Breast/Chestwall/S'Clav Planning Flashcards
Why multiple fields? (3F)
Third field = SCF nodal irradiation
Indicated when:
4+ lymph nodes positive*
25%+ lymph nodes positive*
Apical node positive
Recommended when:
1 – 3 lymph nodes positive
*out of excised axillary nodes
NB: Surgeons do not access supraclavicular site
Why multiple fields? (4F? 5F?)
A PA field may be used to increase dose to axilla
An electron field may be used to treat internal mammary chain nodes
slide 12
LN levels
I - infraclavicular nodes, supraclavicular nodes, apical nodes
II - central nodes
III - lateral nodes
+ IMC, subscapular nodes, pectoral nodes, subareolar plexus
Level 1 axilla overlap with chestwall, usually not included in sclav field as tend to be covered in chestwall field
Field Junctioning - what is it? positioning importance? types of junctions?
Means matching of adjacent treatment fields
Multiple isocentres or monoisocentric
Requires accurate planning and treatment setup techniques
Need to be very accurate in patient positioning, because junctions could result in hot/cold spots
Patient stability and setup accuracy are extremely important when treating with junctions to avoid areas of:
Overdose = risk increased side-effects
Under-dose = risk treatment failure
Types of junctions in breast/chestwall RT
Photon – photon
Photon – electron
[Electron – electron]
If field junction is matched at skin
Beams diverges at depth, overlap, then create a high dose region in the shape of a spike where beams overlap underneath skin
Unacceptable
- Solution use half-beam blocking (non-divergence at central axis) OR add gap between fields
Slide 17
If fields have 1cm gap
Note, beams still diverging, but have 1cm gap between beam edges, region of overlap happens far below the skin…
Looks better, but need to be careful of ‘cold spots’ now
95% separated? Tumour coverage?
Have to be careful of junctions, role: don’t junction over areas of confirmed disease
Junction only when disease is NOT superficial
slide 18
Field Junctioning for Breast/Chestwall
Several options: Geometric matching and/or half-beam block
Beams are angled to match along a straight edge
Using gantry, collimator, couch rotation (or combination)
Half-beam block shields half of the beam
Produces non-divergent beam edge
Stationary block or asymmetric jaws create block
Multiple isocentres: setup error?
Examples on slide 19
Field Junction w/ multiple isocentres
Might see if breast/chest wall field is required to be longer than 20cm
Multiple isos:
High chance of setup error
Patient movement bc longer tmt and shifting couch
Hot/cold spots along junction
Matching using couch rotation
SCF field:
Asymmetric
Half-beam block along INF beam edge
Mono-isocentric technique
Single iso at junction
Separate dpts for SCF and tangents
Half-beam block
One jaw closes to central axis, creates non-divergent beam edge
No couch movement needed to match planes
Advantages:
Simpler and faster patient setup
Image verification needed for one point only
Disadvantage:
Limited beam length to half of maximum field size
Dose point placement SCF field
(mono-isocentric)
Placed at effective centre of the field
At prescribed depth (given by RO) and in tissue as per ICRU guidelines (density 1.0g/cm3)
Above bone, not in density interface
May be prescribed to a specific depth or to cover volumes (SCF/axilla nodes)
Depends on department and RO preference
Dose point placement tangents
(mono-isocentric)
Placed at effective centre of the field/PTV
Where the isocentre would be for a symmetric technique
In tissue as per ICRU
Move closer to junction if dose to SUP part of field is not adequate
Patient Positioning
Similar to tangents but some specific instructions
Breast board
Patient flat as possible to achieve a flat sternum – minimises dose to lung from SCF field
Elevating breast board may be considered for larger patients if breast settles too superiorly
Must fit through CT scanner
Head RO preference:
Head straight (chin extended) to avoid SCF field or
Turned to contra-lateral side
Arm position
Both arms up or one arm up (affected side)
Must ensure patient fits through CT scanner
Wide SCF/axilla field: contra-lateral arm down
Adjust arm position to reduce axillary folds
Other considerations:
Kneefix
Ensure patient is straight, level, stable and that position will be reproducible
Planning junctioning/technique cannot use collimator angle as SUP border of field (junction) must be straight and vertical
SCF Field borders
Half-beam block to INF border using asymmetric jaws
Field borders depend of position of involved or at risk lymph nodes
SUP B: Include all SCF (indentation above clavicle), approximately 5 cm SUP to SN
INF B: Isocentre/junction; avoid placing over involved nodal groups; at angle of Louis to cover the clavicle; just SUP to palpable breast tissue; or at a point > 2 cm SUP to original mass
MED B: Midline (unless nodal groups extend over ML); exclude spinal cord [and thyroid gland as per RTOG guidelines]
LAT B: To include 2-3 cm humeral head, wider if axilla involved
SCF Field Shielding
Shielding: larynx [if indicated], humerus
Field length <10 cm, minimise amount of lung in field (50gy to whole field, so need to be careful with amount of lung in field)
Note other OARs, what are they?
- cord
Possible gantry angle to avoid larynx/cord (AP vs. APO)
If APO (e.g. 5°), medial beam edge is vertical
PA axilla field rationale
Scenario 1:
Increase dose to mid-axilla (prescribed dose = 50 Gy) in patients who have not had Level III lymph nodes dissected.
Scenario 2: Large patient separation when dose from SCF alone is insufficient
Increase dose to mid-axilla to 45-50gy
PA field parallel opposed to SCF field, medial border reduced, need to be at 100FSD when using varian (risk of collision)
Separate dose point
AP field won’t achieve this (50Gy to mpd/midaxilla) without getting too much ANT dose.
Need to ‘boost’ from PA
PA field, dose pt. added at mid-plane, but dose contribution small to ‘top up’ to midplane point (i.e. 0.42Gy/#)
Disadv – hot anteriorly, but getting 50gy to where we need to
