1. Comparison between VMAT and VMAT with 3D AP/PA lung sparing in Stage IIIA Mediastinal Lung Cancer Case By
Nathaniel Miles

2. Patient History Rectal cancer in 1999 Bladder Cancer in 2015
Treated with chemoradiotherapy and surgery
Bladder Cancer in 2015
Treated with 4 cycles of MVAC chemotherapy
Lung Cancer in right upper lobe of lung found in 2016
Treated with SBRT
Tobacco use for last 55 years
Approximately 1 pack per day
Continues to smoke
The patient we are discussing today is a 76 year old female
She worked for the casinos for over 30 years and is now retired
She has smoked approximately 1 pack per day for the last 55 years and continues to smoke despite being on oxygen
She has a recurrence of a stage III non-small cell carcinoma which has metastasized to her subcarinal nodes
She has had three cancers including rectal cancer in 1999 and bladder cancer in 2015 which appear to be remission
She has had previous SBRT to a nodule in her right upper lobe in 2016
The CT scan from her previous treatment has been registered with the current CT and the total composite dose will need to be under constraint tolerances
The previous PTV structure was transferred and would be avoided with entrance dose if necessary
She also has a suspected brain metastasis that will be not currently be treated, but will be watched

3. Patient Presentation Emergency room with chest pain
Patient had atrial fibrillation
Asymptomatic
Pain radiates to anterior neck and jaw
Pain resolved after a few hours
Patient was sent for a CT and chest angiogram
The patient initially presented months ago to the ER with chest pain
The pain radiated to anterior neck and jaw
The pain resolved itself after a few hours
The patient was given a myocardial perfusion imaging test which caused atrial fibrillation
Subsequent stress tests were cancelled and patient was given a chest CT and angiogram

4. Imaging and Diagnosis
Angiogram was negative for pulmonary embolus but did show significant coronary artery calcification
CT scan showed enlarged precarinal lymph node
Biopsy of enlarged precarinal lymph node performed
Positive for non-small cell carcinoma
PET performed
Increased uptake in precarinal lymph node SUV 6.93
Brain MRI performed
Showed possible metastatic deposit
Left paramedian posterior frontal lobe
Angiogram was negative for pulmonary embolism but did show significant coronary artery calcification
CT also showed precarinal lymph node
Biopsy of the precarinal lymph node was performed
The node was positive for non-small cell carcinoma, which had recurred from her previous SBRT
A PET scan was also performed which showed increased uptake in precarinal lymph node, but no uptake in previous SBRT site
Finally a Brain MRI was performed looking for any brain mets as this is a common site for metastatic migration of lung tumors
A possible metastatic deposit was found in the left paramedian posterior frontal lobe, but will only be monitored for now and not treated

5. Location and Extent of Tumor
PTV is 3.9cm x 2.6cm x 4.6cm
The tumor is located in the patient’s mediastinum just inferior to the carina.
The tumor measures 3.9 X 2.6 X 4.6 cm and begins inferiorly at the aortic arch and superiorly at the junction of the right and left brachiocephalic veins into the superior vena cava
The GTV covers the entirety of the enlarged precarinal node
A 1cm margin was placed on the GTV to form the CTV and a 0.4cm Margin was placed on the CTV to from the PTV

6. Treatment Plan and Prescription
Prescription is 200cGy daily for 30 fractions
Total of 60 Gy
Two plans are generated
3D plan using AP/PA beams (avoiding lung)
IMRT plan with 2 full arcs
Two separate prescriptions written
3D and IMRT prescription
Change number of fractions between 2 plans
Example: 10 fractions of 3D and 20 Fractions of IMRT
Doctor wrote 2 prescriptions
One 3D plan and one IMRT
1 Plan will be generated 3D and 1 will be generated IMRT
The plans will be summed and then the fractions will be changed to balance spinal cord and total lung dose
The doctor prefers a 10 fraction 3d to be treated first followed by 20 fractions of IMRT to be treated next
Both prescriptions will be 200cGy just the number of fractions will change depending on how the plans look

7. Planning Techniques 3D
The 3d plan will be just a simple AP/PA beam setup
A 15 MV beam was used posteriorly and a 10 MV beam was used anteriorly. This is due to the anterior position of the tumor and the need to shoot through the spine
The MLC block was created using a 0.7cm margin
The beams were weighted to reduce the hot spot as much as possible and equalize the hot spot anterior and posteriorly.

8. Planning Techniques IMRT
The VMAT plan used 2 full arcs with 6 MV energy. A 0.5 Margin was placed on the PTV in order to set the jaw size. Jaw tracking was also used during optimization to further reduce the leakage through the leaves.
The beams were collimated at 15 and 345 so that the leakage through the leaves is not in a single plane

9. Organs at Risk and Contouring
The organs that were at risk for this treatment included the spinal cord, for which the whole spinal canal was drawn and the canal was expanded with a 0.5cm margin to ensure the constraint would be reached even with patient movement.
The ribs and great vessels were contoured in order to ensure that the dose to these structures was not exceeded especially since these structures received large amounts of dose from the previous SBRT treatment
The other organs that were at risk and contoured include the heart, lungs, and esophagus
The CT scan from the previous SBRT treatment was registered to this current CT scan and the previous PTV was brought over in order to track dose to the previously treated volume and possibly spare lung tissue around the volume
Oral contrast was used in order to better visualize the esophagus and this contrast was contoured and the CT value was changed to regular esophagus houndsfield unit

10. Optimizer Constraints IMRT- Planning Objectives
The plan was optimized to reduce dose to the lungs, esophagus, expanded cord, and previously treated great vessels.
A constraint was placed on the previous ptv in order to minimize dose to previously treated volume.
Finally the 115% isodose line after the fist iteration was made into a volume and a constraint was placed on this volume in order to try to move the hot spot away from the great vessel and possibly reduce the overall hot spot

11. Plan normalization Plans normalized the same Did not base dose plan
100% of the dose covers 95% of PTV
Did not base dose plan
Plans are run nonconcurrently
First 3D fractions are given then IMRT fractions
Both plans were normalized in order to have 95% of the PTV volume receiving 100% of the prescription dose
The IMRT plan needed to be normalized up 1.2% in order to achieve this PTV coverage
The 3D portion of the plan was not used as a base dose for the IMRT portion as the plans will not be treated concurrently
This prevents portions of the ptv receiving less than tumor lethal dose on a daily basis as the IMRT tries to make up for hot spots in the 3D plan

12. Coverage 3D
The 3D plan coverage has a large amount of high and intermediate dose with full dose covering the entire tissue from anterior to posterior.
Hot spots are near the beam entrances and within the great vessels especially superiorly
The anterior beam is weighted heavier in order to keep the hot spot from the being within the spinal canal
The dose is kept out of the lungs, but the sacrifice is in the larger amounts of high and intermediate dose as well as the high amounts of dose within the spinal canal

13. Coverage IMRT
The IMRT coverage is much more conformal around the tumor with the 100% isodose lines staying tight around the PTV and dose dropping off in rings around this structure.
The only negative within this plan is that there is quite a bit of low an intermediate dose within the lungs

14. Plan Comparison 3D fractions = Spinal Dose
VMAT alone is compared to VMAT with 3D for up to 10 fractions
Lung Dose
3D fractions =
Spinal Dose
The reason that the physician likes to use some fractions of 3D AP/PA instead of purely IMRT is because he wants to lower the overall lung dose in order to spare some lung tissue for the patient
The patient is already on oxygen and has issues with COPD, the doctor is worried that a high V20 might damage the small amount of healthy lung that the patient has left
Therefore he uses some 3D, despite the large amount of high and intermediate dose, in order to spare the lungs
As he does so, it is important to keep an eye on the spinal canal dose. As you increase the amount of 3D fractions the spinal cord dose will increase

15. VMAT alone
Here is an isodose distribution if the patient was treated with VMAT alone.
As you can see the dose distribution is highly conformal, but the lung dose is substantial.

