Oncol355- Lecture 2 (Characteristics of a Radiation Beam and Field)
What is an isodose curve?
A locus of points of equal dose value; the % depth dose values are the same along the line.
What does "geometric" mean?
A region where there's 50% of the dose. This is why we always want to have a margin around the actual tumour so that we know that the tumour isn't in the penumbra. Also see slide 12!
What do electronic compensators do?
A static field, but the MLC leaves are dynamic, and so they create a non-uniform distribution across the radiation field. They modify/modulate the intensity. The electronic compensation allow the MLC leaves to move in and out, which allows for that dynamic and non-uniform field! They are used with STATIC radiation fields.
What is a physical wedge (PW)?
A wedge that's made of stainless steal and is inserted into the head of the machine, and has graduated thickness. They're available in 15/30/45/60 degrees. These are rarely used now.
LEAD: 1) What is an advantage? 2) What is a disadvantage?
Adv: good attenuator because it's really dense Dis: very high melting point, so it's hard to make; straight-edged, made standard blocks, was toxic
What are the advantages and disadvantages of cerrobend?
Adv: lower melting point, so its much easier to make blocks with, you can make custom blocks Dis: toxic
What is a beam modifier?
Anything placed in the path of the beam to modify it and alter the dose distribution within the patient, by attenuating the beam to change the isodoses (shape and depth) These are used because contour irregularities or target volume location/shape require modifications to the beam to alter the distribution of the dose within the patient.
Why must you keep the MLCs/wedges/shielding a distance away from the patient?
Because if you had them close to the pt the scatter off of the stainless steel wedge would cause contamination within the patient.
Here are the Pb HVLs for various energies: Co60 12mmPb- 60mm 6MV 13mmPb- 65mm 15MV 15mm Pb- 75mm 18MV 14mm Pb- 70mm So why does the 15MV have to be thicker than the 18MV?
Because you start doing pair production at 18MV, and all of the other energies are in the Compton range so there's a change to the linear attenuation coefficient, which decreases the HVL. With PP, you dont need as much shielded to block out the radiation.
What are standard blocks?
Blocks that you could use for any patients and were NON-divergent Custom blocks were for ONE specific patient and were divergent. They were always mounted up away from the patient, so that the scatter didn't reach the patient.
How can we visualize the dose depositing/attenuation process?
By looking at an isodose chart!
How is Co60 made?
Co59 is taken and put in a nuclear reactor, where it's bombarded with neutrons to make it unstable. This is because we need unstable for therapy. It will then actively decay until it gets back to its stable state (Co59).
What is the equivalent square formula?
Eq sq. = (4 x area)/perimeter = (2(length x width))/(length + width)
What are compensators? What are the 3 types?
Externally placed/located devices, in the head of the treatment unit that alter the beam before it reaches the patient. All of the compensators are retracted, so that there isn't electron contamination. (Electrons will scatter and disappear BEFORE they hit the patient, making sure there's no skin reactions. Must be 15-20cm away from the patient!). 1. Retracted/wax compensators 2. Lead compensators 3. Electronic compensators
Where are cerrobend and lead still used, when it comes to field shaping?
For electron and orthovoltage for superficial treatments!
What is HVL?
Half Value Layer; the thickness of an attenuator that reduces the intensity of the incident beam to half of its original value; this depends on the energy of the beam and the type of attenuating material. Similar to HVT; Half value tenth
What is the half life and photon energy of Co60?
Half life: 5.26 years Photon energy: 1.17 and 1.33MeV (with 1.25 average)
Are photons directly or indirectly ionizing?
Indirectly! This is because the photons don't directly deposit their dose; instead, they deposit their energy to electrons, which will then travel through the tissues and deposit their dose at different depths.
What is the disadvantage of the above method for finding the effective area missing?
It accounts only for the change in field area. So, the location of shielding within the field is not accounted for. This is important, because the shielding distance from the central axis makes a difference, such that the shielding along the periphery has less affect on the dose rate than more central shielding.
Is the penumbra a bad thing?
It gives a dose beyond the treatment volume, may encroach onto a critical organ, produces the "blurred" effect at the edges of images, and the tumour should NOT be in the region of the field. So, it isn't a good thing.
What kind of radiation is the geometric penumbra due to?
