Head and Neck IMRT/VMAT planning

Question 1

Independent of treatment site:

Answer 1

Remove couch and set laser localisation
• Contouring by RO and RTs to define target volumes and OARs
• Add beams
• Isocentre typically in middle of lowest dose PTV - otherwise will be off midline and cause collision risks
• Similar principals as for 3DCRT but use non-opposing beams
• Collimator angles chosen to maximise optimisation parameters
• For head and neck starting jaw setting of 10x10
• Add prescription and set up dose grid and isodose lines - 0.25cm is clinically acceptable not 0.4cm

Question 2

Create density overrides for high density artefacts 11

Answer 2

High density artefacts caused by
fillings
• Dose calculation accuracy negatively
impacted
• Need to be contoured and density
overrides applied
• Need to consult with physics
regarding correct density values for
metal objects

Question 3

IMRT beam set up:

Answer 3

Use 9 non-opposing fields, static, use MLC – ”yes”, - different degrees of modulation.
• 6MV – Why? - Skin-sparring effect
• G180, G140, G100, G60, G20, G340, G300, G260, G220
• Coll = 0 for all beams except G180, G100 and G260:
• G180: Coll rotated to 270 so MLC can shield spinal cord - reduces number of segments
• G100 and G260: Coll rotated to miss shoulders
• Make jaws asymmetric

Question 4

Create contracted external contour 24

Answer 4

CONT_EXT = external contour (8 slices above
most sup aspect of PTVs and 8 slices below
most inf aspect of PTVs) contracted 3mm in
AP and LR directions only
Created to ensure not trying to push reference
dose into build up region
If RO wants dose to skin surface, wax must be
applied to relevant region

Question 5

Two distinct steps pre-optimisation

Answer 5

Creation of dose-controlling ROIs
• PTV and OAR objective ROIs
• Must avoid expansion outside of external (occurs for most head and neck plans if steps not taken to
avoid it)
• Some planning systems do not allow overlap of objective ROIs (Pinnacle does allow ROI overlap)
• Ring contours
• Used to conform dose to PTV
• Non-target normal tissue contour
• Used to conform dose to PTV and avoid dose dumping in tissue in the irradiated volume that has
not been contoured
• Creation of IMRT optimisation dose objectives

Question 6

Clean dose controlling ROIs –

Answer 6

When subtracting overlapping volumes and avoiding expanding into others can often get very small isolated
contours – delete them as they can cause problems with optimisation

Question 7

Biological objectives

Answer 7

In Pinnacle can select EUD-based objectives
• EUD = equivalent uniform dose
• The dose if given uniformly to a ROI that will give the same biological response as the planned
heterogeneous dose distribution for the ROI
• Dependant on the “a” value (measure of seriality)
• a < 1 will decrease cold spots
• Use for targets
• a = 1 cold and hotspots considered equally
• Use for parallel OARs
• a > 1 will reduce hotspots
• Use for serial OARs
• Benefits of use:
• One EUD objective can replace multiple objectives often required for targets and OARs

Question 8

Prior to starting optimisation process

Answer 8

In IMRT parameters window select optimisation parameters such as
• Optimisation type – use direct machine parameter optimisation (DMPO) : optimise and convert in the same process
• ALLOW jaw motion during optimisation, except for G100 and G260
• DO NOT allow beam splitting
• Total number of iterations, number of iterations before CC calculation etc
• Maximum number of segments and minimum MU
• Minimum leaf pairs and leaf end separation

Question 9

IMRT PARAMETERS FOR Head N Neck

Answer 9

Max iterations: 70
Convolution dose iteration: 35
Max no. of segments: 108 (10-12 segments per beam for a 9 field IMRT HN plan)
Min segment area: 4
Min segment MUs: 5
Min no. of leaf pairs: 4
Min leaf end separation: 2

Question 10

IMRT plan evaluation

Answer 10

Required after each optimisation run
• Evaluate dose distribution
• Can very quickly tell you where the plan is too hot or cold
• “Odd” dose distributions means something is wrong with an objective or the ROI relating to an
objective
• Check max point dose of plan and location
• Should be in target requiring the highest dose
• Interrogate DVH/Scorecard to see if specific IMRT planning goals met
• Be aware that large bins in DVHs can result in errors in DVH dose reporting
• Pinnacle Scorecards automatically use an appropriate bin size
• Check objective values – adjust weights?

Question 11

IMRT plan evaluation

Answer 11

Required after each optimisation run (cont)
• Evaluate segment shape
• Have a lot of small thin segments been created?
• Check max number of segments per beam
• Should be in target requiring the highest dose
• Aiming for approx 10 – 12 segments per beam for HN IMRT
• Check total MUs
• If have unusually high hotspots in unexpected regions, lots of small thin segments, high
segment numbers per beam, high total MUs check:
• Field geometry (consider adjusting), dose controlling contours, dose objectives
• Prescription and dose grid set correctly

Question 12

VMAT Head N Neck

Answer 12

Same as IMRT with iso + ROIs + prescription etc

Col: 5 degrees to redice interleaf leakage

Question 13

VMAT PARAMETERS

Answer 13

Allow jaw motion
Create 2 arcs
Gantry spacing = 4 (total arc angle must be divisible by gantry spacing)
Constrain leaf motion

Question 14

VMAT plan evaluation 59

Answer 14

Same as for IMRT plans PLUS
• If using a dual arc
• Check MU of both arcs
• If total MU of second arc very low ?only need one arc
• If not getting planning goals after adjusting objectives and their weights etc
• Try increasing maximum delivery time in the SmartArc IMRT parameters window (90 seconds ok in
most cases)