IMRT/VMAT PROSTATE Flashcards
Why cant you use opposing beams in IMRT?
It gives you less ‘optimisation space’. Less options for optimisation
Why do we rotate the collimator from to 270 or 90?
This is done to allow shielding from SUP to INF. MLC’s can only travel a certain length (for Varian). If the collimator was left on 0 the MLC’s wont be able to move far enough to shield OAR and thus will create 2 segments to achieve adequate shielding. This results in more MU and increased dose to the patient.
What are the differences in prescribing for IMRT and 3DCRT.
For IMRT we prescribe to a volume not a reference point (ICRU83). This is because the RP wil most likely be under shielding (which is against ICRU 50 RP guidelines).
What is the purpose of Ring contours?
To conform mid-range dose levels to the target volumes. To bring dose in nice and tight and prevent dose dumping.
What are the types of IMRT objectives in Pinnacle?
Uniform Maximum and minimum dose Max and Min DVH Biological objectives Contrain
What is the ODM?
Opening density matrix.
Displays the intensity across the beam.
Darker=less intensity
What is the theory behind beam splitting?
MLC’s for Varian are not part of the primary collimation they are an external application (thats why the tube head is a lot bigger). The leaf can only travel a maximum of 14.5cm. Therefore may have to split field into 2.
What is the process of conversion?
Segments and MU created for each field
MUs are then rounded
Compute with adaptive convolution when developing plan
Compute with CC convolution when final plan created.
What are the advantages of DMPO. (Direct machine parameter optimisation).
Includes the linac MLC configuration in the optimisation process.
No separate conversion process after optimisation.
Results in less MU than using a two-step optimisation process.
What is a warm and cold start?
Warm: do not rest the beams prior to beginning another optimisation
Cold: reset the beam prior to beginning another optimisation.
What is an EUD based objective.
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
Benefits: One EUD objective can replace multiple objectives often requires for targets and OARs.
How do you control the EUD objective.
Use the ‘a’ value (alpha).
a less than 1 will increase cold spots (use for targets)
‘a’=1 cold and hotspots considered equally (use for parallel OARs).
‘a’ more than 1 will reduce hotspots (use for serial OARs).
What is a consideration of the gantry?
Gantry rotation speed constant or variable. Constant can limit modulation.
