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Planning Assignment (Lung)

Target organ(s) or tissue being treated: Apex of Right Lung; Non-Hodgkins Lymphoma

Prescription: 4 Gy x 3 Fractions = 12 Gy

Organs at risk (OR) in the treatment area (list organs and desired objectives in the table below):

Organ at risk Desired objective(s) Achieved objective(s)

Bronchus

No objective in Planning

Directive;

Typically max dose 105%

Max Dose 102.5%

Lungs

No objective in Planning

Directive;

Typically mean 20Gy

Mean Dose 13.7 Gy

Spinal Cord No Hot Spots in the cord No Hot Spots in the cord

Contour all critical structures on the dataset. Place the isocenter in the center of the PTV (make

sure it isn’t in air). Create a single AP field using the lowest photon energy in your clinic. Create

a block on the AP beam with a 1.5 cm margin around the PTV. From there, apply the following

changes (one at a time) to see how the changes affect the plan (copy and paste plans or create

separate trials for each change so you can look at all of them).

**In all figures below, the shaded purple shape is the PTV and the blue area inside is the GTV.

Plan 1: Create a beam directly opposed to the original beam (PA) (assign 50/50 weighting to

each beam)

a. What does the dose distribution look like?

The dose distribution is fairly uneven in regards to covering the PTV. The 100% and

105% isodose lines have a tendency to break up thereby creating a more randomized dose

distribution rather than a more conformal and ideal looking distribution. The max dose, or

hot spot, is 116% and is located superiorly and posteriorly on the patient. This is shown by a

bright green isodose line in Figure 1 and is at the center of the crosshairs. Also seen in

Figure 1 below, there is a large hot spot of 110% superior with two additional spots located

more inferiorly on the anterior and posterior borders. This can be seen in the white isodose

lines.

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Figure 1: AP/PA of RT Lung at 6x energy level.

b. Is the PTV covered entirely by the 95% isodose line?

No, the PTV is not entirely covered by the 95% isodose line. However, 98.9% of the PTV

is covered by the 95% isodose line.

c. Where is the region of maximum dose (“hot spot”)? What is it?

The region of max dose is located superiorly and posteriorly. The maximum dose is

116% or 13.91 Gy in terms of absolute dose. As noted before, this can be seen as a green

isodose line at the location of the crosshairs in figure 1.

Plan 2: Increase the beam energy for each field to the highest photon energy available.

a. What happened to the isodose lines when you increased the beam energy?

The isodose lines were pushed superficially, thereby making the area they cover larger.

The isodose lines have less of a tendency to break apart and are more conformal and

concentric to each other. The PTV is still not completely covered by the 95% line. The

110% hot spots that were located more inferiorly on the anterior and posterior borders of the

patient have disappeared and the overall max dose has been decreased to 111.4%. There is a

larger 105% hot spot located inferiorly and more posteriorly than it’s anterior counterpart as

seen below in the sagittal view of Figure 2. They are shown by the pink isodose line.

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Figure 2: AP/PA of RT Lung at 16x energy level, evenly weighted fields.

b. Where is the region of maximum dose (“hot spot”)? Is it near the surface of the

patient? Why?

The region of max dose, 111.4%, is located at the superior and posterior portion of the

fields and PTV. In terms of anatomical structures, the region is located at the inferior of the

patient’s neck lateral to the spine. Additionally, the hot spot is not near the surface of the

patient but is located more posteriorly due to the varying densities between the lung and

tumor. The fact that the hot spot is located superiorly is due to that being the region where

the patient is thinner.

Plan 3: Adjust the weighting of the beams to try and decrease your “hot spot”.

a. What ratio of beam weighting decreases the “hot spot” the most?

The ratio of AP= .544 field weight and 253 MU while PA=.456 field weight and 221 MU

is the best ratio for this particular plan. With this weighting, the 105% pink outlined hot

spots are more evenly weighted to the anterior and posterior borders while decreasing the

110% white hot spot found in the superior region of the fields. This is best exhibited in the

sagittal view of figure 3 below. The max dose is down to 110.8% or 13.3 Gy.

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Figure 3: AP/PA of RT Lung at 16x energy, weighted fields.

b. How is the PTV coverage affected when you adjust the beam weights?

The PTV coverage is better now that the fields have been weighted to give more power to

the AP beam. There is now 99% of the PTV being covered by the 95% isodose line. The

figure below represents the problematic area that is not covered. Aside from this region, the

PTV is covered.

Figure 4: AP/PA 16x weighted fields PTV coverage by orange 95% isodose line.

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Plan 4: Using the highest photon energy available, add in a 3rd

beam to the plan (maybe a lateral

or oblique) and assign it a weight of 20%

a. When you add in the third beam, try to avoid the cord (if it is being treated with the

other 2 beams). How can you do that?

