Kayla Tedrick 1Case Study 4/8/18

Parotid Sparing Whole Brain Irradiation

History of Present Illness: The patient, SM, was a 29 year old female who in July 2017,

presented a complaint of abdominal pain. She expressed her recent decreased appetite leading to

a weight loss of 20 pounds. She also presented with bumps on her head and neck region as well

as her groin. The healthcare provider at this visit noted a slight deviation to the right of her

tongue. A chest x-ray on July 3, 2017, showed a right paratracheal lesion. On the next day, a CT

of the abdomen and pelvis revealed a right ovarian lesion, enlarged para-aortic nodes, external

iliac nodes, inguinal nodes, liver lesions, lytic lesions on iliac bone, and lumbar and thoracic

vertebrae lesions. Next was an MRI of the brain on July 5, 2017, where there was found to be

multiple parenchymal masses with involvement of the right temporal and occipital bone, a mass

of the right sphenoid sinus into the right orbit, and the largest lesions resided in the sella and

pineal gland. An MRI of the abdomen on the same day further discovered liver and spleen

lesions, a left adrenal mass, a pancreas mass, and upper abdominal lymphadenopathy. This day

concluded with an image guided liver biopsy, noting a neuro-endocrine carcinoma (NEC). Upon

meeting with medical oncology on July 7, 2017, she was diagnosed with stage IV metastatic

NEC of an unknown primary. Systemic chemotherapy and local external beam radiation was

recommended and discussed. The next day, SM began chemotherapy comprised of cisplatin and

etoposide.

After a couple months passed of chemotherapy, on September 28, 2017, she underwent

another MRI of the brain which revealed no evidence of metastasis in the parenchymal region or

the mass in the sphenoid sinus and pineal gland, however, extensive osseous metastasis was

found in the cranium and skull base. Days later on October 4, 2017, she switched her

chemotherapy regimen to carboplatin due to paresthesias on her hands and feet. At the end of

November, it was noted she had almost complete response to chemotherapy and was then

referred to radiation oncology. In February of 2018, an MRI of her brain presented with

numerous new parenchymal metastasis.

Past Medical History: SM had Hepatitis C that had been going untreated for an unknown

amount of time. She also stated that she, along with her mother, tested positive for the BRCA-1

gene. This was found later to be incorrect, that she instead had the BRCA-2 gene. Lastly, she had

complained of severe anxiety and had been on multiple different medications for it in the past.

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Social History: SM was unemployed for an unknown amount of time, with two children. She

had a smoking history of 0.50 packs/day for the past 5 years. She also noted a previous problem

with substance abuse of opioids. Her mother had previous breast cancer, in which she was 42

years old at the time of onset.

Medications: SM had been taking a number of medications throughout her journey once

diagnosed. First, she had been taking duloxetine to aid in controlling her anxiety. While she was

hospitalized she was given morphine for pain alleviation. Sucralfate and pantoprazole were

medications to help relieve stomach issues such as GERD and acid buildup. Lastly,

dexamethasone, a corticosteroid, is a medication she was taking to reduce swelling in her brain

that can typically occur with brain tumors.

Diagnostic Imaging: After presenting with abdominal pain and weight loss, the patient

underwent a chest x-ray on July 3, 2017 which revealed a right paratracheal lesion. On July 4,

2017, a CT of the abdomen and pelvis showed multiple metastatic lesions which then led to the

decision to take an MRI of her brain the next day, which to no surprise, presented multiple

metastatic lesions as well. An MRI of the abdomen on the same day, for better soft tissue

viewing, confirmed those lesions. The image guided liver biopsy later that day furthered her

workup with findings of a high grade NEC. After a course of chemotherapy, an MRI of her brain

on February 16, 2018 once again revealed extensive brain metastasis.

Radiation Oncologist Recommendations: The radiation oncologist saw SM on February 20,

2018 and due to her diffuse brain metastasis, recommended whole brain radiation therapy

(WBRT) and a short steroid taper. The side effects, risks, benefits, and alternatives to this

method were discussed and SM signed the consent form to radiation therapy to her brain. It was

made clear that this is in palliative nature, emphasizing the fact that radiation will not be of

curative intent. A follow up MRI was scheduled for 2 months later for surveillance.

