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. PTV coverage is very poor. The anterior portion of the PTV is not even full encompassed

by the 70% isodose. The 100% line bisects the PTV at its approximate half-way point. There is also an approximately 10 centimeter deep zone of isodose from the posterior aspect of the body that represents dose of at least 110%. According to the DVH, 48.4% of the PTV volume is receiving 100% of the dose.

Where is the hot spot and what is it? The hot spot is 171.3% and is located in the extreme posterior aspect of the patient. It

resides superior within the field. What do you think creates the hot spot in this location?

When using a single beam, the maximum dose will be located around the expected DMAX depth of the energy that you are using. For 6MV this depth is 1.5 cm, and in the plan the hot spot is located at an approximate depth of 1.4 cm.

Plan 2: Change the field to a higher energy and calculate the dose. Describe how the isodose distribution changed. The lower isodose lines are significantly deeper. The PTV is now encased easily by the

70% line, and the 50% reaches to the patients anterior. The higher isodose lines are only marginally deeper. The percentage of PTV receiving 100% of the dose increased marginally to 51.7%. The 110% isodose extends slightly less deep at 9.7 cm instead of 10cm.

Where is the hot spot and what is it? The hot spot is now 145.3%. It is still located in the posterior aspect of the patient and is

slightly superior in the field, but is located deeper than the hot spot in the 6MV plan. What do you think creates the hot spot in this location? As stated previously, when using a single beam, the maximum dose will be located

around the expected DMAX depth of the energy that you are using. For a 10 MV beam the DMAX depth is 2.5 cm. The hot spot is located at an approximate depth of 2.52 cm.

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. The distribution has become much more conformal as compared to the single beam

plan, but still has plenty of issues. The isodose lines create a box-like area in the center of the pelvis, with extension to each beam entry point. The left and right sides of the patient have ovals of higher isodose levels ranging from 90%-105% near the skin surface. The 70% line encompasses the full PTV. According to the DVH, the 90% line encompasses approximately 93% of the volume, and the 100% covers approximately 49% of the PTV volume. The 110% isodose has been significantly lessened, and no longer reached to the posterior skin surface.

Where is the hot spot and what is it? The hot spot is 113.7% and is located in the posterior aspect of the patient. It lies to the

far right corner of the field. What do you think creates the hot spot in this location? With no AP contribution, it makes sense that the hottest area of the plan would be

posterior. The hot spot could be located to the right due to the patient’s body habitus. With equal weighting it appears that more of the dose is concentrated to the right side within the boxed section of the pelvis.

Plan 4: Change the 2 lateral fields to a higher energy and calculate the dose. Describe the impact on the isodose distribution. The largest apparent difference is the loss of the higher isodose levels in the left and

right laterals of the patient. The highest isodose reaching the skin surface laterally is the 70% line. The 90% isodose encompasses more of the PTV anteriorly and now covers approximately 97.4% of the PTV volume according to the DVH. The 100% isodose looks more like a straight line anteriorly, rather than the appearance of having lateral horns that it had in the previous plan. Coverage has only marginally increased with 100% of

the dose covering 50.4% of the PTV volume. The 110% isodose has increased its coverage area as compared to the previous plan.

Where is the hot spot and what is it? The hot spot is 113.6% and is located in the same posterior and right location as notated

in the previous plan. What do you think creates the hot spot in this location? As previously stated, the hot spot is located posteriorly due to the lack of AP

contribution. The hot spot could be located to the right due to the patient’s body habitus. Even with the changes in energy, with equal weighting it appears that more of the dose is concentrated to the right side of the pelvis.

Plan 5: Increase the energy of the PA beam and calculate the dose. What change do you see? The isodose lines have pushed anteriorly. The 90% line almost fully encompassing the

PTV. According to the DVH, 99% of the volume is receiving 90% of the dose. The 95% has also pushed significantly anterior. The 95% isodose now covers 87.9% of the PTV. The 100% has pushed slightly anterior with the 100% of the dose going to close to 55% of the PTV volume. It has also regained its lateral horn appearance. The 110% isodose has significantly decreased as compared to the previous plan, and is now only located in the right posterior aspect of the field.

Where is the hot spot and what is it? The hot spot is now 110.9% and is still located in the right posterior aspect of the field.

What do you think creates the hot spot in this location? The hot spot is created by the overlap of the posterior beam with the lateral beam. It

appears that the patient is slightly thicker on the posterior left aspect rather than the posterior right aspect, which is the most likely reason that the hot spot gravitates toward the right corner of the field. The beam weighting needs to be adjusted more to the left to compensate for this.

Plan 6: Add the lowest angle wedge to the two lateral beams. What direction did you place the wedge and why? The left lateral wedge was placed 10 IN, meaning that the heel of the wedge was

posterior to the patient. The right lateral wedge was placed 10 OUT meaning that the heel of the wedge was also posterior to the patient. I placed the wedges in this direction because the hottest area of the plan is posterior, meaning the wedge can help cool the posterior aspect of the plan and distribute the dose to encompass more of the anterior aspect of the PTV.

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..) The addition of the wedges helped to distribute my dose toward the anterior aspect of

the patient. This means that the posterior aspect of the field has cooled, while the anterior aspect of the PTV has gained isodose coverage. The 90% isodose very nearly encompasses the full PTV, and the PTV has had a significant increase in anterior coverage by the 95% isodose. According to the DVH, 61% of the volume is now receiving 100% of the dose, approximately 94.6% of the volume is receiving 95% of the dose, and approximately 99.7% of the volume is receiving 90% of the dose.

