Lung Clinical Lab AssignmentPrescription: 60 Gy in 30 fractions to the PTV

Planning Directions: Place the isocenter in the center of the designated PTV (make sure it isn’t in air). Create a single AP field using the lowest photon energy in your clinic. Create an MLC block on the AP beam with a uniform 1.5 cm margin around the PTV. Apply the following changes (one at a time) as listed in each plan exercise below. After making the adjustments requested for each plan, answer the provided questions. Tip: Copy and paste each plan after making the requested changes so you can compare all of them as needed.

Plan 1: Create a beam directly opposed to the original beam (PA). Assign equal (50/50) weighting to each beam. What does the dose distribution look like? o The dose distribution appears to be in an hourglass shape in regards to the 95% line at isocenter. The

PTV seems covered decently well, however the MIN for the PTV is low at 89.2%. The entrance and exit points of both fields are hot, but dose does cool down a bit in the lungs.

Is the PTV covered entirely by the 95% isodose line? o No. In this plan roughly 84.1% of the PTV is receiving 95% relative dose. Throughout the PTV, the lateral

portion of the PTV is consistently outside of the 95% line, but inside the 90% line. Some lateral superior and lateral inferior portions of the PTV also fall outside the 95% isodose line. Relating to the hourglass shape of the distribution, the anterior and posterior portions of the distribution are completely encased in the 95% line, with many points being much hotter.

Where is the region of maximum dose (“hot spot”) and what is it? o The hottest region in this plan is in the posterior region of the plan. In this area, the 110% isodose

encompasses back muscles and fat, skin, and small portions of the rib and lung. The hot spot is in back muscle at 115.2%.

What do you think caused the hot spot in this location?o The hot spot is here due to a number of reasons. Being opposed AP/PA beams with the same MLC

margin around the PTV, this spot is unblocked from both beams. Additionally, a 6MV beam has about a 1.5cm dmax depth. The build-up region in the tissue causes the hot spot to fall here. Thirdly, the hot spot falls in a plane including the inferior portion of the PTV. On this slice, the PTV does not extend laterally as much, resulting in radiation from the AP beam to only travel through lung, where radiation falls off less.

--The left image shows the window at isocenter, while the image on the right shows global dose max --

Plan 2: Adjust the weighting of the beams to try and decrease your “hot spot.” What ratio of beam weighting decreases the “hot spot” the most?o AP/PA weighting of (53.1/46.9) decreased the hot spot the most to 112.9%. The hot spot also is now in

the anterior chest, while the posterior chest still has a spot at 112.9%. The maximum dose region of at least 110% is evenly split between anterior and posterior portions of the field.

How is the PTV coverage affected when you adjust the beam weights?o The PTV coverage is ever so slightly better with the AP beam being weighted heavier. At the new

weighting of (53.1/46.9) vs (50/50) the MAX for PTV increases .1% to 102.1%, the MIN for PTV increases .2% to 89.4%, and the MEAN for PTV increases .1% to 97.7%. According to the DVH, roughly 85.1% of the PTV is receiving 95% isodose. With a heavy weight on the AP beam (65/35) PTV coverage can be increased at the cost of creating a very large hot spot >120%.

--The top image shows the window at isocenter, while the image on the bottom shows global dose max --

Plan 3: Add a 3rd beam to the plan (a lateral or oblique) and assign it a weight of 20%. Try to avoid the spinal cord. How did you decide on the location of your 3rd beam? o After evaluating plans with gantry angles of 72, 90, 135, 113, and 97, I decided on an LPO at a gantry

angle of 97. Since the AP beam is weighted higher than the PA, and a constant weight of 20% for the 3rd field, my 95% line was slightly anterior and lacking posterior at all angles attempted. At gantry angles of 72, 113, and 135, the PTV coverage was similar to 97 but gave excess radiation to the anterior L lung. I chose a collimator angle of 0 so I could ellipse fit the MLC around the PTV with a slightly smaller margin anteriorly to spare lung. The X1 jaw was fit with a .75cm margin and the others were fit with a 1cm margin. At this beam location, I was able completely block the spinal cord, partially block the superior heart, increase lung sparing, and keep all significant hot spots within the PTV.