It's the region in space which can be irradiated by PRIMARY photons only; so, there is NO scatter. This penumbra is from the primary photons that come directly from the beam still.
What are MLCs?
Metal (tungsten) "leaves" or rods which move independently in and out of the field to shape it. They're located in the head of the gantry, either before or after the collimator jaws, depending on the make of the Linac (Varian has them AFTER the collimator jaws). You can have 52, 80, 120, or 160 leaf systems available, and the leaves are 0.5 or 1cm in widths are most common, and the height is 5.5-7.5cm thick. (80,120 leaves are common)
With linear accelerators, do we still calculate a treatment time?
NO! We calculate a monitor unit because of how x-rays are produced in the Linac. That is, the time fluctuates due to dose rate; the number of ionizations are related to dose and the machine knows when to shut off after a certain number of these units have been reached.
What is the geometric penumbra formula?
Penumbra = (source size x (SSD-SDD))/SDD
What is primary radiation?
Photons that haven't interacted with anything other than air before they reach the patient.
What's the difference between PW and EDW
Physical wedge has attenuation; it's a physical device that decreases the amt of radiation getting to the pt. EDW: changes the dose rate of the radiation reaching the field, but it's not attenuation. There's a change of dose from one side of the field to the other.
What is scatter?
Primary photon comes in and transfer energy to the electron, which will then scatter OR it can be the primary photon that scatters as it goes through an object. Ejected electrons + projected photons
What is scatter radiation?
Primary photons have hit something and interacted, and the fallout from this interaction is what reaches the patient; this involves a change in direction of the particle.
What is the geometric field size?
Roughly passes through the centre of the geometric penumbra, and corresponds to the 50% isodose value at skin surface
What is SPR? What is SMR?
SPR = Scatter Phantom Ratio SMR = Scatter Maximum Ratio
What is SAR?
Scatter Air Ratio; the ratio of scattered dose at a given point in the phantom to the dose in free space (air) at the same point. This depends on energy, depth and radius. Doesn't include the primary beam. It ONLY considers scattered information!
What is the shielded area? What is the treated area?
Shielded: the total area of all shielding within a field size Treated area: the remaining unshielded area within a field size. The remaining open area of the field (unshielded area) must be calculated and new field dimensions determined.
How do beam divergence and magnification effects relate to shielding?
Shielding is up in the machine and the patient is down further. So, any errors in the shielding up in the machine have a magnified impact on the patient down below, due to magnification. This was a big issue when we had shielding blocks that had to be hand mounted.
What is shielding? Why do we shield out some of the treatment field?
Shielding: physical devices put into the path of the radiation to "block" it from reaching the pt. We shield to protect tissue/organs that don't need treatment or those that are critical structures, and to shape the radiation field (gives us different distribution shapes)
What are non-divergent blocks?
Straight-sided blocks that will cause more transmission penumbra, so some of the rays get through These make it so that the edge on the medial part didn't attenuate all of the beam, because the divergence of the beam will make it so that the beam goes through less block, and thus is less attenuated. (Slide 45!)
So, based on the previous flashcard. What's a better method to finding the effective area? Just name it and what it uses.
The Clarkson Method! It uses SAR- Scatter Air Ratio
What is the isodose curve of an electron beam like?
The Dmax is at a depth, but we get a high dose rate at the skin surface too. So, these are used for disease at the skin's surface. However, an advantage is that right after the 80% curve, the isodose drops off really quick, so there's less dose to surrounding organs.
How does SDD affect the size of the geometric penumbra?
The SDD is the source to diaphragm (the diaphragm was used to trim down the penumbra!). So, the larger the SDD the smaller the penumbra.
Explain beam divergence and central axis.
The beam at the central axis will NOT diverge!! As a result, all of our data is based on measurements taken at the central axis. So, the central axis is the non-divergent part of the field. Beam divergence is when the beam literally diverges away from the central axis, and the farther the divergence from the central axis the more the dose is affected.
How does the thickness for a cerrobend HVL relate to the thickness for a lead HVL?
The blocks of cerrobend have to be even thicker to reach the same HVL as lead.
What is Dmax?
The depth at which 100% of the dose is deposited; the depth of maximum ionizations
The amount of beam divergence depends on what?
