Oncol355- Lecture 4 (POPs)
T/F: with normalization, the isocentre is always placed at the isocentre.
FALSE. Sometimes the normalization point is not placed at the isocentre. This is because the normalization point should be localized in an area which best represents the scattering characteristics of the rest of the field. Sometimes where we place our isocentre isn't the best place to drive 100% of the dose to, because the scattering properties of this tissue are not true to the rest of the tissue in the field.
What is ICRU?
International Commission on Radiation Units; provides treatment planning guidelines and ensures uniformity of practice around the world
See slide 6 for a POP isodose curve. Explain what it looks like.
It has a characteristic hourglass shape, which is created by the 100% isodose curve. So, it doesn't have a dose gradient throughout the treatment volume, instead it's a lot more uniform. So, the 95% isodose is like a rectangle around the volume. There's a larger irradiated volume to a large dose. You also have a dose coming in from the top and from the bottom, and exit does coming out from the top and the bottom, which creates a high dose region that is more than the prescribed dose, at the entry points.
In eclipse what is the norm point called?
It's called the reference point.
What is the difference between imaging for midplane POP vs non-midplane POP?
Midplane: one image only on the anterior Non-midplane: orthogonal images to confirm the iso location. This is because the iso was picked for a reason in this case. However, if the patient loses weight then the iso needs to stay at that anatomical location.
What is beam weighting often like? What is 2:1 weighting?
Often the tumour is located more to one side and a better distribution through the tumour can be achieved by giving more dose from one side. 2:1 weighing means 2/3 of dose comes from one beam and 1/3 from the other.
What is midplane POP?
The centre of the patient's separation; isocentre is placed at midplane. This has equal FSDs on each side. Make sure the plane is on sup/inf and left/right. However, we don't have to worry about where isocentre is placed ant/post (ie don't have to take lateral images), because we set the isocentre with the SSDs. So on first day we would confirm that we get the same SSD on both fields (AP and PA) and then we would set couch vert for the rest of the treatment days. Iso is placed in the middle of the patient and it stays there. We modify so that iso is always in the centre.
What is equal beam weighting?
The same does is given from each mean; so, 50% of the prescribed dose is given from each beam (for a POP). For a homogenous treatment volume this gives a uniform distribution of 1:1. This gives you high dose regions at the skin with the typical hourglass shape.
POP- what do standard isodose distribution allow for?
The treatment plans are not necessary but planning POPs does allow for optimization of dose distribution.
Why aren't POPs used on breast tangents?
They look like they're directly opposing beams coming in at the chest, but often they vary by a couple degrees from each other and that's because we match beam divergences. This is the same for extremities.
Clinical aspects: What is the separation? What is it used for?
This is the patient separation at the treatment field centre. You use callipers or FSDs to measure this. This provides confirmation of treatment depths for mid-plane set ups. You can read an FSD on the anterior surface, which will tell the depth from ant to isocentre. Then you can use TTH for the lateral, which will tell you the depth from post-isocentre. Then you can add them together and this will tell you the separation.
Beam Weighting: what is the formula to find the total dose per field?
Total dose per field = (total dose/total weight) x (field weight)
What happens to MUs when you change the reference point to be the normalization point at a deeper depth than Dmax?
You have to get more MUs because you have to give more dose in order to get 100% dose to the normalization point. So, changing the normalization points will change the MUs.
The patient separation can often dictate the minimum beam energy. What is the patient separation for the following: a) Cobalt b) 6MV c) 15MV
a) Cobalt: <20cm (very thin patient) b) 6MV: 20-25cm c) 15MV: >25cm You also have to take into account tissue inhomogeneities.
What are the 2 considerations to take into account during treatment planning?
1. Beam angles- shortest route 2. Beam weighting- without affecting the other tissues, we want to drive the beam to the tumour but avoid the healthy tissue.
What are 2 factors that affect the POP hour glass shape?
1. Energy of the beam used- higher energy = less pinched in hour glass because the doses travel deeper before depositing their energy, so the beams are less interactive at isocentre. *So dose uniformity increases as energy increases*. 2. Patient separation- as the patient thickness gets bigger, the isodose pinches get squished in. *So, dose uniformity decreases as separation increases.*
The norm point should NOT be in what 4 places?
