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Craniospinal Irradiation (CSI) Assignment

Prescription: 1.8 Gy/day for 20 fx to a total of 36 Gy

Planning Directions: Research current methods to plan a CSI case in the prone or supine

position. Review the methods with your preceptor/clinical instructors and solicit their input.

Design a plan that you feel is the best you can attain and create a detailed write-up with helpful

screen captures.

Patient Positioning

The CT scan used for planning was imported from an outside source. The patient was

positioned supine, arms by their sides, and scanned from above their head to their mid-thigh. The

exact immobilization devices used for the patient is unknown. If the CT had been performed at

AU Georgia Radiation, the patient would have been supine on a Type-S™ overlay board with a

clear head holder. A full head and shoulder aquaplast mask would have been created, in addition

to a Vac-Lok™ device positioned and molded for the patient’s lower body.

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Technique

The technique that I chose to employ for planning was a SAD multi-isocenter VMAT.

This technique is used at my clinical site for the treatment of pediatric craniospinal irradiation

(CSI) for a few different reasons. Studies have found that the use of IMRT/VMAT provides

dosimetric advantage, when compared to traditional 3D conformal CSI, by reducing higher doses

to normal tissues.1,2 With traditional CSI planning, gapping and feathering was also necessary for

the overlapping region of the spinal fields. This is associated with a high risk of over or

underdosing in the overlapping region, which can lead to radiation injury or incomplete

response.3 A multi-isocentric VMAT plan replaces junctions and match lines with overlapping

sectors, in which the dose from each arc is automatically accounted for. Though VMAT plans

are associated with a higher integral dose than 3D, research has shown that in cases of CSI

planning, IMRT planning can reduce the absolute risk of secondary cancer in pediatrics.1 One of

the dosimetrists at my clinical site did the research and helped to initiate VMAT as our new

primary way of treating pediatric CSI.

Contouring

For CSI cases, nearly every organ in the body has to be contoured. In our case,

contouring for normal structures had already been completed, along with CTV and PTV

designation for the brain and spine. I had to make a few minor adjustments. The body contour

had to be adjusted around the nasal area and near the most inferior portion of the PTV_Spine. At

the inferior portion of the PTV_Spine the body contour was picking up a large air gap, as can be

seen below in the blended image of the before and after body contour.

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I created three additional structures. The first was a shell that we typically use to help, in

addition to our normal tissue objective (NTO), to constrict the dose fall-off into normal tissues. I

first summed the PTV_Brain with the PTV_Spine. I then created a ring, shown in pink, by using

the wall extraction tool in Eclipse that began 1.5 cm away from the PTV and ended 4 cm away.

After looking at the ProKnow constraints, I decided to create a PTV_Brain optimization

structure, to crop it 0.5 cm away from the optic nerves, and to create lens PRV structures. I

created the PRV structures by expanding the lenses 0.2 cm.

The final adjustment that I had to make involved the CT data set. The data set ended

exactly at the top of the body contour, which restricted the use of any form of vertex-type arc.

Physics dictated, upon consultation, that to allow the software to properly calculate a vertex arc I

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had to extend the CT data set to include air above the body. I did this by duplicating the first

slice of the data set and assigning it a fill of -1000 HU.

Plan Parameters

For this case, 3 isocenters were planned, all for a Varian 23IX machine with 6 MV

energy. Isocenter one (IC1) was placed in the central and inferior aspect of the brain.

Isocenter two (IC2) was placed 25.5 cm inferior at the approximate level of the fifth thoracic

vertebrae.

Isocenter three (IC3) was placed 24 cm inferior to isocenter two at the approximate level of the

third lumbar vertebral body.

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Shifts in the anterior/posterior direction and the left/right direction were kept zero for simplicity

of treatment. Isocenter locations were chosen that could accommodate the lack of

anterior/posterior and left/right shift.

I utilized three arcs for IC1, two full 360° arcs and a partial arc of 180°-330° with a table

kick of 90°. A partial arc with a table kick is not in our normal planning repertoire for CSI, but I

found that I was having issues meeting lens and optic nerve constraints without making the

PTV_Brain hot. The addition of the partial arc alleviated this issue. The first partial arc clock-

wise (CW) rotation with the table kick was assigned a collimator of 0°, the counter-clock-wise

(CCW) rotation was assigned a 350° collimator rotation, and the second clock-wise (CW)

rotation was assigned a 10° collimator rotation. The maximum length that the MLC leaves can

travel is 15 cm. Without the use of jaw tracking on this machine, I was thus limited to a 15 cm

field width for my arcs. The established width needed to be greater than 15 cm for both of my

360° IC1 arcs, thus, to compensate, the fields were mirrored. On the CCW rotation, the X2 was

left as established and the X1 was changed to make the total X equal to 15 cm. On the CW

rotation, the X1 was left as established and the X2 was changed to make the total X equal to 15

cm. I set the total X for the partial arc to 15 cm, equally distributed between the two jaws.