Did you do anything special to avoid the spinal cord? (ie. Adjust the gantry angle, tighter blocked margin, decrease the jaw alongside of the cord) o All gantry angles attempted primarily avoided the spinal cord. If a beam marginally clipped the cord, I

would have used jaws and/or MLC to block the cord. Adjusting the collimator angle is another option. What does the dose distribution look like? o Visually, the dose distribution conforms fairly well to the PTV. The 95% line is primarily lacking in the

very superior portion of the PTV. The MIN for PTV is found here at a low 88.3% isodose. There are other portions of the PTV that fall out of 95% line on the edges of the PTV laterally and posteriorly, but these appear insignificant. Additionally, the anterior and posterior portions of the body that were covered by the 100% line on the AP/PA plan have been reduced to in between 80% and 90% isodose.

Is the PTV covered entirely by the 95% isodose line?o No. According to the DVH, roughly 84.6% of the PTV is receiving 95% isodose. Adding the 3rd beam at

20% weighting changed the weighting to (42.5/37.5/20) AP/PA/LPO. The MAX, MIN, and MEAN for PTV are similar to the AP/PA plan, at 103.1%, 88.3%, and 97.7% respectively.

Where is the region of maximum dose (“hot spot”) and what is it? o The hottest region of the plan is consistently within the PTV and the 100% line does not extend outside

the PTV. The 100% isodose line is most present in the inferior portion of the PTV. The hot spot was reduced to 103.1% from 112.9%.

What do you think caused the hot spot in this location?o Giving 20% dose to a lateral beam removed the hotspots from the exterior body because the AP and PA

beams had a much lower entrance and exit dose. The hotter region of the PTV is most likely inferior due to the beams passing through more normal tissue superiorly.

--The left image shows the window at isocenter, while the right image shows the BEV of Field 3, the LPO. Notice the shaping of the MLC on the anterior side and the off cord position of the field --

Plan 4: Alter the weights of the fields and see how the isodose lines change in response to the weighting. Describe the ideal beam weighting to achieve the best PTV coverage. o To strictly focus on PTV coverage, I ignored dose to other structures when working on this plan 4. After

playing with dozens of different weight combinations with the above beam geometry, I found that the only way to increase overall PTV coverage was to lower the LPO weight and increase the AP beam weight. To allow the 95% and 100% isodose lines to reach maximum coverage of the PTV, the best AP/PA/LPO weight ratio is (48.4/33.2/18.4). With this weighting, I was able to reach 86.0% PTV coverage with the 95% line and 21% PTV coverage with the 100% line. The MIN for PTV increased from 88.3% to 88.6%. In another plan, I changed the weighting to (50.3/41.7/8) which did increase PTV coverage depending on what you are looking for. This plan raised the MIN for PTV to 89.5%. By increasing the PTV coverage of the 95% line to 86.6% with this plan, the 100% decreased to cover 18.2% of the PTV. Both plans could not correct the lack of superior coverage from before.

Where is the region of maximum dose (“hot spot”) and what is it? o The hot region of the plan remained mostly unchanged, and remains within the inferior portion of the

PTV. The 100% isodose line only extends outside the PTV on a couple slices. The hotspot slightly increased to 103.2%.

What do you think caused the hot spot in this location?o By keeping significant weight in the LPO, the hotspot remained within the PTV and out of the chest wall

or back. As you remove weight from the LPO, the hotspot will begin to re-appear in the chest and back, depending on where you increase weight.-- The left image shows planes at isocenter, while the image on the right shows planes at global dose

maximum. Only the 95% and 100% lines are shown to show emphasis on PTV coverage --

Plan 5: Try inserting wedges for at least one or more beams to improve PTV coverage. List the wedge(s) used and the orientation in relation to the patient.o On the AP beam I added the wedge EDW10OUT. I changed to collimator to 90 so the heel is medial. On

the LPO at a 97 degree gantry angle, I added an EDW20IN. The heel is inferior. Describe how the isodose lines changed.o The isodose lines were adjusted to slightly push dose away from the anterior medial region, and to add

dose to the superior portion of the PTV. The adjustments were slight, but 95% PTV coverage increased from 86.0% to 88.1%. Additionally, the MIN for PTV increased from 88.6% to 89.8%. The chest wall has small portions in the center of the field that reach 95%.