The distance between the source and the target; the larger the distance, the more the divergence.
What happens after Dmax is reached?
The electrons will continue to travel deeper into the tissue, depositing their doses at various depths.
What is the effective field size?
The equivalent rectangular field dimension of the treated area with shielding accounted for. It's also the size at the calculation depth.
What is the effective equivalent square?
The equivalent square field with shielding accounted for at the calculation depth; AKA blocked equivalent square or axis equivalent square THIS IS THE SIZE USED IN OUR MU AND TIMER CALCULATIONS FOR TISSUE ABSORPTION FACTORS OF Sp, TAR, TPR, TMR, and %DD because these are the ones that take scatter into consideration!!
How does SSD affect the size of the geometric penumbra?
The larger the SSD, the larger the penumbra, due to divergence. So, the further away you are, the more divergence you have, and therefore the larger the penumbra will be.
As the beam energy increases, what happens to penetration and attenuation?
The penetration ability of the beam increases, so the beam deposits more dose at deeper depths. And the attenuation decreases at shallower depths, which also pushes the dose deeper so that there's less at the surface.
What's the difference between a 4MV and a 10MV isodose curve?
The penumbras are both really tight, but there's a difference in Dmax in that on the 10MV there's a higher penetrating ability, so the dose goes deeper. The 10MV will also have more exiting dose.
What is electronic equilibrium?
The point at which the number of electrons leaving a volume equals the number entering it; this is where dmax occurs!
What is the % shielded?
The ratio of the total shielded area to the area of the open field.
What is a penumbra?
The region at the edge of a radiation field over which the dose rate changes rapidly as a function of distance from the beam's central axis. AKA: geometric penumbra.
What is a transmission penumbra?
The region irradiated by photons which have traversed part of the thickness of the collimator. This is also related to the passage of the beam through the edges of the shielding blocks. See slide 45 for diagram!
What is the collimator setting or collimated field size?
The size of the opening for the xrays to pass through; this determines the treatment field when no "shielding" is used. There's a minimum field size that we can cone down to (4x4) and there's a max we can open up to (40x40)
What is the advantage of a deeper dmax?
The skin sparing effect! This is if the max dose is deposited below the skin then we can spare the skin from the higher dose that occurs at dmax.
Eq. Sq.: What happens when we add shielding to a field?
The volume of tissue irradiated with an open beam is different than the total volume of tissue irradiated when shielding is used to block out parts of the beam. There's also a change in scatter in the treated volume. The part with the shielding will have less tissue impacted by scatter radiation, because less scatter can make it back to the central axis. There's a smaller volume so there's a change in PDD, because the amount of radiation getting to this point is more with an open field.
What is the equivalent square?
There is a square field size for every non-square field size in terms of radiation characteristics. Equivalent square tables are recommended when one field dimension exceeds the other by 2-3cm
What do isodose curves look like for 200kVp and why?
There's a big bulge at the edges, because lower energies are less forward penetrating, and so you get a lot of scatter. SEE SLIDE 25
What's the difference between a Co60 and a 4MV isodose curve?
There's still a shallow Dmax, but the penumbra is much smaller for the 4MV. This is because the average energy of a 4MV is 1.33MV, which isn't far off from Cobalt's 1.25. Because there's less penumbra, the 4MV has a higher quality beam.
What do flattening filters do?
These flatten the beam at a specified depth; without this, the isodose is curved like on slide 25. But, these filters attenuate the beam in the middle, so that you get a much smoother isodose curve.
What are divergent blocks?
They match the divergence of the radiation beams; the further the divergent block is from the central axis the more angled it will be as the beam's divergence will also be greater.
What is the thickness vs the width of the MLCS?
Thickness: what the radiation needs to travel through; the height of each MLC Width: how wide each MLC is
What's the definition of inverse square? Formula?
This intensity of a beam varies inversely as the square of the distance from the source. I = 1/d2 OR I1/I2 = ((d2/d1)2)
What is bolus? What are the 3 cases that it's used for?
This is a tissue equivalent material used for: 1. Reducing the skin sparing effect 2. Bringing all isodoses up closer to the surface (raising the high dose region/dmax closer to the skin surface) 3. Compensating for missing tissue
What is the Co60 isodose curve like?
