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Matching Electron and Photon Fields in Tangential Breast Treatment

History of Present Illness:

Patient SM is a 68 year old female that received a routine mammogram on March 28,

2016. The scan revealed irregular densities in the left breast and a six moth follow up was

requested. On October 17, 2016 the follow up diagnostic mammogram and ultrasound revealed

a mass in the left breast and heterogeneous calcifications. Approximately one month later on

November 7, 2016 the patient underwent a biopsy of the left breast which revealed invasive

ductal carcinoma with mucinous features. The biopsy also showed that her estrogen receptors

(ER) were 90% positive, progesterone receptors (PR) were negative and Her2 were negative.

The staging CT and bone scan completed also reported negative results. At this time, the patient

transferred her care to the University of Michigan Hospital where the remainder of her treatment

would take place. A bilateral diagnostic mammogram was completed on December 5, 2016 and

revealed a mass and calcifications in the left breast with a negative right breast.

On December 22, 2016 SM underwent a lumpectomy and SLNE. The surgery revealed a

1.3cm adenocarcinoma with negative margins and microcalcifications present within invasive

carcinoma. Of the sentinel and non-sentinel lymph nodes, 0/1 and 0/2 lymph nodes respectively

were positive. On January 9, 2017 the patient had a post-lumpectomy mammogram that revealed

two groups of calcifications that were deemed suspicious. SM underwent a second surgery on

January 26, 2017 to remove these calcifications. This revealed foci of residual high grade DCIS.

At her post-operative checkup, SM denied having any surgical complications. Additionally, at

this time adjuvant radiation therapy was recommended to reduce risk of recurrence after the

second excision. It was suggested that she undergo hypofractionated radiation therapy with a 10

Gy boost after the completion of her primary radiation treatments. The patient has had no

previous radiation therapy treatments.

Past Medical History:

SM has a history of diabetes mellitus, and restless leg syndrome. Her surgical history

involves a hysterectomy and a foot surgery. SM has a history of skin cancer that was deemed

superficial melanoma. The melanoma was treated surgically only and did not require further

action or radiation therapy. Additionally, SM is allergic to adhesive. She experiences skin

redness and blistering if exposed to adhesive glue.

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Social History:

SM has never smoked and is a non-drinker. She is a retired dental hygienist with 2

children and is currently married. The patient reports the following familial medical history.

Her mother, father, and sister have hypertension. Her father, maternal uncle, and paternal

grandfather have heart disease. The patient’s father also has diabetes. In terms of cancer, her

sister had breast cancer and her paternal grandmother had melanoma.

Medications:

The patient was previous on a Hormone Replacement Therapy for 3 years and took an

oral contraceptive. Currently, the patient takes a multivitamin, Metformin, Metamucil, and

Zoloft.

Diagnostic Imaging:

The patient underwent a routine mammogram on March 28, 2016 that revealed irregular

densities in the left breast and a six moth follow up was requested. On October 17, 2016 the

follow up diagnostic mammogram and ultrasound revealed a mass in the left breast and

heterogeneous calcifications. On November 7, 2016 the patient underwent a biopsy of the left

breast which revealed invasive ductal carcinoma with mucinous features. The results of the

biopsy expressed that she was 90% ER positive while being PR and Her2 negative. The staging

CT and bone scan completed also reported negative results. A bilateral diagnostic mammogram

was completed on December 5, 2016 that confirmed a mass and calcifications in the left breast

with a negative right breast.

On December 22, 2016 SM underwent a lumpectomy and SLNE. The surgery revealed a

1.3cm adenocarcinoma with negative margins and microcalcifications present within invasive

carcinoma. Of the sentinel and non-sentinel lymph nodes, 0/1 and 0/2 lymph nodes respectively

were positive. On January 9, 2017 the patient had a post-lumpectomy mammogram that revealed

two groups of calcifications that were deemed suspicious. SM underwent a second surgery on

January 26, 2017 to remove these calcifications and foci of residual high grade DCIS was

revealed. Finally, SM received another mammogram before beginning radiotherapy.

