Ian Zoller

DOS 773

Craniospinal Irradiation (CSI)

Patient Positioning, Setup, and Treatment Fields

For this assignment, I chose to download the CT data set with the patient in the supine

position. My clinical site, the James Cancer Hospital, also treats patients in the supine position

utilizing a setup similar to what can be seen from the given CT. In this position, the patient has

his or her arms down by their side holding on to hand pegs. The patient’s head is positioned in a

head rest with the depression for the back of the head chosen to extend the chin. Having the

patient’s chin extended helps to avoid unnecessary exit dose to the mandible.1 An aquaplast

mask extending over the patient’s shoulders is made to better immobilize the head and upper

spine. In addition, the patient is given a knee and foot sponge not only for comfort but also to

help level the curvature of the spine (Figure 1).

Figure 1. Example of immobilization used for CSI treatment at the James Cancer Hospital.

When setting up fields, I chose to use an SAD setup so that I could use VMAT arcs when

planning. I realized that the total length of the PTV could not be encompassed within two

isocenters. For this reason I chose to create three isocenters that were centered laterally in the

middle of the spine. In the anterior and posterior direction, I chose a number that placed all three

isocenters close to the vertical axis of both the Brain and Spine PTVs. In this way, only shifts in

the superior and inferior direction would be needed when moving between isocenters.

I started by creating two spine fields to cover the entirety of the spine PTV, trying to keep

both fields close to the same length. The first field focused on the lower extent of the Spine

PTV, giving margin to the PTV inferiorly and laterally. Next, I added an upper spine field that

overlapped with the lower spine field by approximately 8 cm. The upper border of this field

extended superiorly to give margin to the upper most portion of the spine PTV. Finally, I added

a field that covered the brain and overlapped with the upper spine field by 8 cm, making sure not

to include the patient’s shoulders (Figure 2). Eight centimeters of overlap between fields was

chosen so that the dose could be stepped down gradually within the optimizer. Gradually

stepping down the dose between fields reduces the magnitude of the change in dose if the

patent’s setup is off.

Figure 2. Image showing three separate isocenters with the borders of each field. Also, the

areas of field overlap can be seen.

Treatment Planning Process

Given the planning objectives from the ProKnow plan challenge, I decided to utilize

VMAT arcs optimizing all three isocenters within the same plan. This way, the areas of overlap

could be modulated by the treatment planning system and the dose could be stepped down

automatically. In a case study completed by Fogliata el al,2 the researchers examined different

approaches to using VMAT in treating CSI across multiple institutions. The authors

demonstrated that uniform coverage could be achieved while limiting dose to anterior organs at

risk by using avoidance sectors around the front of the patient for the spine fields. With the

patient positioned supine, I had to create full 360 degree arcs (gantry 181-179 and 178-182) with

an avoidance sector for the spine fields beginning and ending at the posterior of each arm. This

allowed for just the posterior portion of the patient to be treated without entering through the

arms or healthy tissue. Originally I decided to use two arcs per isocenter, but after attempting

the first optimization I added a third arc to the brain isocenter to help with coverage. The arcs

around the brain used no avoidance sectors.

Each arc used an energy of 10 MV. This energy was needed for the spine fields due to

the depth of the PTV. However, an energy 6 MV may have been more appropriate for the brain

arcs due to the PTV being superficial. Unfortunately since all of the arcs were optimized as part

of the same plan, I was unable to change the energy just for the brain fields. Each arc utilized a

collimator rotation of 0 degrees except for the third brain field. The third arc around the brain

utilized a collimator turn of 90 degrees and was focused just around the superior 15 cm of brain

PTV. From clinical experience, the Eclipse treatment planning system creates a cleaner, less

jagged dose distribution in areas of overlap when the collimator is at cardinal angles. For this

reason, the angles for collimator were kept at either 0 or 90 degrees. The addition of the third arc

helped with coverage of the brain and also provided another angle for MLCs to block organs at

risk.

Figure 3. Summary of machine settings for each arc. No couch rotations were used within the

plan.

After setting up the treatment arcs, I began creating structures to use within the optimizer.

I first separated the Brain and Spine PTV into the parts that were encompassed by the fields for

each isocenter. This gave me three separate PTVs: PTV Lower Spine, PTV Upper Spine, and

PTV Brain Out (Figure 4). Using these structures, I could input different objectives into the

optimizer to better account for lack of coverage and hot/cold spots instead of trying to push on

the entire structure itself. In addition, I made a PTV Brain IN structure that is a 1 cm inner

margin from the Brain PTV. I used this in an attempt to control the area within the brain

receiving greater than prescription dose. Next, I cropped the brain optimization structures away

from the optic nerves so that I could place objectives on the nerves without conflicting with

targets (Figure 5). Originally I created rings around the PTVs to push for conformity, however, a

conformity index was not evaluated within ProKnow so the objectives for these were lowered

(Figure 6).

Figure 4. Image showing the composite of PTV Lower Spine, PTV Upper Spine, and PTV

Brain OUT.

Figure 5. Image showing PTV Brain OUT

and PTV Brain IN, both cropped away from

the optic nerves.

Figure 6. Image showing an example of the

ring structure around the cervical spine.