This is more forward penetrating than a kV energy, but compared to MV there's larger penumbras. Also, the 100% dose range is VERY shallow. SEE SLIDE 26
What are wedges used for?
Tissue compensation caused by a sloping surface, and/or to alter the distribution around the target volume. "Missing tissue" means that the ideal patient would be like a block of tissue. So these wedges are used to compensate for the areas where there is less tissue in regards to where the beam is going through. These trick the beam into thinking it's a flat surface. This will allow for the isodose to be pulled back up, and allow for a more uniform distribution.
What is the size of the collimated field relative to? (What is the collimated field size)
To the size of the field at the isocentre The collimated field size if the field size at the isocentre!! EG: 10x10 field: the collimators move to a certain position, so that with the divergence form the isocentre, the field is 10x10
What are enhanced dynamic wedges? (EDW)
Wedging achieved by closing the Y jaw during dose delivery. The Y jaw closes at different speeds depending on size of the wedge needed and field size. So, the effect is the same as the physical wedge because it blocks out the dose to the missing tissue area while allowing dose to the area with more tissue to still be delivered. These are available in 10/15/20/25/30/45/60 degrees.
How does the penumbra affect our dose?
Well, ideally we would want 100% of the dose in the treatment field, and then immediately after we want the dose to be 0%. But, in reality this doesn't happen because of the penumbra. So, the GEOMETRIC penumbra is the area at the edge of the field that lacks the sharpness of dose.
When does the field size = the collimator size (collimated jaw size)?
When the defined location is at the isocentre distance, and there's no shielding in the treatment field.
When will the field size NOT equal the collimator size? (2)
When the extended SSD technique is used, and when shielding is used in the treatment field
What are the 2 dimensions of the collimator field size?
Y and X; Y1, Y2 and X1, X2 -It can be symmetric (two Y jaws are the exact same, X is the exact same) or asymmetric, and it's LxW or WxL (always based on the patient; L is always from head to feet and W is always across; but they aren't necessary tied to the jaw themselves).
Can there be a combination of the thickness of the leaves?
Yes! The thinner ones will be in the centre, and the thicker ones out to the periphery.
What are 3 disadvantages of MLCs compared to blocks?
1. "Jagged" shaping; squared off ends (cerrobend would be able to curve around the field perfectly) (inbound, outbound & crossbound) 2. Not beam divergent; rounded shape in the thickness dimensions which causes penumbral effects, because the rays coming in from the medially aspect won't be attenuated as much. 4. Can experience mechanical issues
What are 4 characteristics of radiation beams?
1. Beam divergence 2. Central axis 3. Inverse square 4. Penumbra
What 2 factors determine Dmax depth?
1. Beam energy: higher energy = greater depth (BIGGEST DETERMINANT) 2. Field size
What are the 3 types of beam modifiers?
1. Bolus 2. Compensators 3. Wedges
What are some things that affect the skin sparing effect, if the same beam energy is used? (3)
1. Bolus (increases skin dose) 2. Beam coming in at an oblique angle (then the dmax is closer to the surface) 3. Shielding blocks (if they're close to the patient's surface because they cause scatter dose that comes in and hits the patient's skin and causes a reaction)
What are isodose curves affected by? (9)
1. Energy (forward penetrating VS not, and how much they attenuate) 2. Type of radiation (photon vs electron) 3. Source size (effects penumbra) 4. SDD (effects penumbra) 5. SSD (effects penumbra and %DD) 6. Shape of the patient surface (the isodose shape follows the pt curve) 7. Inhomogeneities within the patient (EG travelling through some tissues/air gap of the lungs- the air won't attenuate as much, so the rays going through that one part will travel further) 8. Flattening filter 9. Normalization point (where we push our dose to)
The amount of scatter changes with what 2 things?
1. Field size 2. Depth in the patient and if there's non-tissue areas (EG scatter in an air cavity VS scatter in tissue VS scatter with bone)
SEE YOUR EXTRA PIECE OF PAPER FOR MANUALLY CALCULATING THE EFFECTIVE F/S & DETERMINING THE EqSq!! This is probably a test question so seriously go look at it. It's in the light blue note book. But list the steps to doing these 2
1. Find the effective area, which is the current length * width of the field size minus the area of each of the collimated areas 2. Then find the new effective FS by finding the effective area/longest axis 3. Finally, find the effective equivalent square, which is = (4A)/P
What are the 5 steps to the Clarkson Method?
