Ellie Hawk

Planning Assignment (3 field rectum)

Use a CT dataset of the pelvis. Create a CTV by contouring the rectum (start at the anus and

stop at the turn where it meets the sigmoid colon). Expand this structure by 1 cm and label it

PTV.

Create a PA field with the top border at the bottom of L5 and the bottom border 2 cm below

the PTV. The lateral borders of the PA field should extend 1-2 cm beyond the pelvic inlet to

include primary surrounding lymph nodes. Place the beam isocenter in the center of the PTV

and use the lowest beam energy available (note: calculation point will be at isocenter).

Contour all critical structures (organs at risk) in the treatment area. List all organs at risk (OR)

and desired objectives/dose limitations, in the table below:

Organ at risk Desired objective(s) Achieved objective(s)

Bladder V70 < 25% Rx < 70 Gy

Bowel Dmax < 65 Gy Dmax = 45.5 Gy

Rectum V70 < 20% Rx < 70 Gy

Femoral Heads Mean Dose = 25 Gy Rt side Mean Dose = 33.6 Gy Lt side Mean Dose = 36 Gy

a. Enter the prescription: 45 Gy at 1.8 /fx (95% of the prescribed dose to cover the PTV). Calculate the single PA beam. Evaluate the isodose distribution as it relates to CTV and PTV coverage. Also where is/are the hot spot(s)? Describe the isodose distribution, if a screen shot is helpful to show this, you may include it.

The single PA beam does not give adequate dose distribution to the CTV or PTV. The CTV does not receive full coverage until the 80% isodose line. The PTV nearly receives full coverage with the 80% isodose line, but its entirety is covered with the 70% isodose line. A very large and very hot 10% hot spot isodose line is located with the single field PA plan. It has a highest level of 32.1% hot and encompasses the back of the patient (within the field size) up to the posterior aspect of the CTV. The 5% hot spot isodose line also covers the posterior portion of patient (within the field size), while including the posterior 1/4 of the CTV and 1/3 of the PTV. Using the single PA beam the dose is centralized to the posterior region and does not provide enough coverage to the CTV and PTV for treatment.

b. Change to a higher energy and calculate the beam. How did your isodose distribution change?

By changing my energy from 6X to 18X there were multiple changes in my plan. Initially I noticed that my hot spot decreased in size and value. The 10 % hot spot isodose line now has a value of 17.8% versus 32.1%. The size also decreased drastically while still being located posterior within the patient. The 10% and higher hot spot isodose line does not infiltrate the CTV but it does the posterior ¼ of the PTV in some slices. The 5% hot spot isodose line covers nearly half of the CTV and PTV in multiple slices. The higher energy allowed the beam to be more penetrating, therefore the isodose distribution shifted. Both the CTV and PTV are covered in their entirety by the 80% isodose line. The 90% isodose line is providing a majority of coverage for both target volumes, especially for the CTV.

c. Insert a left lateral beam with a 1 cm margin around the ant and post wall of the PTV. Keep the superior and inferior borders of the lateral field the same as the PA beam. Copy and oppose the left lateral beam 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 this dose distribution.

Upon making the above changes to the original single PA beam plan the plan’s isodose distribution altered drastically. There is still a very large posterior hot spot with a maximum value of 22.5%. This hotspot does not encompass any of the CTV or PTV structures or margins, but stretches to the width of the field size posteriorly. The 5% hot spot isodose line is located posterior encompassing the posterior PTV margin that was added, along with invading nearly 0.5 cm of the original PTV posteriorly on multiple slices. The 5% hot spot isodose line also appears on the right and left lateral aspects of the patient, where the lateral beams are positioned. The 90% isodose line provides a majority of coverage to the CTV, PTV, and PTV with margin, but full coverage begins with the 80% isodose line.

d. Change the 2 lateral fields to a higher energy and calculate. How did this change the dose distribution?

Increasing the right and left lateral beams from 6X to 18X created a change in the isodose lines distribution. The greatest hot spot was reduced to 15.2%. The CTV, PTV, and PTV margin are not within this 10% hot spot isodose line. It is still located posterior within the patient and extends to the width of the field size. The 5% hotspot isodose line follows suit and is located posteriorly within the patient. The posterior aspects of the PTV and PTV margin are covered within the 5% hot spot, but the CTV is not. The CTV and PTV are fully encompassed by the 90% isodose line, but the PTV margin still is not covered entirely until the 80% isodose line. The patient’s right and left lateral sides had a major decrease in dose. The sides no longer have 5% hot spot isodose lines; their highest isodose line is now the 80%.

e. Increase the energy of the PA beam and calculate. What change do you see?

With all 3 beams now possessing 18X energy, there is a shift in isodose lines. The highest hot spot is 8.5% and this hot spot isodose line is located posterior and covers some posterior aspects of the superior PTV and PTV margin. The lower ¼ of the field size does not possess any hot spot regions. The CTV, PTV and PTV margin are all covered by the 90% isodose line.

f. 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..)

