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Heather Maurer

March Case Study

March 21, 2014

Coplanar Oblique’s for Ewing Sarcoma in the Tibia

History of Present Illness: Patient LL is a 13 year old girl who initial complained of left leg

pain in 2013. The pain was conservatively managed without relief which prompted bilateral

lower extremity ultrasound and x-ray. These tested raised suspicions of a mass on the left leg

and an MRI was ordered. The MRI was preformed May 2013 and revealed a long segment of

abnormal bone marrow within the tibial diaphysis measuring 11cm in the craniocaudal

dimension. Connected to this abnormality was an additional soft tissue mass posterior to the

tibia measuring 3x2x11cm. A Biopsy of the soft tissue mass was obtained with results showing

a high grade Ewing sarcoma. The typical onset of Ewing’s sarcoma is between 15 and 30 years

of age1. A Computed Tomography (CT) scan of her chest was then preformed and appeared

clear for metastatic disease. Bone marrow aspirations of both the left and right iliac crests were

both negative for malignancy. She was started on chemotherapy, alternating Vincristine

Doxorubicin Cyclophosphamide (VDC) and Ifosfamide Etoposide (IE) though had some troubles

with her first rounds experiencing abdominal pain, emesis and thrush. She has also been

hospitalized multiple times since July 2013 for hypotension which has responded to fluids or

packed Red Blood Cell (pRBC) transfusions.

Another MRI was done in September 2013 of just the left lower extremity showing a decrease in

the size of the abnormality in the tibial diaphysis as well as the soft tissue mass. After 6 cycles

of chemotherapy she underwent a wide local excision of the left tibia, allograft reconstruction

and planting, soft tissue reconstruction and split thickness skin graft in September 2013.

Pathology revealed a resected high-grade Ewing Sarcoma with some effects from treatment. The

proximal and distal marrow margins were negative but there was invasion through the cortex into

soft tissue with a positive proximal margin. Patient LL recovered well from all surgeries and

continued chemotherapy.

LL was referred to radiation oncology in October 2013 for consideration of adjuvant radiation

post-chemotherapy due to the invasion of soft tissue and positive margin. At one time these

tumors were believed to be radioresistant1. In the 1960s trials of postoperative high-dose

radiation therapy proved the residual was in fact radiosensitive1. With LL being a minor, options

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were discussed with her and her parents. The radiation oncologist recommended adjuvant

radiation to maximize local control while sparing a strip of tissue to help prevent edema. The

logistics, benefits and risks of the radiation treatments were explained, all questions were

answered at time of the discussion and the parents wished to proceed

Past Medical History: Prior to the above stated Ewing sarcoma LL has had an unremarkable

medical history. Since the discovery of the Ewing sarcoma in May 2013 she has had multiple

pRBC transfusions, suffers from hypotension, and underwent numerous surgeries. The surgeries

include tibia soft tissue biopsy, Broviac port placement, WLE of the left tibia, reconstruction and

skin grafts all occurring in September 2013 along with bone marrow aspirations of both the right

and left iliac crests. LL and her parents have also stated she has no know allergies.

Social History: LL is a currently an 8th

grade student with excellent grades living at home with

her parents and siblings. LL states she has never smoked, used smokeless tobacco, or consumed

alcohol. Her mother has thyroid disease along with a maternal aunt. Her maternal grandmother

and grandfather have hypertension, heart disease, and kidney disease and her grandfather also

has prostate cancer. Her Paternal Grandfather has lymphoma.

Medications: LL has been prescribed the following medications: acetaminophen (Tylenol)

650mg, amitriptyline 25mg, cholecalciferol (vitamin D3), Dextrose 5% and 9% NaCl, docusate

sodium (Colace) 100mg, heparin flush (porcine) 10unit/mL solution, ondansetron (Zofran) 8mg,

polyethylene glycol 3350 (Miralax) 17g packet, promethazine 12.5mg, sulfamethoxazole-

trimethoprim (Bactrim DS) 800-160mg, cephalexin (Keflex) 500mg, diazepam (Valium) 5mg,

diphenhydramine (Benadryl) 25mg, oxycodone (Roxicodone) 5mg, pantoprazole (Protonix)

20mg, and senna (Senokot) 8.6mg.

Diagnostic Imaging: In May of 2013 LL underwent an MRI reveling abnormalities and a mass

in her left tibia, these were then biopsied and discovered to be Ewing sarcoma. With this finding

a CT scan was performed and appeared clear for metastatic disease. She received another MRI

in September 2013 after multiple rounds of chemotherapy showing a decrease in size of initial

abnormality and mass.

Radiation Oncologist Recommendations: After examination of LL’s images, reviewing her

history and pathology and speaking with the family, the radiation oncologist recommended

adjuvant radiation post-chemotherapy. The radiation would be delivered using a simplistic plan

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of two oblique fields covering the area with a small margin of healthy tissue yet allowing a strip

of tissue to remain untreated to prevent circumferential radiation1.

The Plan (prescription): The radiation oncologist prescribed a dose of 50.4 Gy in 1.8 Gy

fractions. The concept was to use a simplistic plan that covered 95% of the planed target volume

(PTV) with 95% of the dose while sparing at least 1cm strip of tissue. It is very important not to

treat the entire circumference of an extremity; it could cause fibrosis and compartment

syndromes1. MLC’s were used along with collimator rotation and jaws to minimize dose to the

healthy tissue. The fields were also coplanar creating a straight edge allowing more tissue to be

spared in the lateral edge of the posterior leg.