1. First the treatment field is divided into segments using radii 2. Each radius is measured from c/a to shielding, or to the field edge if shielding is not in the path 3. Then using a SAR (or SPR or SMR) data table, each radius is looked up and a SAR is found at the depth of interest 4. Add up all SARs and divide by the number of radii (segments) to find the average SAR 5. The field size with the same average SAR is the EqSq used in treatment calculations
What are the 3 ways we create shielding today?
1. Lead blocks (Pb) 2. Cerrobend blocks 3. MLCs (main type of shielding used today) *The first 2 are essentially obsolete today in Linacs, but are used in electron & ortho treatment.
What are the 4 planning considerations with MLCs?
1. Leakage between leaves (we use our collimator jaws & drive them in as close as we can to prevent this leakage) 2. Shaping discrepancies (the jagged shape; if you need a really conformed & non-jagged fields then you'd use a machine with a smaller width of MLCs (eg less than 0.5cm)) 3. Non-divergent MLC leaves (if there's too much dose coming in then we will overshield to bring the dose down) 4. Single plane MLC system (leaves only come in on the X-jaw, not the Y; so to get by this you turn the collimator to 90 or 270 to get them in on the Y dimension)
I'll give the energy, you give the Dmax depth: 1. Orthovoltage KV 2. Cobalt 60 (avg 1.25MV) 3. 4MV 4. 6MV 5. 10MV 6. 15MV 7. 18MV
1. Ortho: 0.0cm (at the surface) 2. Co60: 0.5cm 3. 4MV: 1cm 4. 6MV: 1.5cm 5. 10MV: 2.5cm 6. 15MV: 3.0cm 7. 18MV: 3.5cm
What are the 2 different techniques to define the field size?
1. SSD Technique: the F/S is defined at treatment SSD (at skin surface) (EG you lower the bed and set the SSD to be 100cm, and then you set the field size you want) 2. SAD technique: F/S is defined at the treatment depth (inside the patient) (EG you want 5cm depth, so the SSD would be 95cm on the pt, and then you set the field size)
What are the 3 factors that affect the size of the geometric penumbra?
1. Size of the source (larger source = larger penumbra) 2. SSD (FSD) 3. SDD (FDD)
What are the 3 reasons that the isodose lines are curved?
1. The penumbra (the dose rate changes as you go across the field) 2. Inverse square (one ray will be more divergent to get to the surface of the patient, so as you go across the field the less the dose will make it to the patient, which is why the curve goes up at the edges) 3. Scatter (there's scatter that contributes to different points, and there's more scatter to the medial points than to the lateral points of the field, so there's more dose near the centre).
What 2 things will determine what kind of scatter there is?
1. The type of radiation (lateral vs forward travelling) 2. The energy of the radiation (the higher the photon energy, the more forward directed it is and the less side scatter)
What are the 2 reasons we need to know activity and half-lifes?
1. To choose the most appropriate source for the procedure 2. When treatment with a radioactive source we calculate treatment TIME, because sources decay with time.
What are 5 advantages of MLCs over blocks?
1. Tungsten = good attenuator 2. No heavy blocks to lift 3. No storage issues 4. Quick to produce 5. Not toxic
% Shielded Example: 17cm2 of shielded is ______% of 150cm2, with the current field size being 15 x 10.
17/150 = 11.3%; 11.3% of the shortest axis (.113 x 10cm) is 1.13cm. So, the new short axis is now 8.87cm (10-1.13cm). Therefore, the original 15x10 field is now 15x9
1 HVL reduces the intensity to what %? What about 3? What about 4?
1: reduces beam intensity to 50% 3: reduces beam intensity to 12.5% 4. reduces to 6.25%
SLIDE 44- Go look at it! Why is there a big bulging penumbra with orthovoltage?
200kVp= orthovoltage range; there's the bulge because lower energies are less forward penetrating and therefore you get a lot of scatter, which is why there's bulges of the isodoses below the skin's surface.
What is the standard accepted level of transmission?
3.125%! So, shielding blocks are made with 5HVLs