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I utilized one CCW 360° arc for IC2 with a 350° collimator rotation, and one CW 360°

arc for IC3 with a 10° collimator rotation. In a normal CSI case, I would have ran the arcs for

IC2 and IC3 with avoidance sectors on the anterior portion of the body to further restrict the low

dose spillage into the anterior cavity. I found that this was not feasible for this plan with the

kidney constraints that were given.

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To establish the field sizes, unlike in most circumstances in which an MLC margin can be

applied, I had to manually adjust overlap and PTV coverage. I adjusted the length of each arc

field so that it overlapped with the other isocenter arc fields. I then played the arcs and manually

adjusted the widths to include the PTV plus a margin.

The exception to this was in the case of the mirrored fields for IC1. I adjusted the widths, but

then adjusted the X jaws as necessary for the mirroring.

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Optimization

I ran the three isocenters together simultaneously within the optimizer. This allowed the

optimizer to compensate for overlap of the fields and to use all of the arcs to accomplish

coverage to the total PTV. This also allowed the elimination of feathering during the treatment

process. I began with a relatively sharp NTO with a priority of 150.

I ended up putting constraints on the PTV_Brain and PTV_Spine separately, because of the

individual coverage challenges that the two faced. The two obstacles to PTV coverage for the

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brain were the optic nerves and the lenses. These were a major challenge before the addition of

the vertex arc but became slightly easier to balance with that addition. The only real challenge,

albeit a major one, to the spinal coverage was trying to also meet the kidney constraint. I ran the

PTV_Brain with a lower of 100% at 3690 cGy and an upper of 0% over 3800 cGy. I ran the

PTV_Spine with a lower of 100% at 3700 cGy and an upper of 0% over 3800 cGy.

I discovered after my first submission to ProKnow that the software does not read the

lens and optic nerve doses the same as my Eclipse software. In some submissions there was up to

a 2 Gy difference in results, even after making my structures high resolution and increasing the

resolution points in my optimizer. For this reason, I had to push these structures an extra amount

during optimization to try and get them to ideal. As an example, instead of targeting less than

700 cGy for my lenses, I optimized at 550 cGy.

I would typically use the ring to target an even lower dose, i.e. the 50%-60% isodose. The

challenges that I had in meeting the kidney constraint made it less realistic to push these lower

doses, so I chose to target the 70% instead.

Normalization

I normalized my plan so that 100% of the dose covered 95% of the PTV_Spine. The

reason that I chose to normalize to the spine is that it was the closest PTV to attaining that level

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of coverage anyway, so it only changed the normalization of the plan to 99.9. The PTV_Brain

had 98% coverage, so if I would have normalized to it, or to the PTV_Sum, the plan would have

gotten cooler. Consequently, my PTV_Spine would not have individually had its coverage on the

DVH.

Plan Evaluation

When evaluating the multiple optimizations that I ran, I first normalized to get target coverage. I

then assessed my max point and my hot spot at 0.3cc. My max point for my final plan ended up

being 113.5%, and the 0.3cc was 110%, which is within my clinical tolerance. I then assessed

my isodose lines throughout my PTV_Sum. In a plan where you are constraining the dose to a

small area and forcing it to rapidly fall off 105% is expected, but 110% should not be making

appearances, so I assessed the plan for pockets of high dose. I then moved on to the organs at

risk. With the NTO and ring, most of the organs were not an issue. The ones that I had to assess

repeatedly were the lenses, optic nerves, and kidneys. I also assessed where my hot spot was

located, and the low dose wash to my anterior cavity.

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Isodose Distribution

My isodose can be seen in the images below.

I feel as though the force on some of the constraints made the 50% isodose, which can be seen in

dark blue below, be much less conformal than it usually is, but it still falls off before the front of

the vertebral bodies.

You can clearly see the lack of 95% coverage in the location of the optic nerves owing to the fact

that I had to optimize them 3 Gy below the constraint of 34 Gy, which was already 2 Gy below

the prescription. I ran the plan with the mindset of being given the constraints by the doctor for a

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reason, so I compromised coverage in that small area, though the PTV still has great coverage

overall.

Hot/Cold Spots

There were not any cold spots in the plan. Optimizing all of the isocenters together, as I

mentioned previously, allows the optimizer to compensate and eliminate hot and cold areas as

the plan is optimized. My hot spot ended up in the bone of the brain, which is an ideal location.

It is just a personal preference at this clinical site to have the hot spot in the brain rather than the

spinal cord. It is also preferable to have it in bone rather than in tissue. It originally gravitated to

the field junction of the CW and CCW arcs of IC1, seen below, so I did have to run a small

avoidance in that area.

