Pelvis Clinical Lab Assignment Prescription: 45 Gy in 25 Fractions to the PTV Planning Directions: Place the isocenter in the center of the designated PTV (note: calculation point will be at isocenter). Create a PA field with a 0.5 cm margin around the PTV. Use the lowest beam energy available at your clinic. Apply the following changes (one at a time) as listed in each plan exercise below. After adjusting 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: Calculate the single PA beam.

• Describe the isodose distribution as it relates to PTV coverage. If a screen shot is helpful to show this, you may include it.

• Where is the hot spot and what is it? • What do you think creates the hot spot in this location?

The isodose distribution is heavily weighted towards the posterior of the patient, with a dose max at the dmax for the beam, 1.5cm for the 6MV beam, and a gradual fall off with little conformity to the PTV. The maximum dose is very high 171%, as the isocenter is within the center of the PTV, requiring the beam to go through 12cm of tissue as well as the sacrum. Figure 1. Plan 1 isodose distribution

Plan 2: Change the field to a higher energy and calculate the dose. • Describe how the isodose distribution changed. • Where is the hot spot and what is it? • What do you think creates the hot spot in this location?

The isodose distribution changed by having a more gradual falloff since the higher energy beam has less attenuation through tissue, as well as a deeper dmax. The hot spot is now 3.5cm from the posterior surface of the patient, as would be expected since that is dmax for an 18x beam. The global maximum decreased significantly to 144%. Figure 2. Plan 2 isodose distribution

Plan 3: Insert a left lateral beam with a 0.5 cm margin around the PTV. Copy and oppose the left lateral field to create a right lateral field. Use the lowest beam energy available for all 3 fields. Calculate the dose and apply equal weighting to all 3 beams.

• Describe the isodose distribution. • Where is the hot spot and what is it? • What do you think creates the hot spot in this location?

Now the isodose distribution is slightly more conformal, but has a concave shape anteriorly, where the PTV is lacking coverage. This is expected since the posterior beam and two lateral beams are equally contributing to the dose creating a cold spot anteriorly where there is no beam. The hot spot for this beam occurs on the right posterior of the patient near the midline of the PTV superior to inferior. The hot spot occurs on the right because of the central axis of the posterior beam is slightly right of midline, and also possibly because of the air within the rectum on the left side. It happens posteriorly because of the posterior beams interaction with the lateral beams. The global maximum decreased significantly however to 111.4%

Figure 3. Plan 3 isodose distribution

Plan 4: Change the 2 lateral fields to a higher energy and calculate the dose.

• Describe the impact on the isodose distribution. • Where is the hot spot and what is it? • What do you think creates the hot spot in this location?

Increasing the energy of the lateral beams decreases the concavity of the isodose distribution, and decreases the dose to the superficial tissue on either lateral side of the patient. The isodose distribution is still heavily weighted to the posterior however, and actually increases slightly in global maximum to 112.6%. Changing the energy did not change the location of the hot spot. Figure 4. Plan 4 isodose distribution

Plan 5: Increase the energy of the PA beam and calculate the dose. • What change do you see? • Where is the hot spot and what is it? • What do you think creates the hot spot in this location?

Increasing the energy of the PA beam again creates a concavity within the anterior portion of the PTV. The concave shape is even more pronounced with the higher energy beam, as the dose fall of is more gradual and therefore creates higher contribution from the posterior beam. The hot spot still has not changed in location with the higher energy since the right posterior portion of the patient is still the site where the highest dose from the 2 beams interact. (The posterior and right lateral beams). Figure 5. Plan 5 isodose distribution

Plan 6: Add the lowest angle wedge to the two lateral beams.

• What direction did you place the wedge and why? • How did it affect your isodose distribution? (To describe the wedge orientation you may

draw a picture, provide a screen shot, or describe it in relation to the patient. (e.g., Heel towards anterior of patient, heel towards head of patient..)

• Where is the hot spot and what is it? • What do you think creates the hot spot in this location?

I added a 10 degree wedge with the heel towards the posterior of the patient. The heel needs to be posterior in order to compensate for the contribution from the posterior beam. By adding the small 10 degree wedge to the lateral fields the isodose distribution becomes slightly more weighted towards the anterior. The concavity of the anterior portion is increased with visible horns formed on either lateral sides of the PTV. The central portion of the PTV is still cool, because of the amount of tissue the laterals and posterior beams needs to travel through. The hot spot is still located in the same area, but it has decreased to

108.6%. The hot spot is still in this location because the wedges are not large enough to fully compensate for the posterior beam weighting. Either the posterior beam would need to be weighted less or larger wedges would need to be used. Figure 6. Plan 6 isodose distribution with 10 degree wedges

Plan 7: Continue to add thicker wedges on both lateral beams and calculate for each wedge angle you try (when you replace a wedge on the left, replace it with the same wedge angle on the right). You may weight your fields to get a better dose distribution.

• What final wedge angles and weighting did you use? • How did each change affect the isodose distribution? • Where is the hot spot and what is it? • What do you think creates the hot spot in this location?