During optimization, the most difficult objective to achieve was coverage to the Spine

PTV while trying to keep the maximum dose below 39.6 Gy for the Brain and Spine PTV. Other

structures that were difficult to achieve were structures within the brain fields such as the optic

nerves and lenses. I believe the constraints to other organs at risk were not as difficult to achieve

because of the choice to use partial arcs – eliminating the delivery of entrance dose.

Figure 7. Optimization objectives for the completed plan.

Following optimization, I first evaluated the isodose distribution in the axial, sagittal, and

coronal plane (Figures 8-11). Mainly, I was evaluating the conformity of the plan along with the

dose distribution within the areas of field overlap. Next, I evaluated the DVH against the

planning objectives given by ProKnow. If I was not able to meet some objectives, I would

reoptimize the plan pushing harder on certain structures or letting up on constraints to help with

coverage. Finally, I evaluated hot and cold spots within the plan. The overall hot spot is 114.9%

and is located very inferiorly at the level of the sacrum (Figure 12). This portion of the PTV is

where the optimizer struggled to achieve coverage, so it makes sense that the hot spot would be

deposited here in an attempt to compensate for lack of dose. The location of this hotspot is

acceptable because it falls within the Spine CTV and it is below the tolerance for the spinal cord.

On the other hand, there is a portion of the Brain PTV that is cool at 47.7% of prescription dose.

This point is located on the very edge of the PTV at a portion close to optic structures and an

area of rapid change in shape of the PTV (Figure 13).

Figure 8. Axial view of the isodose distribution within the Brain PTV.

Figure 9. Axial view of the isodose distribution within the Spine PTV.

Figure 10. Coronal view of isodose distribution.

Figure 11. Sagittal view of the isodose distribution.

Figure 12. Sagittal view with viewing planes indicating the position of the global hot spot.

Figure 13. Axial view with the viewing planes indicating the position of the dose minimum

for the Brain PTV.

Plan Normalization

During optimization, the goal was to achieve equal coverage between the Brain and Spine

PTVs. Unfortunately, target coverage of the spine was lacking in comparison to the coverage for

the brain. Due to this, I chose to normalize to a volume and selected the Spine PTV as the target

volume for the plan. In this way, I could renormalize to achieve 100% of the prescription dose to

cover 95% of the spine knowing that that brain PTV would be slightly over covered. If there

were to be a boost planned for this patient, the excess coverage from this plan could be

compensated for when planning the boost.

Plan Scoring

Figure 14. ProKnow plan scoring sheet.

For the most part, the objectives from ProKnow were fairly easily met. There were some

structures that were very difficult to lower the dose to, such as the optic nerves and lenses. These

structures were either very close to or within one of the PTVs, making it a challenge to limit the

dose without compromising coverage. Another difficulty I had was keeping the hot spot within

the Brain and Spine PTVs below 39.6 Gy. At my clinical site, we use Acuros as our calculation

model within Eclipse. It has been explained to me from the dosimetry group that it is difficult to

get the hot spot for any plan under 110% of the prescribed dose; even when full arcs are able to

be used.

There was a discrepancy between the plan that was achieved in Eclipse versus how it was

scored by ProKnow. Within Eclipse, I was able to limit the max dose to the left optic nerve to

3,350 cGy and the right optic nerve to 3,405 cGy. ProKnow scored both of these structures as

having received greater than 35 Gy. This is an area where I could have achieved more points

and gotten closer to a score of 127.

DVH

Figure 15. Final DVH for the submitted plan.

Reflection

After completing this assignment, I realized that there are a lot of things to consider when

creating a craniospinal plan and there are many ways to go about it. After researching

techniques and speaking with my clinical preceptors, it seems that utilizing IMRT as opposed to

3D conformal is the popular option. Using this technique, the optimizer is able to handle the

stepping down of dose on its own in order to create an even dose distribution at field junctions.

With that being said, caution needs to be taken with letting the optimizer determine how the

overlapping fields should contribute in the overlap region. For example, in the plan created for

this assignment the arcs for the brain contributed to the overlap region with the spine much more

than the spine fields contributed (Figure 16). If a perfectly even contribution from each set of

overlapping fields is desired, then manually stepping down the dose using reduced fields in a

‘dummy’ plan may be needed. This way, the planner can manually force fall off at the overlap

region. The dummy plan can then be used as a base dose for the other isocenters and the

treatment planning system can be used to match the overlap.

Figure 16. Image showing the dose profile for the overlap of the brain and upper spine fields.

Overall, I think the plan that was created is something that could easily be delivered in a

safe way. Utilizing VMAT really helped to keep the dose fairly conformal at the cost of more

tissue getting lower dose. This is major factor to consider since craniospinal irradiation is

commonly performed on pediatric patients. It would be interesting to compare the volumes of

the body contour receiving low doses compared to other techniques such as using static IMRT

for the posterior spine fields.

References

1. Khan F. Treatment Planning in Radiation Oncology. 2nd

ed. Philadelphia, PA: Lippencott

Williams & Wilkins; 2007.

2. Fogliata A, Bergstrom S, Cafaro I, et al. Cranio-spinal irradiation with volumetric

modulated arc therapy: A multi-institutional treatment experience. Radiother Oncol.

2011; 99(1): 78-95. http://dx.doi.org/10.1016/j.radonc.2011.01.023