I added a 10º wedge to the right and left lateral beams in efforts to reduce the hot spot value and size, while also trying to extend the isodose lines to provide better coverage to the target volumes. I placed the wedges with the heels towards the patient’s posterior, and the toes to the patients anterior. By positioning the wedges this way the hot spot located posterior was reduced to 6.7% and its size was decreased. The wedges also helped to extend the isodose lines. The CTV, PTV, and PTV margin are fully covered beginning with the 90% isodose line, while the 95% isodose line gives a significant amount of coverage to the target volumes but not full coverage.

g. 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). What wedge angles did you use and how did it affect the isodose distribution?

15º wedges: Hot spot reduced in size and value to 5.7%, still located posterior within patient. Isodose lines shifted slightly towards the anterior, but the 90% isodose line is still the only beam providing full coverage of the CTV, PTV, and PTV margin.

20º wedges: Hot spot is now 4.8%, so it is no longer displayed by the 5% hot spot isodose line. These small spots of hot dose are still located posterior. Again, the isodose lines shifted anteriorly, where the 95% isodose lines provides full coverage of the CTV, PTV, and PTV margin. Using these higher wedges has caused patches of 90% isodose lines to appear on the lateral sides of the patient.

30º Wedges: Using these higher wedges created a 5.8% hot spot, which is higher than with the 15 and 20 degree wedges. Although the hot spot increased, the 100% isodose line extended more anteriorly providing better coverage. The 98% isodose line now provides full coverage to the CTV, PTV, and PTV margin. The steeper wedge angle of 30º has now increased the dose on the patient’s lateral sides, displaying patches of 95% isodose lines.

45º wedges: Utilizing 45º wedges created a dramatic increase in the hot spot to 11.8%. A large 5% hot spot isodose appears anteriorly within the patient. Also, the isodose lines shift anteriorly so much that the posterior portions of the target volumes do not receive full coverage anymore. The lateral sides are also affected by spots of 100% isodose lines appearing. The dose distributions created from the use of 45º wedges are undesirable and unhelpful in planning for this patient.

Inferring from the 45º wedge isodose distribution, I do not feel that 60º wedges will be beneficial in this plan therefore I did not implement them.

h. Now that you have seen the effect of the different components, begin to adjust the weighting of the fields. At this point determine which energy you want to use for each of the fields. If wedges will be used, determine which wedge angle you like and the final weighting for each of the 3 fields. Don’t forget to evaluate this in every slice throughout your planning volume. Discuss your plan with your preceptor and adjust it based on their input. Explain how you arrived at your final plan.

Upon experimenting with a few wedge angles, I found the 30º wedges to be most beneficial in this patient’s pelvis treatment plan. Since these wedges did create spots of 95% isodose on the patients lateral aspects, I adjusted the weighting. By adjusting weighting I was able to greatly reduce the size of the 95% isodose lines within the patient’s lateral aspects. By using 32% weighting from both laterals and 36% weighting from the PA beam; the CTV, PTV, and PTV margin are now fully covered by the 100% isodose line on a majority of slices. If they are not receiving full coverage from the 100% isodose line, the target volumes are covered with the 98% isodose line. Below I have included a screenshot of the slice with the worst coverage, yet the target volumes are still covered with the 98% isodose line. I also have included the dose volume histogram (DVH) to display the CTV, PTV, and PTV margin’s coverage.

i. In addition to the answers to each of the questions in this assignment, turn in a copy of your final plan with the isodose distributions in the axial, sagittal and coronal views. Include a final DVH.

Final plan in axial, sagittal, and coronal views with PA beams eye view (BEV):

Final DVH with all contoured structures included:

4 field pelvis

Using the final 3 field rectum plan, copy and oppose the PA field to create an AP field. Keep the lateral field arrangement. Remove any wedges that may have been used. Calculate the four fields and weight them equally. How does this change the isodose distribution? What do you see as possible advantages or potential disadvantages of adding the fourth field?

After adding a 4th beam in the AP position, removing all wedges, and giving all beams equal weighting of 25%; the isodose distribution shifted yet again. The isodose lines became conformal to the field edges, and the hot spot was reduced minimally from 5.1% to 5.2%. The patient’s lateral side’s dose has been reduced to patches of 60% isodose lines, with a majority of 30% and 50% isodose lines. The AP beam created more dose within the anterior of the patient reducing dose from the lateral beams, and the femoral heads. The anterior is also encompassed with 30% and 50% isodose lines with 60% isodose lines extending into the bowel area. This new dose within the bowel I find to be a disadvantage since radiation can harm the bowl and its functions. I find the reduced lateral dosage and greater coverage delivered to the CTV, PTV, and PTV margin to be advantages of the 4 field beam. The visual coverage is very similar to the 3 field, 30º wedges plan. Although the dose distributions appear to be similar since both have full coverage with the 98% isodose line, and a majority of coverage with 100% isodose line, the DVH shows that the 4 field plan provides a very slight increase in mean dose to the target volumes of less than one percentage. The mean dose was also increased in the bladder and bowel space. With the 4 field plan the bladder received a mean dose of 87% while the 3 field had a mean dose value of 75.9%. The bowel’s 4 field mean dose was 17.3% versus 13.3% with the 3 field. In contrast the mean dose was reduced in both femoral heads. The right femoral head decreased to 53.2% (from 74.7%) and the left femoral head decreased to 57.8% (from 80.1%) by using the 4 field plan.

3- Field Plan 4-Field Plan

Plan comparison DVH: 3-field plan represented by triangles ( ), 4-field plan represented by squares

( ).