Principles of Planning Flashcards
What happens when you increase FSD?
1) Dose at depth increases - because the increased amount of air between the source and the patient acts as a filter removing more of the softer x-rays
2) Dose rate at depth decreases - because of the inverse square law: Intensity is inversely proportional to distance squared. Greater considerations for patients treated at extended FSD as a greater area of tissue is irradiated.
It takes longer to treat the patient as there is decreased dose at depth. Extended FSD is used to treat at depth or to treat larger field size.
Why are wedges used?
They are used to preserve dose uniformity when there is missing tissue or if there is another beam at an angle to the first.
Wedges cause a progressive decrease in intensity across the beam, resulting in tilting the isodose curve from its normal position. The degree of tilt depends upon the slope of the wedge.
The wedge shapes the beam to conform to the shape of the contour, eliminating air gaps. Wedges preserve the skin sparing effect as interaction still starts at the skin surface as no matter interaction occurs. So wedges are better than bolus.
Wedges can be used to disperse hot spots, decreasing dose to that point. For a H&N plan there is a small target volume and separation, so DMax is still quite deep (1.5cm with 6MV). The wedge can disperse the dose better which can reduce skin reactions (80% of dose goes near the skin).
Wedges conform dose to a sloped criteria to get a more rounded edge at a point. It produces a gradient on the dose. The heel of the wedge reduces dose to the patient and tilts the isodose curve. A smaller wedge angle is used for smaller tilting isodose curves, allowing more dose through the heel than a larger wedge angle would.
The wedge isodose angle is the complement of the angle through which the isodose curve is at the 50% point to a perpendicular line.
The hinge angle gives an indication of the wedge angle that should be used. It is the angle that is produced when two beams are placed on a contour and are less than 180 degrees. The dose distribution where the two beams meet is affected when less than 180 degrees to each other so the hinge angle between them indicates what wedge angle should be used to account for this.
What types of wedge are there?
Fixed wedges have fixed angles of 15, 30, 45 and 60 degrees.
Motorised wedges can produce any wedge angle, and they move in and out of the beam. The collimator angle is important to ensure the wedge goes in and out of the beam in the right direction.
Dynamic wedges can produce any wedge angle and use the jaws to create the wedge effect. The jaws move across the beam to alter beam intensity by giving a dose gradient.
What is a tissue compensator?
Something that compensated for differences in separation through the body. Compensators adapt the intensity of the beam at smaller separation points without affecting the skin sparing effect as they are not placed close to the skin. They are made of lead/steel to absorb radiation dose.
Compensators are individually made beam attenuators shaped to compensate for the contour of the patient. They are placed in the head of the gantry in the beam on a tray close to the source or target.
Compensators shape the beam in two planes; they are unique to each patient; costly; time consuming to make.
Tissue equivalent materials
There are lots of types of bolus materials. A tissue equivalent material is a substance chosen to ensure that ionising radiation will interact within it in the same manner, and to the same degree, as would if the material was replaced by a patient. it is used to unify the shape of the patient.
The beam starts interacting when it meets the bolus so Dmax is brought up to the skin surface. Energy may need to be increased if still want skin sparing.
Bolus is used to reduce the depth of the maximum dose for superficial lesions.
What is the difference between bolus and a compensator filter?
Bolus is placed directly on the patient’s skin surface so the beam starts interacting when it hits the bolus; skin sparing effect is lost.
A compensator will achieve the same dose distribution but is not placed on the skin, but close to the source. Due to the large air gap the skin sparing effect is maintained.
What is a DVH?
Dose Volume Histogram.
A graphical representation of the dose received by rendered 3D volumes as a percentage of the maximum dose. DVHs check the correct dose is being received by the PTV and OAR. Cumulative DVHs are used in the department (as opposed to Differential), which take into account integral dose when assessing OAR.
The ideal DVH will show >95% of the dose being received by all of the tumour (homogeneous) and low dose to OAR.
What are problems with DVHs?
DVH doesn’t reflect the position of dose in the tumour/OAR or the difference in functioning cells within the volume (serial/parallel). If part of a parallel organ is damaged it will still continue to function (i.e. kidney, lung, liver), whereas a serial organ will not (i.e. spinal cord, oesophagus).
Discuss tissue inhomogeneity
Tissue inhomogeneities affect dose distribution due to different densities within the body, which can affect beam energy.
I.E. the lung is full of air so there is increased transmission and decreased scatter, whilst the bone is more dense so the beam is attenuated, dose is reduced (scatter).
What is the purpose of image fusion?
Optimises the target volume: functional (PET) and structural (CT). More information is given which enhances the PTV volume allowing tighter margins so healthy tissue receives less dose. It is also useful for verification and follow up to check the response of radiotherapy.
To register the images identical points or lines on the images are identified. the computer registers these points, changing the scale, location and orientation of one image.
What is absolute absorbed dose?
Dose calculated to a certain point - relates to the output of the machine
What is relative spatial distribution of dose?
Due to scatter there are profiles of dose in a field due to where scatter contributes. It is related to the point of determination of absolute absorbed dose
What is the beam profile?
Measurement of dose at different positions in a cross-sectional plane; measured at a defined depth
What does an isodose curve tell you about?
Penumbra width, divergence of the beam, scatter, beam hardening, how close different percentages are (i.e. 30% to 60% for skin sparing)
What factors affect PDD?
Beam type, beam energy, field size, field shape, shielding, effective distance, filtration, collimation, wedges, compensators, build up - bolus, attenuating medium