The Plan (prescription): The radiation oncologist prescribed 30 Gy in 10 fractions for WBRT.

A 2D arrangement would be utilized for treatment planning with a parotid sparing technique in

order to minimize side effects. SM had her CT simulation on February 20, 2018 and began

treatment two days later.

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Patient Setup/Immobilization: SM’s setup for radiation therapy treatment was determined in

CT simulation. She was simulated supine with her hands relaxed comfortably at her sides and a

sponge underneath her knees for extended comfort and lower back relief. A short aquaplast mask

was utilized for immobilization of her head (Figures 1-2). Reference marks were placed on the

mask at this time for the purpose of shifting to isocenter on treatment day.

Anatomical Contouring: Utilizing the Eclipse treatment planning system, a number of contours

were performed by the medical dosimetrist. These included the body, which must include

everything that the treatment planning system needs to take into account for calculating dose,

including her headrest and mask, as well as her brain, eyes, lens, parotids, and spinal cord. The

doctor is not required to draw any contours for WBRT at my facility. However, the doctor does

check each contour performed by the dosimetrist before treatment planning begins.

Beam Isocenter/Arrangement: SM was scheduled to be on a Varian Truebeam SD machine.

Prior to calculating a plan, the isocenter was placed midline in the center of the patient’s brain

(Figures 2-5). Before placing beams, 6 MV was chosen as the energy for all beams due to the

small separation in the head area. Next, a lateral beam was placed so that both left and right lens

of SM were in the same plane in the DRR. The angle chosen was 271 degrees, labeled RAO

(Figure 6). No collimator angle was used due to the enhanced ability to block out certain

structures such as the eyes, lens, and parotids with no turn of the collimator. The dosimetrist then

created the field borders by moving the independent jaws in the treatment planning system.

Inferiorly, the jaw was moved to the bottom of C2. Superiorly, 2 centimeters of flash beyond the

brain was provided. Anteriorly, 1 centimeter of flash was left to cover the cranium but to avoid

the left and right lens. Posteriorly, the jaw was placed to leave 2 centimeters of flash from the

brain. Next, MLCs were utilized to block out essential structures that were vital to spare,

including the eyes and lens, most importantly. Anteriorly, the MLCs were brought in, staggering

down from blocking just above the eye to blocking the parotid gland. The same process was

done for the opposed angle, LAO, which turned out to be 83 degrees and no collimator turn

(Figure 7).

Treatment Planning: This treatment plan was completed using Varian Eclipse version 13.6.

The prescription from the radiation oncologist was placed into the system, 30 Gy in 10 fractions.

Using the beam parameters mentioned above, the plan was calculated to get 100% of the dose to

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isocenter. After the first calculation, hot spots arose in the anterior portion of the skull and brain,

leading to the addition of two reduced fields covering these hot spots. After finding the ideal

beam weighting, an acceptable hot spot was then found and the plan was normalized down to

99.8% IDL to increase coverage of the brain and achieve the objective of 100% of the brain

receiving 95% of the prescription dose (Figures 8-10). The final beam weighting was 44% on the

RAO, 5% on the RF RAO, 46% on the LAO, and finally 5% on the RF LAO. The completed

plan was compared to the dose objectives given by the radiation oncologist in the prescription,

noting the importance of keeping the lens dose as low as possible and less than 10 Gy, as well as

keeping the mean of both the left and right parotid glands below 15 Gy (Figure 11). The DVH

showed that both lens were not receiving any more than 5 Gy, and the parotid means were 8.82

Gy and 12.85 Gy for the right and left, respectively (Figure 12).

Quality Assurance/Physics Check: The final plan was then passed on to a second dosimetrist to

be peer-reviewed. After that, the plan was inserted into a quality assurance course where a

calculation point was then placed within the brain, and then exported to RadCalc. RadCalc is

utilized in this department as a monitor unit check, in which our department has a standard of no

more than a 3% difference. The physics department then checked over the plan and monitor unit

checks and approved it for treatment.

