Lec 2 Plan Op & Ev Flashcards
(44 cards)
What things does the radiation oncologist prescribe?
number of fractions
point or volume dose is to be prescribed
indicative energy (this can be changed if necessary)
overall treatment time in weeks
What is the normalisation concept?
dose expressed as percentage of dose to a reference point
Specify dose at the reference point = 100%
(this process is called normalisation)
What point should the 100% isodose line run through?
through the reference point
How does normalisation to a specific point affect the plan?
depending on the point, makes a huge difference to the isodose visualisation
What are the principles of plan optimisation?
lowest dose to critical tissues
fulfilling prescription
plan is efficient to deliver on the treatment machine (i.e. least number of beams and accessories- wedges)
plan is treatable
What are the two techniques of planning?
field-based planning
volume-based planning
What is field-based planning?
old technique
define the field using bony landmarks
not very conformal
What is volume-based planning?
current technique involves defining the fields using contoured PTV (GTV to CTV to PTV) volumes a port (automatically generated treatment field using PTV) a margin to account for beam penumbra is used (PTV to MLC margin).
What is coplanar and Non-coplanar?
coplanar: floor rotation on zero
non-coplanar: floor rotation not on zero
What are class solutions?
certain beam arrangements may give the optimal dose distribution for certain PTVs
(known arrangements e.g. sunrise-prostate, wedged fields)
What does beam quality affect?
depth dose
Why is a lower energy beam used for the thorax?
build-up effect after air spaces
Why is a lower energy beam used for head and neck?
to reduce the exit dose on skin
Why is a higher energy beam used for deep-seated tumours?
increases tumour dose relative to incident dose
What things can modify beams?
wedge (physical/ dynamic)
bolus
MLCs
What things need to be considered during plan evaluation?
impact on patient (quality of life)
impact on department efficiency (workflow)
What are the ideal dose distributions?
dose gradient across the volume minimised (95% to 107%) PTV dose (coverage, conformity, homogeneity) Integral dose should be minimised (try to avoid as much tissue as possible_ Dose to critical structures (within specified tolerances) Shape of treated volume closely matched to the PTV (implies a conformity index close to 1)
What is minimum dose (Dmin)
smallest dose in a defined volume
minimum PTV dose is the lowest dose in the PTV
What is a hotspot?
represents a volume outside the PTV receiving dose larger than 100% of specified PTV dose
only clinically significant if volume >1.5cm
if it occurs in a small organ (lens) then a smaller dimension than 1.5cm needs to be considered
What is the ICRU criteria for reference point?
should be clinically relevant and representative of the dose through the PTV
The point should be easy to define in a clear and unambiguous way
The point should be selected where the dose can be accurately determined
The point should be selected in a region where there is no steep dose gradient
What is the conformity index (CI)
this is a ratio of 95% isodose to the volume of the PTV
CI = (VRI/ TV)
VRI: Volume of the reference isodose
TV: Target Volume
What is the homogeneity index (HI)?
An objective tool used to analyse the uniformity of dose distribution in the target dose HI = D2%- D98%/Dp D2% = dose to 2% of PTV D98% = dose to 98% of PTV Dp = Prescribed dose
What is a DVH?
a representation of the three dimensional dose distribution as a plot of the volume of a ROI versus dose
What is a cumulative DVH?
a plot of the percentage of the total volume of an ROI receiving dose within each specified dose interval against the dose range of the plan
(most commonly used DVH’s as they match dose objectives and constraints given by RO’s on the prescription)
(display the total volume that received a given dose or more)
