Parotid Planning

Question 1

Parotid Glands Anatomy

Answer 1

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

Question 2

Salivary Gland Tumours are:

Answer 2

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

Question 3

Treatment options

Answer 3

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

Question 4

RT techniques

Answer 4

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

Question 5

Dose and fractionation - Radical 3DCRT

Answer 5

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/#

Question 6

Patient Positioning

Answer 6

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

Question 7

Organs at risk

Answer 7

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

Question 8

Field borders - parotid

Answer 8

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

Question 9

Target volumes - IMRT

Answer 9

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

Question 10

Photons - wedged pair

Answer 10

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

Question 11

Photons - neck field

Answer 11

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

Question 12

IMRT

Answer 12

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

Question 13

Direct Electrons - Indicated when?

Answer 13

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

Question 14

Direct Electrons - field borders

Answer 14

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

Question 15

Direct electrons - energy + considerations

Answer 15

9 – 12 MeV electrons
Might need to consider different energies for different phases
Consider pinna:
needs taping down?
EAM packing?

Question 16

Direct electrons - two phases

Answer 16

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

Question 17

Direct electrons - patient position + dpt placement

Answer 17

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

Question 18

Direct electrons - offsetting cutout

Answer 18

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

Question 19

Bolus

Answer 19

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)

Question 20

Adding a bolus in Pinnacle

Answer 20

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

Question 21

Bolus function (in Beams)

Answer 21

1. Override density of any markers etc. before adding bolus
2. Click ‘Add Bolus’ and edit name
3. Select ‘Constant thickness’
4. Enter ‘Density’ and ‘Thickness’
5. Click ‘Define two bolus edges in clockwise direction’
6. Define bolus on transverse slice
7. Enter SUP and INF distances so bolus covers required area
8. Click ‘Build Bolus’
9. Remember to ‘apply’ bolus to relevant beams

To edit bolus select no to constant thickness. Smooth/trim bolus with contouring tool