Parotid Planning Flashcards
Parotid Glands Anatomy
Largest of salivary glands
bilateral structure
Made up of glandular tissue
Divided into superficial and deep lobes
POST to masseter muscle
INF to ear, size 50-60 mm
Role: digestion, tooth protection
Other salivary glands
Submandibular and sublingual
Salivary Gland Tumours are:
Very rare tumours (~5% of H&N Ca)
~80% are parotid tumours
Of these, most are benign (~ 80%)
Pts w/ malignant tumours present w/ swelling & facial palsy
Parotid tumours & Ca of middle ear are treated (planned) in a similar way
Treatment options
Surgery first
RT indicated to parotid bed if:
Inadequate surgical excision (positive margins)
High grade malignancy
Lymphomas
Nodal involvement (treatment will include neck)
T3/T4
RT-related issues:
Healing time after surgery; skin flap and scar tissue – skin reaction
RT should begin 4-6 weeks post-surgery, no later than 8 weeks
RT techniques
Depends on intent (radical/palliative)
Modality & beam energy depends on depth of tumour or tumour bed:
Photons – wedged pair or IMRT/VMAT imrt equally spaced beams entering from one side to spair other parotid
Mixed modality – photons/electrons
Electrons – direct field/s
Bolus
Overcomes skin sparing effect if superficial disease or close to scar
Reduces risk of skin recurrence
Dose and fractionation - Radical 3DCRT
Multi-phase treatment
Phase 1 – Parotid + elective neck (s’clav) to 50 Gy in 25#
Phase 2 – Boost to parotid bed:
60 Gy if completely excised
64 – 66 Gy if microscopic residual disease
70 Gy for high risk / macroscopic residual disease
eviQ single phases 6mv
tumour bed CTV60-66Gy/30-33#
surgical bed CTV54-56Gy/30-33#*
elective regions CTV50Gy equivalent/30-33#*
*2Gy equivalent/#
Patient Positioning
photons AND electrons:
supine, mask, no tongue depressor/mouth bite, wire on scar/palpable disease, consider bolus
photons: head straight, chin extended
electrons: head turned, aim for flat ‘open’ neck, shoulders down, wire around field edge/s
Organs at risk
Spinal cord
Brainstem
Optic chiasm
Orbits
Cochlea (mean dose < 45 Gy)
Contralateral (uninvolved) parotid ( < 24 Gy)
Contralat parotid – might not necessarily be in the field, but need to be aware of location and dose, because complete loss of function in involved parotid due to disease and treatment
Field borders - parotid
Target volume to include whole parotid bed
SUP B: zygomatic arch
INF B: below inf. pole of parotid gland and include upper deep cervical nodes
ANT B: incl. masseter muscle
POST B: mastoid process
MED B: to cover parapharyngeal space
LAT B: to cover palpable disease and scar
Target volumes - IMRT
CTV60-66 Tumour bed including pre-op GTV + ≥ 5-10mm
CTV54-56 Surgical bed
CTV50 equivalent Elective areas
PTV CTV + 3-5mm as per department protocol
Photons - wedged pair
Suitable for ensuring dose deep into parotid bed
Treating to base of skull
Deep positive margins
Lesion in deep lobe of parotid
6 MV photons
Bolus if indicated
Approx. 90° between beams (no dose to opp side)
Avoid entering through oral cavity or brainstem (no AP)
Avoid contralateral parotid
Wedges (or step + shoot)
Dose point – away from bone/air
Half-beam block SUP/INF? - for s’clav, but also for sup edge if extends to eye
Ok to have dpt close to mandible, because bone is behind it, beams pass through tissue first
Wedges optimising dosimetry
Increasing angle between beams = limits dose through brainstem, but 95% isodose larger Ant/Post
Lateral field – would improve the generous ant/post 95%
Disadvantages = Extra dose to contralat parotid and brain stem
Photons - neck field
Supraclavicular nodes irradiated when:
SCC histopathology, other tumours with high risk of nodal spread, patients with palpable nodes
Treatment is to ipsilateral nodes only - drainage or spread to contralateral nodes rare
AP/PA field, half-beam block (non-divergent) SUP border junction
If neck field is included, SUP border of neck field junctions with INF border of wedged pair/parotid field
IMRT
Odd number of beams
Entering from affected side
Can treat to 2-3 different PTV dose levels
More contouring for targets and OARs to enable sparing
Direct Electrons - Indicated when?
Superficial lobe tumours with no perineural invasion (i.e. no need to treat nerve to skull base), treating parotid bed only to a depth of 3 – 4 cm
Parotid node metastases from cutaneous SCC
Possible to treat parotid and neck nodes within single direct field
Direct Electrons - field borders
SUP B: zygomatic arch (parotid)
INF B: thyroid notch (parotid), clavicle (neck)
ANT B: anterior border of masseter (parotid)
POST B: mastoid process
Study diagram on slide 10
Direct electrons - energy + considerations
9 – 12 MeV electrons
Might need to consider different energies for different phases
Consider pinna:
needs taping down?
EAM packing?
Direct electrons - two phases
2 phase tmt
Note cannot avoid sloping surface, large field/sloping neck
Note dose homogeneity
Where is 90% isodose?
Advantage – dose fall-off, minim dose to cord, need to be aware of e- energy, because higher energy could go into cord
Direct electrons - patient position + dpt placement
Head turned, shoulders down
Aim to get a flat field (as much as possible)
Dose point placement
Centre of field if can
Do not place under scar wire, ear, any ‘bumpy’ areas
Place under flat area of tissue
Most likely need separate dose points for each phase
May need to prescribe to 90% isodose to achieve PTV coverage
Direct electrons - offsetting cutout
Difficult to get into the field, thick neck – probably a thick chest too, cutout inside cone like this (normal orientation) would most likely collide with patient
Improve by offsetting field inside cutout, might help with setup
images slide 12
Bolus
Considered if there is skin involvement
Compensates for:
Surface contour variations
Air gaps (electron unpredictability) such as in EAM and behind the ear
Types of bolus used:
Full field bolus, constant thickness (electrons)
Personalised bolus over scar only (photons)
EAM: wet gauze, putty (electrons and photons)
Adding a bolus in Pinnacle
As a ROI (in Contours)
Easier/quicker for smaller boluses e.g. EAM
Create a ‘bolus’ ROI then override density
Contour is present for all beams and entire treatment
Using ‘Bolus’ function (in Beams, Modifiers)
Easier/quicker for full field boluses
Option of applying to some beams/phases
Can be defined in 2D or 3D views
Bolus function (in Beams)
- Override density of any markers etc. before adding bolus
- Click ‘Add Bolus’ and edit name
- Select ‘Constant thickness’
- Enter ‘Density’ and ‘Thickness’
- Click ‘Define two bolus edges in clockwise direction’
- Define bolus on transverse slice
- Enter SUP and INF distances so bolus covers required area
- Click ‘Build Bolus’
- Remember to ‘apply’ bolus to relevant beams
To edit bolus select no to constant thickness. Smooth/trim bolus with contouring tool
