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 isodose distribution displayed for a 6MV plan treated with equal

weighted AP/PA fields, looks like an hourglass shape. The lines start out wider near the surface of the patient and narrow as you get closer to the patient center. There are hotter regions near the anterior and posterior surfaces of the patient.

Is the PTV covered entirely by the 95% isodose line?o The PTV is not entirely covered by the 95% isodose line. Currently, the 90% is

covering almost all of the PTV. Where is the region of maximum dose (“hot spot”) and what is it?

o The hot spot for this plan is located near the posterior surface of the patient with a dose of 6927.3cGy.

What do you think caused the hot spot in this location?o The hot spot is located in this region because of the low energy of the beam.

The 6MV beam has a Dmax of 1.5cm, meaning that 100% of the dose is deposited at a 1.5cm depth, after which the dose will decrease with increasing depth.

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 The weighting that decreases the hot spot the most is the PA beam weighted 47.6%, and the AP beam weighted 52.4%. This gives a max dose of only 6818.7cGy.

How is the PTV coverage affected when you adjust the beam weights?o The PTV coverage increased slightly with the adjusted weighting. The coverage

increased from 20.63% to 22.51% of the PTV receiving 60Gy.

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 I chose a lateral beam for my 3rd field. My goal was to avoid going through the

spinal cord and only giving exit dose to the heart. 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 I adjusted the gantry angle of my 3rd beam to a straight lateral and checked the

blocks to ensure that the cord was avoided. What does the dose distribution look like?

o The isodose lines are starting to look more conformal around the target volume, forming a tighter square around the PTV. The patient surfaces that were previously hot now have lower doses. The image below portrays these differences in isodose distribution at isocenter. Note: AP/PA plan is portrayed on the left and 3-field plan is portrayed on the right.

o

Is the PTV covered entirely by the 95% isodose line?o No, the 95% isodose line is not covering the PTV entirely. The DVH shows that

only 92.86% of the PTV is receiving 5700cGy. Where is the region of maximum dose (“hot spot”) and what is it?

o The hot spot is now located within the target volume at 6274.3cGy. What do you think caused the hot spot in this location?

o The addition of the 3rd beam changed the location of the hotspot by evening out the dose distribution.

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 By adjusting the beam weighting, it was found that decreasing the weight of

the PA beam produces the greatest increase in PTV coverage. I ended up removing the PA beam completely and adjusted the weights of the remaining two beams. The PTV coverage went from 38.39% to 55.44% receiving full prescription dose. The weighting on the AP and LT LAT beam only varied in coverage by a couple percent, so I just focused on reducing hot areas near the patient surface. The final weighting I chose was an AP beam of 53% and LT LAT beam of 47%.

Where is the region of maximum dose (“hot spot”) and what is it? o The hot spot is now within the lung in the left upper corner where the two

beams converge. The hot spot reads 6900cGy. o

What do you think caused the hot spot in this location?o This hot spot location makes sense because of the intersection of the two

beams at this location.

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 I chose to use wedges for both beam angles. The AP beam has a 30° wedge with the heel pointing to the patient’s left, and the LT LAT beam has a 55° wedge with the heel pointing toward the anterior surface of the patient. The

wedge heels were positioned in this fashion to cool off the hot spot and spread the dose out.

Describe how the isodose lines changed.o There are no longer 110% isodose lines in the plan. The PTV coverage also

increased with the 6000cGy line covering a more uniform portion of the PTV. The addition of wedges also increased the coverage of the PTV from 84.56% to the 96% being covered by 5700cGy isodose line.

Where is the region of maximum dose (“hot spot”) and what is it? o The hot spot for this plan is located in the medial and inferior portion of the

PTV and is 6392.1cGy. What do you think caused the hot spot in this location?

o I think that the weighting and arrangement of the wedges led to the location of the hot spot. More dose is able to pass through the thinner portions of the wedge.

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 tried to change some of my beams to 18MV to cool off the plan; however I

lost coverage on my PTV. Therefore, I kept the beam energy of 6MV. What is the final weighting of your plan?

o For my final plan, I was still lacking coverage posteriorly, so I reintroduced the PA beam. The final weighting of my beams was 38% for the AP beam, 38% for the LT LAT, and 24% for the PA beam. This makes sense that the PA beam has a lesser weighting because from previous trials it was found that it does not contribute much to the overall PTV coverage.

Where is the region of maximum dose (“hot spot”) and what is it?o The hot spot is located within the inferior portion of the PTV with a reading

of 6660cGy. What do you think caused the hot spot in this location?

o The effect of manipulating the wedge angles and beam weighting caused the hot spot to occur in this location. For example, if I were to increase the weight of the PA beam, the hotspot would move more posterior and outside of PTV.

What impact did you see after normalization? Why?o In order to increase PTV coverage, I normalized the plan to the 95% isodose

line. This really heated up the entire plan creating a large area of 105%, and a couple of small spots of 110%. However, the coverage of the PTV is greatly affected in comparison to normalizing to the 100% isodose line. The difference was an increase of 30.8% in PTV coverage receiving 60Gy.

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.

o The OAR’s and PTV are displayed in the images below. The plan has 99.6% of the PTV being covered by the 95% isodose line. And 95.28% of the PTV covered by the 100% isodose line. In discussing further planning options with my preceptor, it was suggested that a different planning method other than 3D conformal be used to achieve the same coverage, and avoid 110% regions:

o

o

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 Planning Objective OutcomeHeart V25<10% (QUANTEC)1 0.60% receiving 25GyEsophagus Dmean< 34Gy (QUANTEC) 1 659.2cGy =DmeanLungs-GTV Dmean <20Gy (QUANTEC) 1 1697.8cGy = DmeanCord Dmax<50Gy (QUANTEC) 1 356.4cGy = DmaxTrachea Data not listed for standard

fractionation

Include a final screen capture of your DVH and embed it within this assignment.

References:

1. Radiation Oncology/Toxicity/QUANTEC. Wikibooks. https://en.wikibooks.org/wiki/Radiation_Oncology/Toxicity/QUANTEC. Edited September 23, 2015. Accessed January 22, 2018.