16. VMAT with 3D
Here is the isodose distribution with 10 fractions of 3D as you can see there is a larger amount of intermediate dose and spinal cord dose, but there is far less 1800 within the lungs. The doctor prefers this in this instance since the patient has so many breathing issues.

17. Treatment Outcomes Current Plan
Here are the treatment outcome of the 2 plans
As you can see both plans meet the constraints and are overall very similar.
The only difference being that with VMAT alone the V20 is 19.3% and with 10 fractions of 3D we are able to lower this to 14.3% of the lung
This does however come at a cost and the spinal cord dose increases from a maximum of 17.9Gy to almost 33.2Gy. This is still within the limits of QUANTEC, which give a constraint of 50Gy, but is a significant increase
To the physician this increase in spinal cord dose is worth it in order to spare this patient the intermediate lung dose that may cause further inflammation and worsening of her COPD

18. Treatment Outcomes Composite Plan with Previous SBRT
This is not the patient’s first irradiation of this location however and in order to get a better picture of her overall dose a composite was made with the previous SBRT treatment. As you can see the lung dose increases 6%, and this is not taking into account the EQD2 of the previous treatment. With this taken into account the VMAT plan alone exceeds 30%, and the only option becomes the VMAT with 10 fractions of 3D.
Another constaint that came very close to being exceeded was the composite dose to the great vessels. A paper on reirradiation of aortic vessels states that the maximum dose after irradiation to the great vessels cannot exceed 120Gy. After the calculating the EQD2 from the SBRT treatment the maximum dose to the great vessel was found to be Gy when combined with the 6494 dose from the current plan the total becomes a whapping Gy, coming very close to the constraint.
This becomes slightly less scary when looking at the actual dose to the specific section of the great vessels in the previous plan compared with the portion of the great vessels receiving the maximum dose from the current plan. After calculating the EQD2 on that portion of the great vessels it appears that the highest dose to the great vessels is nearer to 90Gy. Still a high dose and giving the patient a risk of aneurism. Especially in a patient that already has had pulmonary heart problems

19. Final treatment choice
Works to spare low and intermediate lung dose while sacrificing dose to spinal cord
Either plan would have met constraints
Doctor chose IMRT with 3D
In the end the physician decided to use the 10 fractions of 3D treated first followed by 20 fractions of IMRT
Either plan would have met the constraints, but with the composite lung dose being exactly at the 30% mark and the patient’s current issues with COPD the doctor thought it would be best to provide the most lung sparing possible

20. Conclusions Might be necessary to use 3D in order to avoid structure
Other ill effects
More low dose and intermediated dose
There may be other ways to accomplish
Don’t have to use full arcs
Static field IMRT
This treatment provides a good example of how 3D or static beams might be necessary in order to completely avoid putting dose through a certain organ at risk. This is especially true on retreats where a patient may have already received the maximum dose to this structure or there are other concerns with the patient to take into consideration
This technique may not have worked however if there had been a previous treatment near the spinal cord or the spinal cord was already near tolerance. This would have made the added dose to the spinal cord from the 3D plan excessive
That being said there may have been other ways to accomplish this including not using full arcs or the use of avoidance sectors on the VMAT plan, or by using a static field IMRT where the lung was avoided, or better yet the healthy lung was specifically avoided in some way.
Finally there are other things to look at beyond the dose constraints to the organs at risk, including overall intermediate and high dose being given by the 3D plan. As you can see the intermediate and low dose on the VMAT with 3D treatment plan is higher than the VMAT alone
Another thing to consider is that the biological damage that is incurred during the first ten fractions might not allow the healthy tissue to repair itself. For this reason it might be a better idea to treat the 3D fractions intermixed with 1 day IMRT followed by 1 day 3D