Pelvis Clinical Lab Assignment

Use the Pelvis CT data set provided in Canvas to complete the following 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 1 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. Each plan will build in complexity off of the previous one. After adjusting each plan, answer the
provided questions. Include a screen shot for each plan to show the isodose distribution along with a
DVH clearly displaying your PTV coverage. Note: Make sure that your plan shows the absolute dose
levels and that each view is large enough to clearly read the needed details. You may want to
screenshot each view separately. Describe and/or show how you read the PTV dose on the DVH. Only
provide the PTV when asked for PTV coverage. When asked for field weighting, show the field
weighting for that plan. Embed the question and then your answers with any associated visuals
within your completed assignment. A good visual image and a thorough description of the isodose
distribution in each plan are critical components. The reader should be able to follow your planning
process/outcome using your visuals and explanations.
• Important: Please do not normalize your plan when making these adjustments until instructed
to do so in the final plan.
• 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 field.

• Describe the isodose distribution (be specific in your description of depth, location, etc).
Using a 6MV beam, the dose is highest within the first few centimeters of beam entry into the
posterior aspect of the patient, which makes sense, as Dmax for a 6MV beam is approximately

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 1.5cm. The max dose is slightly (3.4cm) to the patient’s left side. The dose decreases as we
move anteriorly.
• Where is the hot spot (max dose) and what is it?
The hot spot is 177.2 cGy, located 3.4 cm to the patient’s left, and 0.9cm deep in the posterior
aspect of the patient.
• What do you think creates the hot spot in this location?
The hot spot created here is due to the characteristic dose of dmax of a 6MV beam
(approximately 1.5cm deep within a patient). Its location slightly to the patient’s left may be
due to a slight variation in tissue density and electron contamination.
• Using your DVH, what percent of the PTV is receiving 100% of the dose? Remember to describe
or show how you read this.

•
Approximately 51.1% of the PTV is receiving 100% of the dose, as shown above as the red line
representing the PTV crosses the 4500cGy dose marker.

Plan 2: Change the PA field to a higher energy and calculate the dose.

• Describe how the isodose distribution changed and why?

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 The isodose lines moved more anteriorly within the patient with the 15MV beam. As you can
see, the 50% isodose line (2250cGy) is within the patient’s abdomen using the 6MV beam, but
extends to the patient’s anterior external contour using the 15MV beam. This is because higher
energy beams have a more gradual dose falloff than lower energy beams.
The maximum dose moved deeper within the patient (to 2.0cm).

Using your DVH to confirm, what percent of the PTV is receiving 100% of the prescription dose?

Approximately 55.56% of the PTV is receiving 100% of the dose.

Plan 3: Insert a left lateral field with a 1 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 fields.

• Describe the isodose distribution. What change did you notice?

The isodose distribution is more conformal, as there is no 50% dose exiting anteriorly into the
patient’s abdomen. There is, of course, now dose coming from each lateral field. The amount of
PTV covered by 100% of the dose has increased.

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 • Where is the hot spot and what is it?
The hot spot is located 7.5cm deep in the patient from the posterior aspect, and on the
patient’s right side, just lateral to the sacrum/ilium.

• What do you think creates the hot spot in this location?

The hot spot is located here due to the convergence of the three beams, as well as the
inhomogeneity of tissue, where the right lateral and posterior beams converge right before
hitting the pelvic bones.

Plan 4: Increase the energy of all 3 fields and calculate the dose.

• Describe how this change in energy impacted the isodose distribution.

This change in energy improved the dose distribution, and also reduced the entrance doses and
the hot spot. (The volume of the PTV receiving 100% of the dose using the 6MV beam was
54.14%, while the volume of the PTV receiving 100% of the dose using the 15MV beam was
61.8%)

In your own words, summarize the benefits of using a multi-field planning approach? (Refer to
Khan Physics for benefits of multiple fields)
Using a multi-field planning approach allows for a more conformal dose distribution, and gives
the treatment planner a lot more flexibility regarding how to accomplish their dosimetric goals.
More beams allow for more adjustments regarding weighting, blocking/ beam modifiers, and
beam energies.

• Compared to your single field in plan 2, what percent of the PTV is now receiving 100% of the
prescription dose? Use a DVH to show how you obtained this response.

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61.8% of the structure volume receives 100% of the prescribed dose with the 3-field 15MV beam,
compared to 55.55% using the 2-field plan.