For this plan, I added in a right posterior oblique beam at a 205 degree gantry angle

(orientation of 0 degrees at the top above couch/patient). Other gantry angles were tried

so as to spare the spinal cord, however, gantry angles that are more lateral expose a

greater portion of the body and the beam goes through a larger portion of the contralateral

lung. Additionally, giving a tighter blocked margin along the cord or decreasing the jaw

along that side blocks nearly half of the PTV due to its placement so near the cord. In the

figure below, it is shown that too much of the PTV is being sacrificed. The green

structure in the figure is representing the spinal cord while the purple shape is the PTV.

Figure 5: PTV coverage with jaw movement.

When the jaw is moved so as to completely block the spinal cord, only 73% of the

PTV is covered by the 95% isodose line. If there is no jaw movement to block the cord,

then the spinal cord only receives a max dose of 12.9 Gy and 99% of the PTV is covered

by the 95% isodose line. In other cases I feel that these tactics would be helpful and cause

a bigger change to the dose distribution, however, for this patient’s PTV the cord cannot

be spared in these ways and still maintain PTV coverage. Typically, the spinal cord has a

dose limit of about 20 Gy when receiving 4 Gy per fraction. The dose of 12.9 Gy actually

being received here, while there is no jaw closure, is well within that limit and I have

therefore chosen to leave the jaw completely open.

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b. Alter the weights of the fields and see how the isodose lines change in response to the

weighting.

When the AP field is given more weight, the PA field is given less weight, and the

Oblique field remains at 20% the 110% hot spot is decreased and is only present at the

superior margin of the field thereby getting rid of the anterior and posterior hot spots. The

max dose hot spot is 110.4% or 13.2 Gy. The coverage to the PTV is very good and

appears to be more covered compared to the other plans. If the PA or Oblique fields are

given more weight over the AP field, then there are much greater hot spots posteriorly as

expected. Therefore, the best weighting of these fields is AP giving 268 MU, PA giving

108 MU, and the Oblique field giving 99 MU. In terms of field weight, this equates to AP

having 57.7%, PA having 22.3% and Oblique having 20% weight.

Figure 6: RT Lung with added Posterior Oblique beam.

c. Would wedges help even out the dose distribution? If you think so, try inserting one for

at least one beam and watch how the isodose lines change.

A wedge was placed on all three beams. A 10 degree outward wedge was placed on

the AP and PA beam while a 10 degree inward wedge was placed on the Oblique beam.

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The maximum dose increased to 113.2% or 13.6 Gy. The 110% hot spots have increased

and become present again on the inferior anterior and posterior borders of the fields. As

seen in the figure below, wedges did not help decrease the hot spots when used on the

plan including an oblique field.

Figure 7: RT Lung with added Oblique field and added wedges.

Additionally, I created a plan that uses the AP/PA 16x energy fields with wedges. I placed

the wedges at 10 degrees with the heel of the wedges at the superior portion of the fields. I

placed them in this direction to subsidize for the thinner aspect of the patient near the neck in

comparison to the body. When the wedges were applied to this plan, the isodose lines become

more concentric and cover a great deal of the PTV. There is still the same superior anterior

portion of the PTV seen in Figure 4 that is not covered, but the values 99% PTV covered by 95%

isodose still remain. Additionally, the 110% isodose lines are much smaller and only located

inferiorly on the anterior and posterior borders of the fields. The two spots are fairly evenly

weighted between anterior and posterior as well. Below are two figures showing the size of the

110% hot spot on the AP/PA wedge plan compared to the wedge plan with 3 beams.

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Figure 8: Hot spot size on AP/PA Figure 9: Hot spot size on 3 field

wedge plan. wedge plan.

Both of these measurements were taken on the same sagittal slice in their respective plans. As

you can see, the size of the hot spot in the AP/PA wedge plan is .85 cm compared to 1.95cm

distance in the 3 beam wedge plan and therefore the AP/PA plan is superior.

Which treatment plan covers the target the best? What is the “hot spot” for that plan?

The AP/PA wedged plan covers the target the best in comparison to all the plans

prepared. The hot spot for this plan is 110.5% or 13.2 Gy. The GTV is completely covered

by the 100% isodose line and thereby also covered entirely by the 95% isodose line. The

PTV receives 96% of the 100% dose and 99% of the 95% dose. While this is similar to the

outcome of other plans, the hot spot size is smaller in comparison. Also, smaller margins of

the patient are being exposed to radiation due to the angled third beam not being present.

Did you achieve the OR constraints as listed above? List them in the table above.

The constraints were met. In the planning directive prepared by the patient’s doctor, the

only constraint given was no hot spots in the spinal cord, and that was achieved. In regards

to typical dose limits for the lungs and bronchus, those constraints were met as well.

What did you gain from this planning assignment?

I gained basic planning skills in Eclipse such as inserting a new course of treatment, a

new plan, fields, blocks, and wedges. Additionally, I gained knowledge on what dose certain

structures can take and tactics to employ in order to meet those desired limits.

What will you do differently next time?

If I were to complete a similar plan in the future, I would consider not going through the

process of adding a lateral beam. If added, I would try changing the collimator angle before

moving the jaw so as to block the spinal cord more while still covering as much of the PTV

with the beam as possible. I might also make use of MLCs in order to block the cord,

decrease max dose, or move the dose distribution.