Radiation Oncologist Recommendations:

After the second excision, the radiation oncologist recommended that SM receive

adjuvant radiation therapy to reduce the risk of recurrence. It was their recommendation that her

treatment utilize intact breast tangents and hypofractionation. Hypofractionated breast cancer

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radiation therapy is completed in 16 fractions of 2.66 Gy to a total of 42.566 Gy opposed to 25

fractions of 2 Gy to a total of 50 Gy. In a study completed by Linares, Tovar, Zurita et al.1 it was

shown that hypofractionated breast radiation therapy is a safe alternative to the more classical

schedule, with good cosmetic results and lower toxicity as well. It has also been shown that

hypofractionated treatment plans have a higher compliance rate from patients due to the shorter

treatment time.2 In addition to the primary course of radiation, it was also recommended that SM

receive a boost of 10 Gy to her tumor bed. This was to be completed in 5 fractions of 2 Gy each.

The radiation oncologist also suggested that SM’s treatment utilize the use of SDX Spirometric

Motion Management System breath holding and monitoring techniques if she could tolerate it.

This suggestion was made due to the fact that her cancer is in the left breast, and precautions

should be taken to reduce heart dose as much as possible. All of the possible options were

explained to SM along with her relatively low risk, and the patient decided to consent to

radiation therapy after this review with her radiation oncologist.

The Plan/Prescription:

Based off the radiation oncologist’s recommendations, the treatment plan for SM was

intact breast tangents completed in a hypofractionated schedule with a boost of 10 Gy at the end

of her primary course of treatment. Due to the fact that SM did not have any positive nodes, she

did not require a supraclavicular or posterior axillary boost (PAB) field. As defined above, the

prescription dose for hypofractionation is a total of 42.566 Gy completed in 16 fractions of 2.66

Gy. The boost prescription was set for 10 Gy to be administered in 5 fractions of 2 Gy. Both the

primary course and boost plans were to be delivered 5 fractions per week.

Patient Set-up and Immobilization:

On March 2, 2017 SM underwent a CT simulation to begin her radiation therapy. SM

was placed in the supine position with her arms above her head which were kept in place by the

cuffs of the breast board immobilization device. The patient’s head was supported by a head rest

that fits into the breast board. A knee cushion fix was placed under the patient’s knees as well.

Finally, the SDX Spirometric Motion Management System was utilized as well to help the

patient perform voluntary breath holds during treatment so as to reduce her overall heart dose.

This reduction in heart dose is seen because breath holds allow for the displacement of the heart

away from the chest wall and treatment area.3 The setup for using SDX involves the breathing

sensor, bacteria filter, mouthpiece, nose clip, and video goggles. The sensor is attached to the

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mouth piece which monitors the patient’s breathing with the use of the SDX software. When the

patient is wearing the video goggles, they see a screen that projects their breathing patterns and a

green band that represents where they need to hold their breath until the timer is up and the red

light appears. In addition to these goggles, the patient is also coached by the radiation therapists

over a speaker in the treatment room (Figures 1-3).

Figure 1: SDX machine equipment

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Figure 2: Patient set up while using the SDX machine for breathing monitoring

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Figure 3: Screen that is seen by patients when wearing the goggles during treatment

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Anatomical Contouring:

After the CT simulation was completed, the CT data was transferred to the Varian

Eclipse radiation treatment planning system (TPS). At this time, the radiation oncologist

contoured the tumor bed within the left breast. After that had been completed, the medical

dosimetrist contoured the breast and the organs at risk (OAR). The OAR included the ipsilateral

lung, heart, and body surface. The breast was contoured to include the entirety of the breast

tissue and then cropped 4 mm inside the body surface to account for the build-up region of dose.

Taking this penumbra into consideration allows for a more accurate dose volume histogram

(DVH) to be calculated and a more real representation of breast tissue coverage to be assessed.