1. Near the skin surface 2. Near a shielding block (too close to shielding so the properties of the tissue don't represent what they should represent; we want them to give full representation of the scattering characteristics if we move it away from shielding blocks) 3. In an inhomogeneity (eg lung- the isocentre is where the scattering properties don't represent the scattering properties of the other tissues, so we move it to a norm point that more represents the scatter) 4. In a hot or cold spot
Compare a POP to a single field (3)
1. The dose gradient is better with a POP than with a single field. (With single field you get high dose from one direction, but with POP you spread out the dose over 2 beams, which creates a larger high dose irradiated volume) 2. Reduces dose to normal tissues- the dose is spread out 3. Larger irradiated volume
What are the 3 downsides to POP compared to single fields?
1. The skin sparing for POPs is less that single fields as the surface dose is equal to the entrance dose + exit dose, and with POPs each side has an entry and exit dose. 2. The penumbra is larger due to divergence of the opposite beam (the exiting beam). So, the exiting beam because of divergence is causing scatter & fuzzy edges on the other side. 3. The max dose occurs at the depth of build up
What do you do if there's a change to separation or depth of treatment, when you measure the separation or FSD?
1. What is the cause? 2. "Temporary" or "permanent"? (eg bladder was really full vs not so full, patient has lost weight, maybe the tumour is starting to grow) 3. What needs to happen?
Beam Weighting: what is the formula to find the daily dose per field?
Daily dose per field = (daily dose/total weight) x field weight
EG: Patient was 30cm thick at the field centre (ie separation of 30cm). This is a mid-plane set up, couch CV was set on day one and we are just checking today. What has happened if we are now reading 87cm?
Day 1: Midplane = 15 cm from A to iso and 15 cm from Post to iso. This tells us that our SSD should be 85cm. Day 15: the patient separation has decreased, because the ant to iso separation has decreased from 15 to 13, but from post to iso it's still 15cm. Some causes could be: temporarily- the patient has a full bladder and it's pushing their bladder up, the patient has lost weight, etc.
What is the hourglass shape of POPs due to?
Due to the two beams interacting with one another, creating a 100% isodose curve that looks like an hour glass.
What is normalization?
Assigning an isodose value to an isodose line (point) and changing all other isodoses proportionally. Used for single fields, POPs, and multi-fields. For SSD, set-ups are typically normalized to Dmax. For SAD, sets ups are typically normalized to the isocentre.
Why are the entry and exit doses of concern?
Because each side now gets an exit and an entrance dose, so each Dmax gets entry and exit. Whereas with single field only one side gets entry and only one side gets exit.
How does non-normalized compare to normalized?
In normalized, 100% is assigned to a more relevant point (eg midplane or a point in the tumour or to an isodose line encompassing the tumour volume). Therefore, 100% of the prescribed dose in normalization goes to this assigned location. However, this means that you can get a really high summed entrance and exit dose on each side.
What is a POP?
Parallel Opposed Pair; two treatment fields which oppose each other by 180 degrees. The angle pairs commonly used are 0 & 180 (AP/PA) and 90 & 270 (Rt/Lt Lats). Or any variations of these, they just have to be 180 degrees apart.
What technique is used for POPs?
The SAD technique, so the field size is defined at isocentre. The isocentre is usually placed at midplane, and the treatment dose is usually prescribed to the isocentre.
What is a non-midplane POP?
Referred to as an AP/PA instead because of unequal FSDs. These also have different imaging protocols when compared to non-midplane POPs. Because this isn't in the middle of the patient there must be a lateral image to make sure we set the patient up correctly ant/post. Iso is placed at a depth and it always remains at that given depth!
What would happen if in the scenario before the patient has lost weight?
Some centres will re-calculate and change the MU for a given area, some centres will move the iso which will cause CV to change. This also causes the FSD to change on each side.
Clinical aspects: What are FSD checks used for?
Used to check separation and confirm treatment depths
What happens on the first day of tx? On day two onwards?
We set up the patient, and then we acquire the CV. Day 2+: set up the patient, straighten and level, and then set CV. If the FSD has changed from day 1 then you know that something has happened.
When is POP used?
When the tumour is at a depth too great for a single field, and is used when more complex planning is not needed. Often used for palliative treatments (whole brain, pelvis, chest, etc), and for when the total dose prescribed is relatively low (eg in palliative- exception is breast tangents). They can be used at almost any body site but critical structures, exit and entry doses must be considered.
What is non-normalized?
Where 100% is assigned to each beam's Dmax. Then, variation on dose at point of interest (eg midplane) depends on energy and separation.
