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Angela Kempen

February Case Study

February 22, 2012

3D Conformal Radiation Therapy for Mucinous Carcinoma of the Breast

History of Present Illness: JE is a 45 year-old Caucasian female who underwent a regular

scheduled screening, which led to the detection of a right breast cancer. She had a screening

mammography that defined an 8 millimeter (mm) irregular density in the right breast at 6 o’clock

posterior depth. She was referred to the Breast Center. The lesion was non-palapable. When a

diagnostic mammography was performed, an irregular 13 mm focal asymmetrical density with

an indistinct margin was seen in the right breast at 6 o’clock posterior depth. On ultrasound,

there was a subtle area of vertically oriented irregular hypoechogenicity in the 6 o’clock location,

7 centimeter (cm) from the nipple corresponding to the mammographic density. She had a

vacuum-assisted biopsy, with clip placement, that same day. The pathology returned as benign

breast tissue with nerve bundles. The surgeon met with the patient and discussed options for

management considering this could be benign, breast malignancy, or a different soft tissue

malignancy. She elected to have an excisional diagnostic biopsy. During the biopsy, the

surgeon felt a suspicious lump and converted the procedure to a formal lumpectomy. The

lumpectomy demonstrated a 1.3 cm, grade 1, mucinous carcinoma with 1 cm of intermediate

grade ductal carcinoma in situ (DCIS). No peritumoral intralymphatic tumor emboli were found.

The anterior invasive tumor margin was 2 mm, inferior margin was 4 mm, and all others < 10

mm. Margins for the DCIS were 6 mm anterior and all other margins were < 10 mm. The tissue

was estrogen receptor (ER) 3+ and progesterone receptor (PR) 3+. The patient was staged with a

T1c N0 M0 mucinous breast carcinoma. She was referred to medical and radiation oncology for

further management.

Past Medical History: The patient has mild acne. She has a long smoking history. No

significant history of alcohol use. The patient has no previous history of previous malignancies

or radiation.

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Diagnostic Imaging Studies: The patient had a diagnostic mammogram which identified a 13

mm abnormality at 6 o’clock in her right breast. She underwent a subsequent ultrasound with

abnormal findings correlating to the mammographic finding. A vacuum-assisted needle biopsy

identified benign breast tissue with nerve bundles. She underwent a right axillary sentinel lymph

node biopsy. A surgical excision was recommended. She underwent a needle-localization open

lumpectomy, where a suspicious lump was noted. The procedure was converted from a

diagnostic excisional biopsy to a formal lumpectomy, in which the surgeon took extra tissue and

performed margin assessment. She had chest x-rays to evaluate any possible metastasis.

Family History: The patient has a significant family history. Her mother was diagnosed with

breast cancer at age 38, colon cancer at age 59, and lung cancer at 64. She is deceased at age 67.

Additionally, her grandmother’s sister was diagnosed with breast cancer at a young age. The P53

sequence analysis showed “no mutation detected”. CHK2 sequence analysis of 1100delC

mutation was not detected. The patient’s father has a history of diabetes and hypertension.

Three of her uncles had skin cancers at ages 68, 63 and 59.

Social History: The patient is married and resides with her husband. She has two children. The

patient is a current every day smoker, approximately half a pack per day. She works as a

technician at Dairyland Power Cooperative.

Medications: The patient uses the following medications: bupropion XL and hydrocodone.

Recommendations: The Radiation Oncologist recommended post-lumpectomy radiotherapy to

the right breast. The logistics of radiation were discussed with the patient, in addition to the

utilization of post-lumpectomy radiotherapy to reduce the risk of recurrence. The risks, benefits,

side effects, and alternatives were explained. The patient understood the benefit of reduced risk

of recurrence and alternatives, including any recommendations from medical oncology.

The Plan (Prescription): The plan is for the patient to receive 50.4 Gray (Gy) in 28 fractions,

with a boost to the planning treatment volume (PTV) lumpectomy of 10 Gy in 5 fractions using

tangential treatment fields. The goal is for 95% of the boost PTV lumpectomy composite to

receive 60.4 Gy.

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Patient Setup/Immobilization: The patient was simulated in the prone position, using the

prone breast board, and a wedge cushion under her ankles for support. A treatment planning

computed tomography (CT) scan was competed with 2.5 mm slices through the chest region.

Anatomical Contouring: The physician drew contours in the treatment planning program,

Pinnacle. The physician contoured the right breast, the right breast – 0.5 mm skin, the

lumpectomy site, and the PTV lumpectomy (lumpectomy + 1.5 cm margin). I contoured the

heart, right lung and carina.