Plan 5: Using your 3 high energy fields from plan 4, adjust the field weights until you are satisfied with
the isodose distribution.

• What was the final weighting choice for each field?

I decided to weigh the posterior field 25%, and each of the lateral fields at 37.5%.
• What was your rationale behind your final field weight? Be specific and give details.
I chose this weighting because it allowed significant influence from the lateral beams while
sparing the skin to some degree. It also maintained some weight from the posterior beam, and
allowing 67.136% of the PTV to receive 100% of the dose, as shown below.

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Plan 6: Insert a wedge on each lateral field. Continue to add thicker wedges on both lateral fields until
you are satisfied with your final isodose distribution. Note: When you replace a wedge on the left,
replace it with the same wedge angle on the right. Also, if you desire to adjust the field weights after
wedge additions, go ahead and do so.

• What final wedge angle and orientation did you choose? To define the wedge orientation,
describe it in relation to the patient. (e.g., Heel towards anterior of patient, heel towards head
of patient..)
I used a wedge angle of 20 degrees on the lateral beams, with the heels of the wedges toward
the posterior aspect of the patient.

• How did the addition of wedges change the isodose distribution? Include a screen shot
(including axial and coronal) of the isodose distribution before and after the wedge placement.

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 Without wedges:

With wedges:

• According to your Khan Physics book, what is the minimum distance a wedge or absorber
should be placed from the patient’s skin surface in order to keep the skin dose below 50% of
the dmax?

15cm is the minimum distance from the skin surface a wedge or absorber should be placed for
adequate skin sparing.

Plan 7: Insert an AP field with a 1 cm margin around the PTV. Remove any wedges that may have been
used. Calculate the four fields. At your discretion, adjust the weighting and/or energy of the fields, and,
if wedges will be used, determine which angle is best. Normalize your final plan so that 95% of the

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PTV is receiving 100% of the dose. Discuss your plan rationale with your preceptor and adjust it based
on their input.

• What energy(ies) did you decide on and why?

I chose to use all 16MV energies, as the dose distribution improved compared to using 6MV.

• What is the final weighting of your plan?

PA: 31.6%
RtLat: 23.3%
AP: 22.8%
LtLat: 23.3%

• Did you use wedges? Why or why not?

I chose not to use wedges, as the dose distribution was not improved by their addition.

• Where is the region of maximum dose (“hot spot”) and what is it?
The hot spot of 110.2% is located just posterior to the patient’s right pelvic bones.

• What is the purpose of normalizing plans?

Normalizing a plan is used to ensure adequate coverage of the tumor volume.

• What impact did you see after normalization? Why? Include a screen shot (including axial and
coronal) of the isodose distribution before and after applying normalization.

The isodose distribution was improved after normalization.

Before normalization:

After normalization:

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

Include a final DVH with PTV and OARs. Be sure to include clear labels on each image (refer to the
Canvas Clinical Lab module for clear expectations of how to format your DVH).

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 • Use the table below to list typical organs at risk, critical planning objectives, and the achieved
outcome. Provide a reference for your planning objectives and a rationale for the objectives
chosen.

Organ at Risk (OAR) Planning Objective Objective Outcome Objective Met? (Y/N)
Bladder 50% <65 Gy Max Dose = 47 Gy Y
Femoral Heads Max dose <50 Gy Max Dose = 47 Gy Y
Rectum 50% <60 Gy Max Dose = 48 Gy Y
Bowel Space 50% <45 Gy V45 Gy < 195 cm3 Y

I chose bladder, bowel space, femoral heads, and rectum for my planning objectives because they are of
common concern to pelvic cases. Other organs, such as the uterus, are not commonly included in a list
of OARs due to their high resistance to radiation. I used the Mobius table for all dose constraints, except
for bowel space, which was found in the radiation oncology journal listed below.

References:

1. Dose Volume Histogram Limits Conventional Fractionation 1 Fraction SRS 3 Fraction

SBRT 5 Fraction SBRT Notes and Sources.
https://thisrtplan.weebly.com/uploads/4/5/7/2/45723995/dvh_limit_table.pdf

2. Marks L.B., Yorke E.D., Jackson A., Ten Haken R.K., Constine L.S., Eisbruch A., Bentzen
S.M., Nam J., Deasy J.O. Use of Normal Tissue Complication Probability Models in the
Clinic. Int. J. Radiat. Oncol. Biol. Phys. 2010;76:S10–S19. doi: 10.1016/j.ijrobp.2009.07.1754

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