Megan Sullivan

Clinical Lab – Rectum

April 25, 2016

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

Whole bladder (QUANTEC) o V80 < 15% o V75 < 25% o V70 < 35% o V65 < 50%

V80 = 0 V75 = 0 V70 = 0 V65 = 0

Bowel space

Contour peritoneal space (QUANTEC)

o V45 Gy < 195 cc

V45 = 40.3 cc

Left femoral head

Femoral head QUANTEC o V50 < 10% o V45 < 25% o V40 < 40%

V50 = 0% V45 = 0% V40 = 0%

Right femoral head

Femoral head QUANTEC o V50 < 10% o V45 < 25% o V40 < 40%

V50 = 0% V45 = 0% V40 = 0%

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.

This plan used a 6MV beam. The CTV and PTV are overly covered. Nearly the entire PTV falls within the 100% isodose line and nearly the entire CTV falls within the 110% isodose line. This plan is very, very hot posteriorly. The mean coverage for the PTV is 122.5% which equals 5513.4 cGy and there is a 3D Dose MAX of 194.7%.

6MV PLAN

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

This plan used a 23MV beam. Overall, it is less hot than the previous plan, but it is still unacceptable. There is a slight skin sparing effect in comparison to the 6MV beam. This is evident by the 110% isodose line pulling away from the surface. This effect becomes more noticeable when viewing higher isodose levels. The PTV is now covered by the 95% line and the CTV is covered by the 105%. The mean coverage for PTV is now 117% which equals 5263.7 cGy and the hot spot is 161.1%. In relation to the 6MV plan, low dose is covering more tissue in the 23MV plan whereas high dose is covering less. Additionally, low dose is pushing more anteriorly with the 23MV beam.

6MV PLAN 23MV PLAN

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.

This plan used 6MV beams. The use of two lateral beams diminished the posterior hot spot as well as the low dose falling anteriorly. However, there are now two hot spots out laterally on either side. This plan is on the right path, but the weighting is favoring the lateral beams too much and resulting in too much unwanted dose laterally. Additionally, this area of the body has too great of a separation for 6MV beams to deliver the dose in an effective manner.

3 FIELD 6 MV PLAN

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

This plan used 23MV beams on the lateral fields and a 6MV beam on the PA field. The use of higher energy on this plan was a positive change as the lateral fields are not nearly as hot and the monitor units on the lateral beams went down significantly. The plan is also less hot posteriorly and significantly less hot out laterally. This is due to the fact that the 23MV beams do not have to work as hard to deliver dose to the prescribed volume as the 6MV beams. However, there are still 95% isodose lines present in the lateral fields which is still too hot. Weighting the beams differently will be effective in achieving less dose laterally.

6 MV PLAN 6 MV & 23 MV PLAN

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

This plan used 23MV beams on all three fields. This plan has benefited from completely using high energy beams because of the amount of tissue the beam must attenuate to deliver 95% to 100% of the target volume. The change to high energy on the PA has reduced the amount of 110% and reduced the number of MUs for the PA beam while still obtaining coverage. The hotspot is solely posterior which makes it easy to distribute this hotspot with the use of wedges.

23 MV PLAN

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..)

This plan used 23MV beams on all three fields. The 15 degree wedge was added to both lateral fields with the heel of the wedge positioned posteriorly and the toe positioned anteriorly on both fields. This wedge orientation requires the wedge to be “flipped” i.e. 15 LEFT on the right lateral field and 15 RIGHT on the left lateral field. This orientation is necessary because the hotspot was solely posterior and having the heel of the wedge lined up with the hotspot allows more attention of the beam to this area of the field thus delivering less dose to this portion. Using wedges has pushed the dose anteriorly and reduced the 110% in the field. The 95% isodose line is still covering the PTV, and nearly the entire CTV is covered by the 100%. Wedging the lateral beams is necessary, but 15 degree wedges are not enough. This is evident based on the presence of the remaining 110% isodose line positioned posteriorly.

23 MV PLAN WITH 15 DEGREE WEDGES

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?

The first wedge increase was to 30 degree wedges. This change resulted in the posterior region to be less hot than with the 15 degree wedges. The 95% isodose line is covering nearly the entire field but the 100% line is not quite covering the CTV. Increasing the weighting of the PA field would improve the dose distribution by decreasing the dose out laterally and providing more target coverage.

The second wedge increase was to 45 degree wedges. This change resulted in the complete coverage of the CTV and PTV. All of the fields are significantly hotter than before, but this makes for a solid starting point because coverage was achieved. Modifications in weighting will play a vital role in improving this plan.

Because of the outcome with the 45 degree wedges, I chose not to continue on to 60 degree wedges. This plan would not benefit from pushing anymore dose anteriorly. This can be observed by the coverage of the 100% isodose line coming close to the posterior aspect of the CTV and the amount of 110% isodose line that was pushed anteriorly.

30 DEGREE WEDGE PLAN 45 DEGREE WEDGE PLAN

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.

The energy used for all three fields is 23 MV. Both lateral fields have a 45 degree wedge with the heels positioned posteriorly in order to push dose anteriorly. A weighting of 2:1:1 of the PA, RLAT, and LLAT beams, respectively, was used.

Some goals to obtain with a three-field prone rectum include minimizing the hotspot to about 106-107%, keep the low dose in the lateral tissue to no more than 80%, and have the PTV covered with the 95% isodose line and the CTV covered with the 100% isodose line.

In order to improve the plan, I added multi-leaf collimators (MLCs) on all fields. On both lateral fields the MLCs were brought in to block out the posterior hotspot. The PA field used MLCs to block out the right and left femurs as well as some normal tissue. These blocks were shaped with the goal to block out the hotspots as much as possible while still including the sacrum and maintaining at least a 1 cm margin around the PTV.

Additionally, I rotated the gantry a few degrees on both lateral fields in order to match divergence anteriorly.

The end result: i. For PTV coverage, 100% of the volume is covered with 99% of the dose

and over 95% of the volume is covered with 100% of the dose. ii. For CTV coverage, 100% of the volume is covered with 100% of the dose.

iii. No more than 80% isodose coverage is spread out into lateral tissue and the quantity is kept to a minimum.

iv. The overall hotspot is 106.1%

LLAT FIELD PA FIELD RLAT FIELD

AXIAL

CORONAL

SAGITTAL

FINAL DVH

PTV

CTV

FEMURS

BOWEL

BLADDER

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?

The isodose lines display a much more even distribution. Adding a fourth field decreases the overall hotspot and eliminates the presence of 105% isodose line completely. Also, this is another field that would be blocking out the femoral heads and could potentially redistribute beam weight from the lateral fields to reduce dose to the femoral heads. Also, there is less dose deposited into normal tissue laterally.

On the downside, because the bladder and bowel are both positioned anteriorly, these OR are now receiving entrance dose from the AP field in addition to the already present exit dose from the PA field. Additionally, the isocenter is often positioned posteriorly resulting in a low treatment table height. An anterior beam could potentially not have clearance with the treatment table.

DVH COMPARING 3 FIELD PLAN WITH 4 FIELD PLAN

3 FIELD BLADDER 4 FIELD BLADDER