Beam Isocenter/Arrangement: I placed the isocenter mid-depth in the superior to inferior

plane in the breast; however, slightly lateral. The calculation point was placed within the

thickest portion of the breast tissue in order to increase coverage in the area closet to the

chestwall. The first beam I placed was a right posterior oblique (RPO) at 68 degrees. The

second beam placed was a left anterior oblique (LAO) at 258 degrees. I matched the divergence

of the posterior field edges of the beams in order to avoid divergence into the chestwall and

lungs. Therefore, the beams were not truly parallel-opposed because they were slightly less than

180 degrees apart at the central axis. A photon beam energy of 6 megavoltage (MV) was used

for each beam. The same beam arrangements and energies were used for the photon boost. A

0.7 cm margin was placed around the PTV lumpectomy for the boost treatment.

Treatment Planning: The physician provided a list of the objectives he would like to achieve to

complete this treatment plan, using the Pinnacle treatment planning system. He listed several

structures and the corresponding instructions. He required 90-95% of the breast (subtracting a

0.5 cm skin margin) to receive 50.4 Gy, in addition to 95% of the PTV lumpectomy to receive

50.4 Gy. For the boost, he required 95% of the boost PTV lumpectomy to receive 10 Gy, and for

95% of the composite boost PTV lumpectomy to receive 60.4 Gy. Other requirements included

25% or less of the ipsilateral lung to receive 20 Gy, and less than 5% of the heart to receive 25

Gy.

After initial placement of the beams, the physician drew blocks to shield the lung. I computed

the dose and reviewed the isodose distribution. I contoured in any hot spots, and optimized the

plan by creating segments with the step and shoot multi-leaf collimator (MLC) to minimize the

hot spots. Since the PTV was in close proximity to the chestwall, it was not adequately covered

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with the prescription dose. I placed supplemental fields, of 4 Gy each, to boost the chestwall

area to the prescription dose. I achieved this by contouring the cold spot located in the PTV, and

then adding a field from each beam angle encompassing only this under dosed area.

Supplemental fields are a way for the breast to achieve a more homogenous dose distribution to

the prescription. The alternative was to move the calculation point; however, this method was

creating more hot spots to segment out and decreasing coverage elsewhere within the breast. In

addition, moving the calculation point created hotter areas medially and laterally in the breast,

where it is not possible to segment them out. The physician evaluated the plan and prescribed to

the 99% isodose line. The boost plan utilized the same beam angles and energy. A 0.7cm

margin was placed around the PTV lumpectomy cavity. Weighting the RPO beam 53% and the

LAO beam 47% optimized the boost plan. I showed him the boost plan and he prescribed to the

100% isodose line. He evaluated the composite and approved the plans.

Figure 1: Transverse, Sagittal and Coronal slices of calculation point location

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Figure 2: Isodose Transverse slices

Figure 3: Rpo and Lao supplemental fields

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Monitor Unit Check: Monitor unit (MU) calculations were performed to verify that the

treatment planning computer was calculating properly. MU check takes several factors into

consideration. These include: beam energy, field sizes (equivalent square), TMR or PDD, output

factor including Sc and Sp, SSDs, physical and effective depths, inverse square, off-axis

calculation point, percent isodose lines and dose. It includes all these factors in a calculation to

determine the monitor units needed for treatment. The MU result is compared to the computer

MU calculation. The difference between the two should result in less than 5%, which it did for

this plan. I did hand calculations for each field as well, where the RPO, LAO, RPO

supplemental, and LAO supplemental had 1.01%, 1.02%, 1.11%, and 1.10% percent differences

respectively.

Quality Assurance: A physicist performed a second check of the treatment plan and monitor

unit check. It is critical to have an independent set of eyes look over the plan and additional

checks.

Conclusion: I chose this patient for my case study because it was the first treatment plan I

completed on my own. Many breast cases are done at Gundersen Lutheran in the prone position,

so I had the opportunity to observe quite a few before attempting to construct a treatment plan

individually. From this treatment plan, I gained a better understanding of how tangential fields

are set up, in addition to how dose distribution is affected by the placement of the calculation

point. I had some assistance with the construction of the supplemental fields, but learned a lot

about this technique. I learned a lot from this patient’s plan and feel I am capable of creating

treatment plans of similar cases in the future.

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Figure 4: Dose Volume Histogram