In addition to the breast and OAR, there were a few other helpful contours created. These

contours included the surgical scar on the patient’s surface, the surgical clips at the site of the

tumor bed, and the carina. The surgical scar was contoured with the help of a wire placed on the

scar at the time of CT sim. The wire was contoured out of the body surface, and then given a set

CT value equivalent to air. The purpose of this is seen during the planning of her boost

treatments. Should SM have received her boost treatment with electrons instead of photons, the

scar would have been delineated as match anatomy for the therapists to line up her electron cut

out. It should not have a tissue density that could affect the treatment planning process and dose

distribution.

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Beam Isocenter and Arrangement:

The patient was treated on a Varian Linear Accelerator machine. The user origin was

placed at the time of simulation by the radiation therapists. This process is done by measuring

the distance between the medial and lateral catheters placed on the patient and finding a midpoint

between them. The user origin is then placed approximately 1cm anterior from the chest wall at

this midpoint. The isocenter of the fields is then placed at the same location as the user origin so

that it falls at (0,0,0). Subsequently, during daily treatment there are no shifts or moves to be

made from the CT reference points marked on the patient at the time of simulation.

The medial and lateral tangential fields were then set and evaluated. At this time it

became apparent that treating this patient with intact breast tangents would result in too much

dose being administered to the heart. SM’s heart was located too far anteriorly even with the use

of voluntary breath holds to displace it away from the chest wall. To compensate for this heart

anatomy complication the tangents were rotated away from the medial edge, and matched with

an electron beam at midline. This decision was made due to the fact that electron beams do not

penetrate at deep as photons and thereby would not expose the heart to as much radiation. This

change in treatment plan also resulted in the patient no longer being eligible for

hypofractionation. SM’s new prescribed dose was 25 fractions of 2 Gy per fraction to a total of

50 Gy with the addition of the electron beam anteriorly at midline.

The medial tangent was a 16MV energy beam with a gantry angle of 330 degrees, a

collimator rotation of 11 degrees, and contained 3 subfields. The lateral tangent was a 6MV

energy beam with a gantry angle of 154 degrees, a collimator rotation of 349 degrees, and

contained one subfield. All subfields were created with the field in field technique and were

16MV energy. The electron beam was 9MeV, had a gantry angle of 340 degrees and a

collimator rotation of 0 degrees. A 20x20 cone and 110 cm source to surface distance (SSD)

were used. The electron field was uniquely matched to the medial tangent with a 10 degree

gantry rotational difference and copper block. As the treatment planning process continued, it

became necessary to add a 6MV posterior (PA) beam with a single subfield. The field in field of

the PA beam was 6MV.

Treatment Planning:

The Eclipse TPS was used to create the following plan for the patient. The dose

constraints given by the radiation oncologist for the heart and ipsilateral lung were a mean dose

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of 1.5 Gy and the lung volume receiving 20 Gy (V20) less than or equal to 33%, respectively.

The target goals were to cover the entire breast with the 95% isodose line and to cover the tumor

bed with the 98% isodose. The objective of the plan was to treat SM with opposed medial and

lateral tangents while matching the medial tangent border with an electron field at anterior

midline. She would be treated to a total of 50 Gy in 25 fractions of 2 Gy per fraction. All three

beams were placed, and the match line between the electron field and the medial tangent was set.

A 10 degree gantry angle separation was determined to be the best location for the match. Once

that border was set, the plan was copied twice. On one copy, only the tangents were kept and the

electron field was deleted. On the second copy, the tangents were deleted and just the electron

field remained. The field in fields would be created on the first copy with only the tangents so as

to expedite calculations and allow for the future medical physics checks. At the end, a plan sum

would be made combining the electron field only plan and the tangents only plan to view the

complete dose distribution that the patient will receive. Both plans were linked under one

prescription.

The electron field was a 9MeV beam with a 20x20 cone size. The edge of the field was

blocked with a custom copper block so as to properly match the medial tangent field. This block

edge was drawn by hand in the Eclipse TPS by the medical dosimetrist and then ordered through

the use of a third party company, .decimal. The electron field was set at a 110cm SSD opposed to

a customary 100cm SSD. This was to allow for the fact that the patient’s arms were above her

head for the breast board set up and a large space would be needed to ensure there was so

collision between patient and machine.