In hindsight, I probably could have also rotated one of the collimators a small amount.

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DVH

Proknow

For the vast majority of the tolerances, the ideal was achievable. As I have mentioned, the

tolerances that were the hardest to achieve were the lenses, the optic nerves, and the kidneys. The

optic nerves were given a constraint that fell 2 Gy below the prescription dose, making it

impossible to achieve the constraint without underdosing the PTV in that area. The lens

constraint, difficult regardless because of the tendency of VMAT to spread low dose, was made

more difficult due to the calculation algorithm of ProKnow. Even after applying high resolution

contours, Proknow calculated the lens and optic nerve doses consistently above what Eclipse did,

making me have to push these structures significantly harder in the optimizer. I could meet the

constraints at first, but not without making my PTV_Brain very hot. The vertex arc made

meeting the ideal for these constraints, while maintaining a cool plan, possible, or very nearly so.

Though Eclipse says that my left lens is at a max of 637 cGy, ProKnow says that I am not ideal

at 720 cGy.

The only other constraints that aren’t ideal, though they still pass are the kidneys and the

volume of the PTV_Spine and PTV_Brain receiving 39.6 Gy. The kidneys were certainly the

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hardest part of this plan. I tried using arc avoidances and I pushed the constraints heavily, but I

could not get the mean down to 2 Gy. The other non-ideal was produced by pushing the

constraints for the kidneys, lenses, and nerves. I tried to balance how non-ideal I was willing to

allow the hot volume to be, in order to achieve ideal or more ideal for the other three organs.

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Reflection

This was the most frustrating, yet fun, challenge that I have yet to tackle. I am much later

than normal submitting my assignment because I didn’t start planning it early enough, and I

didn’t compensate for my inability to leave a puzzle alone. I have worked and tweaked and tried

something else again amidst my clinical schedule. I think I ran this plan 10 times, and for anyone

else who optimized multiple isocenters, you know that it takes forever and another day to

optimize and calculate.

I think the hardest part of the plan for me was trying to meet the kidney constraints and

trying to balance the dose calculation differences of the ProKnow software. If I had more time to

work with it, still trying to achieve these same constraints, I think I would try doing multiple

partial arcs for my IC3. I am not sure if I could find a configuration that would work, but the

addition of the vertex made a big difference for my optic constraints, so beam geometry could

also potentially help with the kidneys. I did try using IMRT one time, in order to use angles to

avoid the kidneys, but I found that every beam for the brain isocenter would split. Realistically,

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this would be a less treatable plan on the machine, since the treatment times for these plans are

already decently long even with arcs.

Reflecting on the plan itself, if a physician here dictated these tight of constraints, the

plan that I created is a good and treatable plan. If it was just the average CSI that comes in, I

would not use this plan to treat them, because the constraints given here and pushed for in this

plan wouldn’t need to be stressed so heavily. For example, a physician here would not want

PTV_Brain coverage compromised in order to meet a max dose of 34 Gy to the optic nerves. The

pediatric physician here would also prefer a slightly higher kidney mean if it means that a much

higher portion of the anterior cavity is spared of low dose. By this I mean that to meet a mean of

2 Gy to the kidneys, all of the dose is forced anterior, as can be seen below.

The 50% is shoved to the front of the vertebral body and the 25%, seen in mint green, is shoved

into the anterior cavity. Normally it is our highest priority to have the dose fall off rapidly and to

avoid low dose in the anterior cavity.

Overall, this was an excellent learning experience. It made me move outside of my

normal box and try some new weird tricks. It was also about prioritizing and balancing the

constraints within Proknow and overcoming some differences in calculation algorithms. I’m still

not wholly happy with my plan, but I really enjoyed this project.

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References

1. Miralbell R, Lomax A, Cella L, et al. Potential reduction of the incidence of radiation-induced

second cancers by using proton beams in the treatment of pediatric tumors. Int J Radiat Oncol

Biol Phys. 2002;54:824-829. http://dx.doi.org/10.1016/S0360-3016(02)02982-6

2. Yoon M, HoShin D, Kim J, et al. Craniospinal irradiation techniques: A dosimetric

comparison of proton beams with standard and advanced photon radiotherapy. Int J Radiat

Oncol Biol Phys. 2011;81(3):637-646. http://dx.doi.org/10.1016/j.ijrobp.2010.06.039

3. Fogliata A, Bergström S, Cafaro I, et al. Cranio-spinal irradiation with volumetric modulated

arc therapy: A multi-institutional treatment experience. Radiother Oncol. 2011;99(1):79-85.

http://dx.doi.org/10.1016/j.radonc.2011.01.023