Patient Setup/Immobilization: LL partook in a CT simulation in the first part of February

2014 for planning of her radiation therapy treatments. During this procedure she laid supine on a

CT simulation couch with, feet toward the gantry. The skin graft on her left lower leg was

outlined with CT wire and wrapped in a full sheet (30 cm x 30 cm) of 5 mm Superflab.

Superflab is a commercially made material that imitates tissue when interacting with radiation,

also known as bolus2. A custom mold called Alpha cradle was made out of Styrofoam, a plastic

bag and some foaming agents for positioning2. The Alpha cradle immobilized her left leg from

mid-thigh to bottom of foot and allowing her right leg to be positioned out of the way for

imaging purposes (Figure 1). The Alpha Cradle was placed on top of a 2 cm foam pad for

patient comfort and her head rested on a pillow with hands on her abdomen (Figure 2).

Anatomical Contouring: The data set was imported into the treatment planning system (TPS)

Eclipse. The resident and staff radiation oncologist contoured the clinical target volume (CTV)

by viewing the CT scan and the wires placed around the skin graft. A planning directive was

completed requiring the medical dosimetrist to create a PTV by applying a .5 cm margin to the

CTV in all directions.

Beam Isocenter/Arrangement: The medical dosimetrist used an alignment tool in the Eclipse

TPS to place the isocenter in the center of PTV. She then tried different beam arrangements that

would allow for the best PTV coverage and at least a 1 cm strip of tissue not getting dose. The

finally result was a LAO (Left Anterior Oblique) field at 316.7 degrees and a RAO (Right

Anterior Oblique) at 141.7 degrees both with a collimation of 90 degrees. The medical

dosimetrist chose these exact angles in order to create a parallel edge with the LAO Y2 jaw and

the RAO Y1 jaw. This parallel edge, also called the coplanar edge, prevents the beam from

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diverging into the 1 cm section in the lateral/posterior portion of her leg that was to be spared

from dose as requested. Multileaf collimators (MLC’s) were used to conform the beams to the

PTV with a 7 mm margin (Figures 7 & 8). This helps block the normal tissue and concentrate

the dose on the PTV. The physician prescribed 6 MV (megavoltage) energy for all fields in the

plan and treatment on a Varian EX linear accelerator. Due to the thickness of most extremities,

lower energies are often used.

Treatment Planning: As requested according to the planning directive completed by the

physician the beams were 6 MV and weighting was adjusted to allow the best coverage of the

PTV. The physician prescribed 28 fractions of 1.8 Gy for a total of 50.4Gy. It was specified that

the 95% isodose line (IDL) should cover 95% of the PTV. It was also requested to keep the hot

spots out of the fibula but not if it sacrificed coverage of the PTV. The medical dosimetrist

accomplished the desires of the physician while working with the CT scan in the Varian

treatment planning system Eclipse. The fields ended up being equally weighted and no wedges

were used (Figures 3-6). The 95% IDL covered 99.4% of the PTV and the hot spots were not in

the fibula (Figure 9).

Quality Assurance/Physics Check: The monitor units (MUs) for the plan were checked using

the Mobius software. Mobius also checks the planning objectives and dose volume histograms

(DVH’s) of the plan. Once the plan has passed the Mobius check the physics staff will do a

visual check against the requests of the physician on the directive and make sure the plan is

ready for treatment (Figure 10).

Conclusion: One of the biggest obstacles in this plan was sparing at least a 1 cm strip of tissue.

Other hurdles were avoiding the opposite leg, preventing a hot spot in the fibula without

jeopardizing coverage of the PTV and immobilization of the leg. It was nice that the 1 cm strip

sparing served as my only critical structure and understanding how that could hinder lymphatic

drainage helped me understand the importance of sparing the tissue. Using an Alpha cradle for

immobilization was helpful for reproducing the angle of the leg and showing exactly where the

bolus material should go, but it does have its downfalls as well. When the physician

contemplated removing the bolus for treatment it was discussed how that would leave a gap in

the cradle around the patient’s leg which would allow for movement. The physician decided to

leave the bolus for reproducibility and do weekly skin checks to assure the skin is not getting too

much dose

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References

1. Antman KH, Eilber FR, Shiu MH. Soft tissue sarcomas: current trends in diagnosis and

management. Curr Probl Cancer. 1989; 13(6):340-367 http://dx.doi.org/10.1016/0147-

0272(89)90015-9

2. Coleman AM. Treatment procedures. In: Washington CM, Leaver D, eds. Principles

and Practice of Radiation Therapy. 3rd

ed. St. Louis, MO: Mosby; 2004:183-184

3. Bentel GC. Radiation Therapy Planning 2nd

ed. New York, NY: McGraw-Hill; 1996.

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Figure 1. Patient’s left leg immobilized in Alpha Cradle.

Figure 2. Patient on CT simulation table

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Figure 3. Four panel screen shot showing isocenter placement and dose distribution

Figure 4. Single panel axial view of isocenter placement and dose distribution

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Figure 5. Single panel sagittal view of isocenter placement and dose distribution

Figure 6. Single panel coronal view of isocenter placement and dose distribution

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Figure 7. LAO DRR showing MLC’s formed around PTV (purple) with 7 mm margin and

isocenter placement

Figure 8. RAO DRR showing MLC’s formed around PTV (purple) with 7 mm margin and

isocenter placement

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Figure 9. DVH showing 95% of dose is covering 99.4% of PTV (purple line)

Figure 10. A section of the plan’s second check through Mobius software