Conclusion: WBRT is typically of palliative intent with an expected short survival of the

patients under treatment. In these situations, quality of life should be kept first in mind, and

xerostomia can be a severe side effect of WBRT that we must be cautious of.2 Parotid glands are

normally regarded as organs at risk when a patient is undergoing head and neck radiation

therapy, but recent studies have been finding that they should be considered for WBRT as well,

even though the overall dose prescribed is significantly lower. It has been found that parotid

gland function can suffer a loss of 5% per 1 Gy of mean dose.1 For one parotid gland, the

recommended mean dose limit for the prevention of severe xerostomia is less than 20 Gy, and

the mean for both parotid glands less than 25 Gy.2 With overall prescription doses for WBRT

ranging from 25 Gy to 35 Gy, these limitations certainly need to be considered. A study done

with 20 patients who previously underwent WBRT observed the dosimetric effects of modifying

the original opposed lateral fields to block out the parotid glands without sacrificing coverage of

the brain.1 It was found that when the average mean gland dose was decreased by about 50% for

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parotid sparing WBRT plans, as compared to the original WBRT plans, the target received

99.2% prescribed dose versus 99.4%, respectively. The difference in coverage is minimal, and

certainly worth it when it has been known that a low dose of 15 Gy can diminish the salivary

production of the parotid glands by about 50%.1

Although it is too early to follow up with SM and assess her side effects, this case

symbolized the goal of dosimetry in that we must optimize dose to the target while minimizing

dose to the surrounding critical structures. If coverage of the brain is not forfeited to a

measurable extent, I think parotid sparing WBRT is definitely a technique that should be applied

in the clinical setting especially if it increases quality of life for those patients. At my clinical

site, currently only one doctor is utilizing this technique, which presents another example of what

dosimetrists must remember. It is our job to take the doctor’s prescription and dose objectives

and achieve them, regardless of the fact many doctors do different things. This is exciting to me

as a dosimetry student because these differences challenge me and allow me to improve. I am

interested to see in the future if parotid sparing WBRT will be more common at this site or at a

different one I may become employed.

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References

1. Shah MM, Barton KN, Yechieli R, Gopal A, Siddiqui F. Parotid gland dose in whole-

brain radiation therapy patients. International Journal of Radiation Oncology.

2014;90(1)S327-S328. doi:10.1016/j.ijrobp.2014.05.1082.

2. Noh OK, Chun M, Nam SS, et al. Parotid gland as a risk organ in whole brain

radiotherapy. Radiotherapy and Oncology. 2010;98(2)223-226.

doi:10.1016/j.radonc.2010.12.013.

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Figures

Figure 1. Full body image showing patient positioning with arms at sides and an aquaplast mask

on the head for immobilization.

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Figure 2. Digitally reconstructed radiograph (DRR) indicating isocenter placement.

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Figure 3. Isocenter placement in the axial plane.

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Figure 4. Isocenter placement in the coronal plane.

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Figure 5. Isocenter placement in the sagittal view.

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Figure 6. DRR of the field RAO. Note the field borders and the blocking of the lens, eyes, and

parotid glands (blue and green).

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Figure 7. DRR of the field LAO. Note the field borders and the blocking of the lens, eyes, and

parotid glands.

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Figure 8. Isodose distribution in the axial plan with 100% of the brain receiving 95% of the

prescription dose.

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Figure 9. Isodose distribution in the coronal plane with 100% of the brain receiving 95% of the

prescription dose. See the parotid glands getting minimal dose.

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Figure 10. Isodose distribution in the sagittal plane with 100% of the brain receiving 95% of the

prescription dose. See the right parotid in blue getting minimal dose.

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Figure 11. Treatment prescription with the objectives in the comments section.

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Figure 12. DVH with each structure noted in the dose objectives given by radiation oncologist.

See the brain getting covered by the prescription dose while keeping dose to the surrounding

structures at a minimum.