The final wedge angles used are 60 degree wedges with a 50% weighting from the posterior beam. As wedge angles were increased the posterior beam weighting could be increased as well. Since there is no anterior beam, large wedges allow less weighting from the lateral beams, and therefore less tissue receiving dose. This is especially true since the patient is relatively large, and therefore the lateral fields need to go through a large amount of normal tissue to reach the volume. Something to keep in mind with this arrangement however is the fact that with increased posterior beam weighting there is more exit dose to the anterior of the patient. If there was significant portions of healthy bowel in this area it would be good to avoid too large of wedges, and the higher weighting of the posterior beam that follows. The maximum dose is still in the same spot but has now lowered to 108% of the prescription. This isn’t a significant change in hot spot, but the amount of health tissue that is being treated laterally has significantly lowered. The hot spot most likely did not change, since we are using the wedges in order to allow us to weight the posterior beam higher, not necessarily to change the dose profile within the PTV.

Figure 7. Plan 7 isodose distribution with 60 degree wedges

Plan 8: Copy and oppose the PA field to create an AP field and adjust the collimators to keep a 0.5 cm margin around the PTV. Keep the lateral field arrangement. Remove any wedges that may have been used. Calculate the four fields and weight them equally. Adjust the weighting of the fields, determine which energy to use on each field, and, if wedges will be used, determine which angle is best. Evaluate your plan in every slice throughout your planning volume. Discuss your plan with your preceptor and adjust it based on their input. Normalize your final plan so that 95% of the PTV is receiving 100% of the dose.

• What energy(ies) did you decide on and why? • What is the final weighting of your plan? • Did you use wedges? Why or why not? • Where is the region of maximum dose (“hot spot”) and what is it? • What do you think caused the hot spot in this location? • What is the purpose of normalizing plans? • What impact did you see after normalization? Why? • Embed a screen cap of your final plan’s isodose distributions in the axial, sagittal and

coronal views. Show the PTV and any OAR’s. • Include a final DVH. Be sure to include clear labels on each image. • If you were treating this patient to 45 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.

I chose to use all 18 MV beams since the volume is so large and the higher energy beams give a more gradual fall off of dose as well as increased skin sparing effect. Since the volume is midline within the patient and there is significant tissue to travel through; the higher energy

allows for less attenuation, especially for the posterior beam, which must travel through the sacrum. The only reason I would change this would be if the physician was concerned about bowel dose anterior to the superior portion of the volume. It would be beneficial in this case to use a lower energy for the posterior beam. The final weighting of the plan gives the highest weighting to the posterior beam, which would be expected since the volume is largely on the posterior portion of the patient. The other beams are almost equally weighted, with just slightly more weight on the right lateral beam, where again there is slightly less tissue to travel through. I did not use wedges in the plan as there was no real constraint that was close to tolerance. If necessary I could have used wedges to weight any one of the beams less heavily thereby avoiding a structure. I think that if I was to do anything, I might use field in field on some of the beams in order to try to lower the hot spot. Especially within the rectum and bowel. The hot spot is located on the left posterior of the volume. The hot spot is very similar in each of the four corners of the target volume in the transverse plane. The hot spot is in this location because the volume is so large that even with the highest energy available, the dose fall off is significant throughout the volume. The only way to maybe avoid this and make the dose more conformal would be to use dynamic arcs to more even distribute the dose and cause less of a hot spot along the border of the volume. Normalizing the plan to 95% coverage is a way to ensure that 95% of the target volume is receiving 100% of the dose. Normalizing not only allows for a means of plan comparison, but also a way to increase or decrease the monitor units from the linear accelerator in order to ensure the target volume receives the necessary dose to effectively treat the tumor. After normalizing the plan, the entire dose distribution increased in proportion to the difference between 95% of the target volume receiving 100% of the prescription and the percent of the target volume receiving 100% of the prescription before normalization. In other words if the volume is covered by the 95% isodose line and we normalize the plan so that 95% of the volumes is receiving 100% of the dose we are making the plan 5% hotter.

Figure 8. Final isodose distribution normalized with OARs

Figure 9. Dose volume histogram for final plan

Table 1. Organs at risk with tolerances and outcome

Organ at Risk (OAR) Desired Planning Objective Planning Objective Outcome Rectum V50 <50% Max=4814cGy Femurs Max<4500cGy Max=4614cGy; Not Met

Possible Hip Fracture; Less than 4500 in surgical neck

Bowel Space V45<195cc 238cc receiving 4500cGy; Possibility for Grade 3+ Toxicity(distention, diarrhea requiring parental support, and blood discharge) Overlap of Bowel and PTV=160cc (Would make this constraint very difficult to meet)

Bladder V65<50Gy Max=4806cGy All constraints are from Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC)1

References

1. Marks LB, Yorke ED, Jackson A, et al. Use of normal tissue complication probability

models in the clinic. International Journal of Radiation Oncology. 2010;76(3):S10-S19.