Where is the region of maximum dose (“hot spot”) and what is it? o The inferior region of the PTV is still consistently the region receiving the maximum dose. The hot spot

was reduced to 102.9% What do you think caused the hot spot in this location?o The wedges were not drastic enough to entirely move the hot spot from the previous plans. Instead,

they were aimed at keeping the hot spots inside the PTV, but shrinking the range between the PTV MAX and MIN. -- The images on the left show isocenter and the images on the right show global dose maximum --

Plan 6: At this point you may adjust the beam energy, beam weighting, and/or wedges to achieve the best PTV coverage. Once you have obtained the best plan possible, normalize your final plan so that 95% of the PTV is receiving 100% of the dose. What energy(ies) did you use and why?o I used energies of 10mV for the AP and PA beams and a 6mV beam for the LPO at 97 degrees. The PA

and LPO have wedges, and the AP does not. The PA beam has a 30 degree EDW and collimator angle to push dose superiorly. The LPO now has a 20 degree EDW and angle to counteract the angle of the body tissue and push dose posteriorly. The 105% line is shaped very well to the ITV.

What is the final weighting of your plan?o The AP/PA/LPO weighting is now (41.3/34.3/24.4). By increasing the weight of the 6mV lateral beam, I

was able to improve overall isodose distribution and significantly decrease dose to the chest wall and back.

Where is the region of maximum dose (“hot spot”) and what is it?o The hot spot is located centrally in the PTV and is at 108.4%. The hottest region of the plan as marked

by the 105% isodose line remains in the PTV. What do you think caused the hot spot in this location?o The use of the wedges on the PA and LPO beams allowed me to centralize dose in the PTV and keep the

hotter dose region from extending inferior. The 10mV beams allowed me to reduce hot spots in the chest wall and back due to the increased buildup region when compared to 6mV.

What impact did you see after normalization? Why?o Normalization greatly increased dose to the PTV, while slightly increasing dose to the critical structures

in the area. Prior to normalization, 8.0% of the PTV was receiving 100% of the dose with the MAX, MIN, and MEAN respectively at 100.9%, 89.7%, and 97%. When normalized, these values increased to 108.4%, 96.4%, and 104.2%, respectively. The mean Left lung dose increased by about 3.5%, with other critical structures had little to no change in dose.

Embed a screen cap of your final plan’s isodose distribution in the axial, sagittal and coronal views. Show the PTV and any OAR’s.

--Top images show isodose at isocenter, bottom images show isodose at global dose maximum-- If this patient is treated to 60 Gy, use the table below to list typical organs at risk, critical planning

objectives, and the achieved outcome. Please provide a reference for your planning objectives.

Organ at Risk (OAR) Desired Planning Objective (Gy) @ (2Gy/Fx)

Planning Objective Outcome (Gy)

Reference

Lungs (mean) 20.00 18.76 RTOG 0623Lungs (V20) <37% 36.8% RTOG 0623Esophagus (mean) 35.00 3.66 RTOG 0920Esophagus (V45) < 33% 0 RTOG 0920Spinal Cord (mean) 45.00 .65 RTOG 0623Spinal Cord (V48) 0.03 0 RTOG 0619Heart (mean) 40.00 1.6 RTOG 0623Heart ( V60) <33% 0 RTOG 0623

Include a final screen capture of your DVH and embed it within this assignment. Make it big enough to see (use a full page if needed). Be sure to provide clear labels on the DVH of each structure versus including a legend. *Tip: Import the screen capture into the Paint program and add labels.