Download Pelvis Lab - Justin Kotrba

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
Bladder
Desired objective(s)
D45Gy<15%
Achieved objective(s)
 D45Gy<15%
Femoral heads and necks
<45Gy, Preferred <40Gy
 <43.5Gy
Bowel
<50Gy, Preferred <5Gy
 <44Gy
*Achieved objectives values obtained from final 4 Beam plan.
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.
A. This planning assignment begins with a 1 beam posterior to anterior field targeting the
rectum with a +1 cm expanded margin as the PTV (red color shade). The energy of the
beam is set at 6MV and the resulting isodose distribution results in a shape somewhat like
a square with the highest dose/hot spots located superficially on the posterior side. As the
dose goes further into the body, the hotter isodose lines fall off one by one.
b. Change to a higher energy and calculate the beam. How did your isodose distribution
change?
B. This step of the assignment the energy of the beam is increased to 15MV and the isodose
lines changed significantly for the better. The overall square shape of the distribution
stayed the same, however the dose has pushed deeper into the body. Using the higher
energy is better for this case since the PTV was only partially covered with the lower
6MV energy beam used in part A. The higher energy beams deeper penetrating ability
also reduced the hot spots on the superficial posterior part of the body.
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.
C. A 3 field beam arrangement was used for this part of the assignment. The original PA
beam was kept intact and 2 lateral beams were added to the plan. All 3 beam energies
were set to 6MV and were all given equal weight distribution. Adding the 2 lateral beams
to the plan created more of a flattening effect to the isodose distribution while also
increasing the dose to the lateral superficial portions of the body. Compared to step B,
this 3 field lower energy beam plan would be unfavorable.
d. Change the 2 lateral fields to a higher energy and calculate. How did this change the
dose distribution?
D. By increasing the energy of the lateral beams to 15MV, the dose to the skin on the lateral
borders was reduced significantly. The flattening effect from originally adding the 2
lateral beams however was minimally reduced.
e. Increase the energy of the PA beam and calculate. What change do you see?
E. Increased the PA beam to a higher energy so now all 3 beam fields are set to 15MV. The
shape of the isodose distribution stayed the same, but decreased the hot spots on the
posterior superficial border significantly.
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..)
F. Added small 10 degree angle wedges to the lateral beams. The orientation of the wedges
are placed with the heels posterior to the patient. I chose to place the wedges in this
orientation because of the hot spots on the posterior corners of the isodose distribution.
The wedges did exactly what I had hoped by reducing the dose to these areas.
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?
G. This step of the process I changed the thickness of the wedges used for the lateral beams.
I evaluated the dose distributions for wedge angles of 30 degrees, 45 degrees, and 25
degrees. As a side note, all of the comparisons were with the heel of the wedges posterior
to the patient.
When comparing the 30 degree wedge angles to the 10 degree angles for step F, the PTV
coverage had better, steeper dose drop off with the 30 degree angle and the hot spots
were reduced on the posterior superficial border. There was a slight increase in dose to
the lateral borders near the femurs however the better isodose distribution for the PTV
makes this wedge angle better than the 10 degree angle.
The next comparison I did was with the steeper 45 degree angle wedges. Since this was
quite a steep angle of wedge, the dose concentration had shifted with the hotter areas
previously on the posterior border to the anterior portion of the isodose distribution. The
dose to the femurs and lateral borders increased even more than it did with the 30 degree
angle wedges thus making this wedge angle not a preferred choice for the plan.
The final wedge angle I compared was the 25 degree wedge. This wedge helped reduce
the hot spots on the posterior superficial border while minimizing the increased dose to
the lateral borders/femurs that the steeper wedge angles resulted in. The DVH also
showed better PTV coverage and dose drop off than the other wedge combinations.
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.
H. My final 3 field plan I chose utilized 15MV energy for all three beams and 45 degree
wedges on the lateral beams. The dose weighting was adjusted to 41% for the PA beam
and 29.5% for each of the lateral beams. This beam weighting combination pulled the
dose away from the lateral borders/femurs while keeping the hot spots as low as possible
on the posterior superficial border.
*It is important to note that these plans were not utilizing blocking or MLCs as were not
instructed to do so.
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 3 beam image:
DVH of final 3 beam plan and 4 beam plan:
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?
By using a 4 field (box) plan, the isodose distribution becomes more of a box in shape. The
dose to the superficial parts of the body are brought in more tightly than they are with the 3
field plan. The advantages of using a 4 field plan is the dose is more tightly compacted into
the treatment target thus reducing the dose to the farther outlying structures such as the dose
to the femurs. By using an evenly weighted 4 field box, the plan is debatably simpler in
design since the comparable 3 field plan has to incorporate wedges and changes in beam
weighting in order to match the efficiency of 4 fields.
The disadvantages of using a 4 field (box) plan is the higher dose to structures closer to the
target PTV. Such as the dose to the prostate, bladder, and small bowel, there is a much higher
dose to these structures when compared to the 3 field plan. The hot spots were very similar
when comparing both plans so the decision of choosing a 3 field or 4 field plan would be
determined by which structures the doctor wishes to reduce the dose for. Both methods result
in excellent PTV coverage for structures of the pelvis.
Image 4 beam arrangement: