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Louise Francis

November Case Study

November 24, 2011

Right Prone Breast Irradiation for Ductal Carcinoma In Situ

History of Present Illness: PH is a 63 year old African-American woman who, on routine mammogram,

was found to have a lesion in her right breast. She underwent a stereotactic-guided biopsy which

showed fibrocystic changes warranting further workup. She subsequently underwent a needle biopsy

which revealed high grade ductal carcinoma in situ. Following the biopsy, PH underwent a needle

localization and segmental mastectomy of a lesion measuring 7.2 x 6.7 x 3.5 cm. The pathology report

revealed ductal carcinoma in situ of high nuclear grade with extension into lobules. The margin for the

specimen was negative, as were the progesterone and estrogen receptors. The patient has recovered

well from the segmental mastectomy and has been referred to radiation oncology for postoperative

irradiation of the right breast.

Past Medical History: The patient’s medical history is significant for diabetes, hypertension, coronary

artery disease and hyper cholesterolemia. She has had two cardiac catheterization procedures and has

also undergone bilateral knee replacements.

Family/Social History: The patient was adopted and therefore does not know her family history. She

denies alcohol or tobacco use. She has a daughter with cervical cancer and hypertension, a second

daughter with cervical cancer and a third daughter who is healthy. The patient is a veteran.

Medications and Allergies: PH currently takes the following medications: Metformin, Labetalol, Aspirin,

Lisinopril, Meclizine and Pravastatin.

Diagnostic Imaging: PH initially underwent a diagnostic mammogram which revealed nonspecific

abnormal findings. She was then scheduled for a stereotactic-guided biopsy to further evaluate the

abnormal findings. A stereotactic biopsy takes samples from a lump that cannot be felt during a breast

exam, but can be seen on a mammogram or an ultrasound.1 The biopsy obtained 6 cores from an upper

outer quadrant right breast mass. A clip was placed in the breast to denote the biopsy area. Following

this procedure a repeat mammogram was performed to verify clip placement. A preoperative

mammographically guided wire localization was placed immediately prior to patient’s segmental

mastectomy. The mastectomy specimen consisted of a 7.2 (medial to lateral) x 6.7 (superior to inferior)

x 3.5 cm (anterior to posterior) mass containing both localization guide wire and clip. The final pathology

report revealed ductal carcinoma in situ.

Oncologist Recommendation: The radiation oncologist reviewed the current findings with the patient

and offered whole breast irradiation using tangent fields. The initial treatment fractionation was 2 Gray

(Gy)/fraction for 25 fractions totaling 50Gy to the entire breast. The boost irradiation scheme would

consist of 2Gy per fraction for five fractions totaling 10Gy to the tumor bed. The overall dose to the right

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breast would be 60Gy. The patient was in agreement with the oncologist’s recommendations and was

given a simulation appointment time.

Simulation: When the patient arrived for simulation and the physician noted the large size of her breast,

he suggested that she would probably be a good candidate for prone breast irradiation. With the patient

in a prone position the irradiated breast volume (breast bridge) would be much thinner than if the

patient was positioned supine and the amount of lung in the treatment field would be reduced. As the

breast bridge increases in size, it can create a larger and much higher hot spot because of the physical

properties of percentage depth dose.2 The patient was assisted into the prone position atop the Q-FixTM

prone breast board (Figure 1). She was positioned so that the breast would fall at least 2 cm away from

the breast board in the superior, inferior and medial direction. The breast should also be at least 2 cm

away from the table top (Figure 2). Close attention was paid to the proximity of the breast to the table

top and prone breast board because scatter from these items had in the past, led to increased skin

reactions. Once adequately positioned (Figure3) the patient was scanned using a Computed

Tomography (CT) scanner. The scan slice thickness was 3 mm and the data was sent to dosimetry for

treatment planning. Prior to the completion of simulation, the patient was given posterior and right/left

setup tattoos for leveling, and instructed to return to begin her treatments.

Figure 1: Q-FixTM Prone breast board

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Figure 2: Patient positioned on the prone breast board.

Figure 3: Full lateral view of patient positioning.

Treatment Planning: The C.T. data set was sent to dosimetry and imported into the Phillips ADAC

Pinnacle treatment planning system. The isocenter was placed in the middle of the hanging breast by

the dosimetrist. This involved shifting the table 25 cm up from the setup tattoos placed during

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simulation (Figure 4). The medial tangent field was then designed with a field width and gantry angle

that covered as much of the breast as possible without hitting any part of the prone breast board. The

lateral tangent field was set using the same criteria, in addition to matching the divergence of the

medial beam (Figure 5).

Figure 4: Isocenter placement and breast to table top distance.

Figure 5: Medial and lateral tangent beams setup.

The prone treatment position displaces the breast tissue away from the rest of the body, and more

specifically, away from critical structures such as the lung or heart, helping to limit dose to these

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organs.2 In this position the breast takes on more of a rectangular shape as opposed to the standard

triangular shape when the patient is supine. This rectangular shape becomes helpful when planning

because it aids in limiting the number of step- and- shoot segments necessary to obtain a homogeneous

plan. Since PH had no risk of chest wall involvement, the physician did not feel the need to construct

tangent fields that encompassed this area. Each tangent field utilized only two step- and –shoot

segments per beam to create a homogeneous plan.

Figure 6: Initial tangent field dose distribution.

Figure 7: Initial tangent field’s step- and- shoot segments.

The initial tangent fields were taken to a dose of 50Gy at 2Gy/fraction for 25 fractions. The prone setup

proved to be very reproducible for the initial fields; therefore the patient was also treated in this

position for breast boost. The breast boost setup consisted of two laterally opposed fields that gave a 2

cm margin around the cavity (Figure 8a & 8b). The total boost dose was 10Gy given at 2Gy/fraction for

five fractions (Figure 9). The total dose given was 60Gy to the right breast. The dose volume histograms

(DVH) reflect the dose given during the initial and final boost treatment plans (Figure 10 and Figure 11).

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Figure 8a: Laterally opposed right breast boost fields.

Figure 8b: Laterally opposed right breast boost fields.

Figure 9: Boost fields dose distribution.

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Figure 10: Initial tangent breast plan DVH.

Figure 11: Final laterally opposed boost plan DVH.

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All plans were second checked by a physicist for accuracy. The second check utilized a spread sheet

program that was expected to agree with the ADAC pinnacle plan within 3%. Each field received an

independent second check (Figures 12-15).

Figure 12: Field 1.1 Right Breast Medial Tangent second check.

Figure 13: Field 1.2 Right Breast Lateral Tangent second check.

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Figure 14: Field 1.3 Left Lateral boost second check.

Figure 15: Field 1.4 Right Lateral boost second check.

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Conclusion: Prone breast simulation and treatment require careful setup considerations. It is important

to remember to check the patient’s position on the prone board before the C.T. scan is done. If this step

is not carefully performed the patient may have to be resimulated because necessary beam angles may

not be achievable without hitting the breast board. This case study was chosen because prone breast

treatments are new to this facility. Patient rotation becomes an issue when trying to adequately cover

masses that are close to the chest wall and positioning very large patients. Often with larger patients,

patient rotation is necessary to accomplish coverage of tumor volumes, while on smaller patients it is

not. For other patients, removal of the cushion from under the contralateral breast may improve setup.

Purchasing commercially available prone breast boards should require the input of all members of

radiation oncology treatment team. Many of the newer models include incremental parts and angled

cushions that provide increased patient comfort and better reproducibility.

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References

1. Stephan,P. Stereotactic Breast Biopsy for Breast Abnormalities. http://breastcancer.about.com/od/

breastbiopsy/p/stereotactic.htm Updated January 16, 2009. Accessed November 20, 2011

2. McKinnes, R, Collins, K. Comparing Critical Structure Dose: Prone vs Supine Breast Treatments. ASRT

Radiation Therapist. 2011;20(2):103-107.