Monaco VMAT treatment Planning Flashcards
Discuss voxelization
- Monaco is voxel-based (Monaco converts contoured structures into voxels based on slice thickness and dose grid size)
- if >/= 50% of a voxel is inside the structure contour, Monaco considers it to be part of structure
- decreasing slice thickness, increases number of voxels
- decreasing dose grid size, increases number of voxels
What are the advantages of voxelization?
controls voxels instead of controlling structures
General Workflow -1
- Import the studyset(s) and assign the CT to ED.
- Open the patient in the workspace.
- (Optional) Fuse multiple studysets.
- Carefully contour all required targets and OARs. Create necessary margins.
- Define and lock scan reference point.
- (Optional) Import applicable treatment devices.
- Start a Monaco plan (load a plan template).
General Workflow -2
- Verify/edit beam geometry, isocenter, machine and energy.
- Verify/edit prescription.
- Verify/edit electron densities and structure layering order.
- Resolve any structure mismatches and edit the IMRT Constraints, Calculation
Properties, IMRT Parameters, and Sequencing Parameters as necessary. - Do the fluence optimization (Stage one).
General Workflow -3
- Use plan analysis tools to evaluate your plan.
- Adjust the parameters and prescription (constraints) as necessary.
- Re-optimize the fluence and repeat evaluation until you have an acceptable
optimized plan. - Do the segment optimization (Stage two).
- Use the plan analysis tools to evaluate your final plan
- If the plan is not acceptable, make edits to the properties and/or prescription
(constraints) as needed. - Verify plan meets all objectives.
- Save and name plan.
- Request proper plan approval.
- DICOM export all necessary data.
- Export plan PDF.
- Create a QA plan.
Plan Set-Up
Contouring • Interest Points • Dose calculation grid - Shifts from CT Ref - Use template to add beams
Beam - Field ID Important - if they have the same plan will not be imported. 1, 2, 3 etc.
Treatment Unit
Monoca does not automatically create a 2nd arc when stating max number of arcs. Have to manually create second arc
Plan set-up
Arc Increment (2-60)
• If you use an increment that is too large, Monaco creates fewer sectors.
• This can produce poor quality plans. When you use an increment that is too
small, Monaco gives you more sectors.
• But this increases planning time and does not significantly improve plan quality.
It is as if you have too few or too many beams in a standard IMRT plan.
• One method to find a reasonable increment is the “Rule of 3”.
• This means you add three (3) to the number of static beams used to treat this
patient. For example, if you used nine (9) beams to plan a standard IMRT plan,
you can create 12 sectors (9+3) for VMAT. 360 deg arc/12sectors = increment of
30. Smaller arcs = more sectors = more treatment time
The sector
Before stage one optimization, the system divides a
sequence into sectors you use to simulate the arc
during stage one optimization.
• Partial Arcs- the system selects the closest
increment value that is uniformly divisible with the
posterior arc increment split into two.
Sweep Sequencer
Sweeping leaf sequencer is that the leaves move from
their start position to their end position in a continuous,
unidirectional manner
• The length they do this is determined by the sector
• Beginning with the first sector, the leaves move to the left
side of the BEV
• They then change direction and move to the right side of
the BEV
• The minimum width of these end segments is hard coded
at 5mm.
Prior to Optimisation:
Segment Shape Optimisation (SSO)
- uses fluence smoothing, MLC sequencing (or clustering) and optimisation of beam weights and shapes to better meet IMRT/VMAT constraints
- ranges from 1-20 where increasing SSO increases treatment plan quality
(1) advantages - decreases number of segments/control points (and increases treatment plan quality)
- decreases treatment delivery time
(2) disadvantages - increases optimisation time
- increases number of MUs
Fluence Smoothing
- a parameter of SSO that controls the smoothing of the fluence in stage one of optimisation
- ranges from off, low, medium or high where increasing fluence smoothing decreases number of segments and increases treatment plan quality
(1) off or low - creates more segments (used for complex treatment plans)
(2) medium - creates average number of segments (used for simple treatment plans)
(3) high - creates fewer segments
Calculation Properties
• The use of Monte Carlo algorithm can cause more
hot spots. Use the cost functions to control these.
• Remember to not be too harsh as this is a more
accurate representation of actual patient dose.
• Statistical Uncertainty Per Control Point is the percent
(%) statistical uncertainty per voxel, on a per-segment
basis, that you are willing to accept for the final dose
calculation
Lower the statistical uncertainty = less noise (use 1% ideal)
Layering Structures
• Do this when setting up your cost functions
▪ Highlight the structure you want to move and click the up or down arrow on the
IMRT Constraints tab in the planning control window.
▪ Targets are typically at the top and the patient skin surface contour at the
bottom.
The layering order determines how the optimizer treats the voxels in the volume
where the structures overlap. It does not imply that one structure’s objectives or
constraints are more or less important.
Essentially, the priority of the structure - layering dictates the allocation of the voxel
Smaller structure inside larger structure needs to layered higher
Optimasation Approach
Pareto mode prioritizes target underdoses on tumor volumes and relax
constraints on healthy tissue. This effectively reverses how Monaco normally
works. -Achieves OAR objectives then try to achieve coverage
• Constrained mode set constraints on healthy tissue while it administers dose to
target volumes. - Vice versa, usually use Constrained
Monaco Cost Functions
- Target EUD - uniform dose • Target Penalty - minimum • Quadratic Overdose - max, modified max dose by adding Root Mean Square (RMS): allows flexibility to DVH tail to target = isoconstraint = use very low RMS • Parallel • Serial • Overdose DVH • Underdose DVH • Maximum dose • Quadratic Underdose • Conformality - Replaces rings for NTT