Where is the hot spot and what is it? The hot spot had been reduced to 108.9%, but is still located in the posterior right

corner of the plan. What do you think creates the hot spot in this location? As described previously, the hot spot is created by the overlap of the posterior beam

with the lateral beam. The patient appears thicker on the posterior left aspect than on the posterior right aspect, which is the most likely reason that the hot spot gravitates toward the right corner of the field. The beam weighting needs to be adjusted more to the left to compensate for this.

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?

The combination that I thought produced the best isodose distribution was a 45° wedge on each lateral. My weighting was established based on isodose lines, and the aim of not allowing any of the 80% isodose to be in the sides of the pelvis. My final weighting was PA/LtLat/RtLat = 51%/24.3%/24.7%.

How did each change affect the isodose distribution? As I increased the wedge from 10°, 15°, 20°, 25°, 30°, and 45°, the isodose lines and

therefore the dose, was pushed more anteriorly. With each wedge change the weighting had to be adjusted. The 45° provided the best distribution as compared with the lower wedge angles. I also tried a 60° wedge. Better GTV coverage could be achieved using the 60° wedges, but the 60° pushed the dose so far anteriorly that it necessitated the weighting of the PA field be much higher in order to cool the overall plan. This caused a build of higher isodoses in the normal tissue in the posterior aspect of the patient as demonstrated by the arrow in the picture below.

60° Wedge 45° Wedge

Where is the hot spot and what is it? The hot spot is located in the left far anterior corner of the field, and is 106.9%.

What do you think creates the hot spot in this location? The hot spot is located in the anterior tip due to the use of large wedges on the laterals.

The wedges attenuate much less dose at the tip of their toe when compared to their heel, thereby pushing the dose anteriorly. It is likely located in the left of the patient due to the unevenness of the patient’s body habitus. The patient is more drastically sloped on their left side than their right, causing the dose to push more anteriorly on the left versus the right side.

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? I used 23X for all four of my fields. Typically a higher energy is used when treating 3D

pelvic fields, because the higher energy allows greater penetration and helps cool the plan when compared to a low energy like 6X.

What is the final weighting of your plan? My final weighting was PA/LtLat/RtLat/AP = 31%/18.8%/22.2%/28%.

Did you use wedges? Why or why not?

I did not use wedges in my plan. The addition of the AP beam evens out the dose distribution, making wedges unnecessary. If I had been doing this plan for an actual patient without the above requirements, I would have most likely adjusted my normalization manually to further increase coverage and then added in some field-in-fields to help reduce the sections of 105% isodose line in the plan.

Where is the region of maximum dose (“hot spot”) and what is it? The hot spot is located anteriorly and slightly to the left within the PTV, and is 106.5%.

What do you think caused the hot spot in this location? Weighting had to be adjusted in this plan to even out the 105% isodose lines, and to

reduce hot areas. The patient’s body habitus is uneven, and thus the largest sections of 105% are in the anterior left portion of the field and the posterior right portion of the field. I chose to weight the fields so that the isodose lines were equally distributed within the field. Though the weighting on the left lateral is considerably lower than the right lateral, due to the deficit in tissue in the patient’s left anterior surface and the overlapping of the fields in this area, the max is still hotter in this area. I could adjust the weighting more the right to change the location of the hot spot, but doing so causes my distribution to less even, and my overall max to rise.

What is the purpose of normalizing plans? Normalizing allows us to manipulate the isodose lines of a plan. If a plan is cool and

doesn’t have adequate coverage, normalizing to a lower isodose than 100% increases the overall heat of the plan, which increases coverage and causes the dose maximum to increase. If a plan is too hot and if coverage allows, a plan can be normalized at greater than 100% to cool the overall plan.

What impact did you see after normalization? Why? The overall plan increased in heat. Normalizing so that 100% of the dose covered 95% of

the volume set the plan normalization value to 98.1%. This means that at every point in the plan, the dose that was being received is now divided by 98.1%, signifying an increase in every point by approximately 1.9%. A visible example of this can be seen with my maximum dose.

104.50.981

= 106.5

The hot spot was 104.5%, and after normalization rose to 106.5%. The treatment planning system chose 98.1% because it was the level necessary to bring my coverage up to 95%. This, as explained, increases the coverage and the overall heat of the plan. As I stated previously, if this were a plan that I was doing without the above requirements I

would have likely adjusted my normalization manually to further increase coverage and then added in some field-in-fields to cool the plan.

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.

Axial View

Coronal View

Sagittal View 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. With a large four field pelvis like this we typically do not fill out the normal check boxes

on our planning directive for the rectum and bladder since we are not going to doses that near the constraints for these organs at risk. We write on our planning directive that the bladder and rectum are encompassed in the field and thus receive the full planned dosage.

Organ at Risk (OAR) Desired Planning Objective Planning Objective Outcome

Rectum V75 < 15% V70 < 25% V65 < 35% V60 < 50%

Met all inherently Max dose = 47.4 Gy

Bladder V80 < 15% V75 < 25% V70 < 35% V65 < 50%

Met all inherently Max dose = 47.4 Gy

Lt Femur Max dose < 50 Gy V45 < 25% V40 < 40%

Max dose = 45.7 Gy V45 = 0.1% V40 = 2.8%

Rt Femur Max dose < 50 Gy V45 < 25% V40 < 40%

Max dose = 46.1 Gy V45 = 0.8% V40 = 3.9%

Bowel Spacea Max dose for small bowel < 54 Gy

Max dose = 47.8 Gy

a We don’t contour and plan using this structure at my clinic, so I just used our small bowel max dose. All of our constraints are based on different separate contours, and all of our protocol constraints that use this bowel space type contour are IMRT, which allows for much more bowel sparing than this four field 3D pelvis. With the planning criteria given, the PTV includes enough of the bowel space to inherently disqualify any of the constraints that I might have tried to compare with IMRT. Our constraint sheet sources