation at level of umbilicus
-dose homogeneity is +/- 10% of Rx, excluding extremities
-single fraction of 10 Gy - perhaps better at killing leukemic cells but more concerns about lung toxicity
-10-15 fractions of 10-15 cGy/fx, low dose TBI
-half body irradiation - 8Gy to upper or lower half of body in single session (for widely disseminated metastatic disease, pain control)
-40 Gy/20 for total nodal irradiation - immunosuppressive agent
-

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issue with thicker patient

-need higher beam energy to get uniform dose

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steps for TMI plan at NSHA

-POP for legs (extended SSD)- go as high as possible
-create base plan from legs to use in VMAT optimization
-optimize all 5 other isocenters simultaneously using base plan
-split total plan into separate plans so unit can use kV/kV match

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same user origin for upper body and legs?

Yes, unless patient is so tall that bed cannot physically move that much, then use 2

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feathering in TMI

optimizer feathers VMAT portion automatically and also VMAT- leg junction
could feather junction at legs but typically don;t

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top priority OARs

lungs, liver, heart, then kidney

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what is typically spared if the patient had previous treatment?

brain

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How are the POPs for the legs verified?

RadCalc

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how are patients scanned at SIM?

head first, then repositioned and scanned feet first
images are registered

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bolus requirements for TMI

1 cm over legs from just below knees to ankles 0.5 cm wrapped around arms between elbow and wrist

collimator angles used at NSHA

-15/345 degrees for head and neck -80/100 degrees for thorax, abdomen, and pelvis use this to cover the widtrh of the body with longer FS

what to inclde in body contour?

immobilization, but no couch

how much to trim OARs back from PTV?

2 mm liver and bowel 5 mm if too close to bone lung 1 cm

where to focus image registration of the 2 CTs?

pelvis get rid of pitch, yaw, roll

what type pf couch?

medium, since we can only use one

why use 90 colli?

shield genitalia with MLC

SSD for POPs

130 cm

where to junction the 2 plans?

shin avoid bones due to reproducibility

usual MU for each POP

170

how to target forehead while sparing eyes?

add another arc with 90 degree colli to help spare eyes

important step before running optimization

adjust baseplan to appropriate location make sure jaws match appropriately with base plan spend time at overlap of arcs- try to optimize this firt (optimal geometry so you spend less time on plan optimization)

AP shift to get to middle of patient's body

about 4 cm post

why are theer less TMI patients now?

bulsufan was previously only available in oral prep with variable efficacy recently available in IV prep- preferred over TMI for some indications (AML)

allogeneic vs autologos

autologos is own blood cells that are treated allogeneic is from someone else former has more chance of putting leukemia back in patient latter has more chance of rejection disease typically determines if autologous or allogeneic is preferred/required

where can bone marrow stem cells be taken from?

peripheral stem cells placental blood/umbilical cord blood donor

mini-transplants

only 2 Gy dose less risky as relies on healthy cells eventually killing the leukemia typically done in older, frail patients

why only up to 12 Gy dose?

15 Gy- people started dying from radiation pneumonitis

why use 45 degree colli rotation for TBI?

make use of pythagorean theorem to get larger FS

most common radiation induced cancer in young women

breast cancer

does our target include CNS?

yes, sanctuary site for leukemia

oral cavity side effect

mucositis -don't get with TMI vs get with TBI

what do we use on lungs in TBI?

attenuators (not shields) -bring dose to within 10% of Rx

when do you use kidney shields?

typically for 18 MV on tomo- TMI

strange type of bolus...

Winnipeg uses sandbags for Co-60

pt at which body starts making new marrow

engraftment -point after TMI when you know it worked

do you measure TMR for extended SSD?

-would have to move entire tank bit by bit... -typially measured PDD and convert to TMR -but at extended SSD, IS law doesn't hold

what can you do about issue with lasers and extended SSD?

get additional laser system

issues with extended SSD

-lasers -whole cable in beam -scatter from wall -extended SSD dosimetry -lose dose rate (need appropriate detectors)

margins in TMI

-targret is bone marrow (CTV is within PTV, which is contoured bone) -additional 1 cm margin for PTV around arms due to trouble immobilizing -ribs and other bone islands are joined together

do most centers play with energy for TBI?

no -image commisioning all those energies at extended SSD...

why might it not matter if some parts of bone marrow aren't well covered?

bone marrow circulates as treatment is happening

how could you assess max permittible shifts for the VMAT plans in TMI?

-look at lung, liver, kidney, heart -model sup-inf shifts in eclipse and determine acceptable effect -also assess hot and cold spots

hardest thing to immobilize in TMI

arms -also can struggle to visualize arms

are standard orthogonals taken to assess vmat positions in TMI?

tend to be at oblique angles so you can see bones, (ex spine would be hard to see with lat orthogonals)

who might not be able to get TMI but could get TBI?

obese patient (weight limit of couch) -may not be able to get lateral FOV in scan

TG17 TBI dose rate to prevent side effects

<20 cGy/min

how did Halifax get big water tank to commission TBI?

moved our water tank around

GE max FOV

65 cm

procdure for measuring dose delivery in large field TBI

-get absolute dosimetry in water tank at TBI conditions -correct for larger beam area and scatter than tank permits -correct back down for patient area and thickness

why avoid in-air measurements (and thus TAR) for TBI?

-measuerments are confounded by scatter from walls and floor of treatment room

3 components of effect of finite patient size

-variation of patient contour on entrance side -lack of lateral scatter in patients that are smaller than total beam size -finite thickness of patient on exit side of beam

issues with inhomogeneity corrections

-corrections factors have errors for large FS unless they are FS dependent -any tissue-air ratio method taken to the power of the density tends to underestimate the correction factor for large field sizes

anthropomorphic phantom for a child

modular water phantom which approximates the body2 9 . shape of the This technique uses a number of small (15 cm x 15 cm2 ) water containers placed side-by-side. The depth of the water in each container is varied to correspond to changes in patient thickness

summary of important checks for water phantom

i) Water is the material of choice. ii) Minimum phantom size should be 30 x 30 x 30 cm3 . Larger phantoms are preferred and can be obtained by placing water equivalent phantom materials about the minimum phantom size. iii) Plastic phantoms will need corrections to convert to water as per TG21. iv) Smaller phantoms will need corrections for the lack of full scatter. These corrections are dependent on phantom size, field size and energy.

summary of important checks for dosimeters

i ) Dosimeter response should be energy independent. ii) Stem and cable effects should be checked and must be minimal.

correction factors for limited tank size

on order of 1.005 to 1.2