The medial tangent was a 16MV energy beam and contained 3 smaller field in fields. All

of subfields were 16MV as well. The lateral tangent was a 6MV energy beam and contained 1

field in field of 16MV energy. The medial tangent was weighted more heavily than the lateral

tangent and was therefore delivering more dose than the lateral beam. This beam weighting was

determined before the field in field process began in order to ensure adequate coverage to the

entire breast and tumor bed. The medial tangent had more subfields than the lateral tangent due

to the fact that the match line with the electron field created a hotter area that needed to be

blocked. All medial and lateral fields contained multi-leaf collimator (MLC) heart blocks to aid

in reducing dose to the heart (Figure 11). Once the tangent field in fields had been created, the

dose coverage to the tumor bed was examined and deemed inadequate. In order to cover the

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tumor bed with the desired 98% of the prescription dose, a PA beam of 6MV energy was added

to the plan. This field served to cover the inferior medial border of the tumor bed, near SM’s

chest wall. The field was shaped with MLCs to surround the tumor bed with a 0.5 cm border.

This was done to only add dose to the specific tumor bed location rather than the entire breast. A

subfield was created to reduce hot spots and spread dose as the medical dosimetrist wanted

towards the needed under dosed region. Overall, the PA field only delivered 12 monitor units

(MU) and therefore did not carry a big impact on the breast treatment as a whole but rather just

enough to cover the tumor bed.

The plan sum was created to express the combined dose distribution of the electron field,

tangent fields, and PA field. The dose constraints and goals were checked using this plan sum.

The heart received a mean dose of 1.4 Gy and the ipsilateral lung received a V20 of 9.56% ergo

both constraints were met. The entire breast was covered by the 95% isodose line and the tumor

bed was covered by 98% as desired. The plan was created using a normalization value of 100.00

and there were two reference points. One reference point was used for the tangent fields and the

second reference point was used for the electron field.

The image guided radiation therapy (IGRT) strategy for SM’s daily treatment was set to

be a breast imaging guideline. This includes a kV orthogonal set field from the anterior (AP),

PA, right lateral, and left lateral directions. These images are reviewed by the MD weekly

offline. These fields are initially created in Eclipse with a digitally reconstructed radiograph

(DRR) so that match anatomy can be drawn on the images for use by the radiation therapists

during daily treatment. The carina and lung are also delineated so help line up the patient’s

anatomy.

After the full 25 fractions of treatment had been delivered, SM received a boost dose to

her tumor bed. The boost included 5 fractions of 2 Gy to a total of 10 Gy. Due to the fact that

the tumor bed could not be covered with 80% using up to 12MeV, the boost was delivered using

photons. This was accomplished by delivering the dose through a 16MV AP beam and a 6MV

left lateral oblique beam with a gantry angle of 155 degrees. There was a 60 degree wedge on

the AP beam with the heel facing out towards the left side of the patient, and a 30 degree wedge

on the left posterior oblique field in the same outward heel orientation. The fields were shaped

with MLCs set at a 1cm border around the tumor bed. Similar to the main whole breast radiation

treatment course, the entire tumor bed was covered with the 98% isodose line.

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Quality Assurance/Physics Check:

As stated above, the course of treatment for SM contained two references points. Neither

reference point had a set location. The reference points only carried the dose prescription for the

MU medical physics checks. While the dosimetrist performs the initial MU calculations while

planning in the Eclipse TPS, it is the medical physics department that performs the QA and MU

calculation checks. All checks are completed through the use of software called Mobius. The

initial calculations and checks must be within approximately 3% of each other to be approved

and ready for treatment.4

Once the medical physicist has double checked the entire plan and set

it’s treatment approval status to ready, a radiation therapist performs a Pre-Start QA (PSQA) task

in which all of the documents are checked before the patient’s first day of treatment. Details

such as daily treatment set-up moves from the CT reference point and IGRT strategy are check

during the PSQA. This process of involving a PSQA before treatment just assures that the

details of each patient’s treatment have been looked over by multiple people and helps to have a

smooth running first day of treatment.

Conclusion:

Due to the fact that no two patients will have the same anatomy or tumor, innovation and

creativity during the treatment planning process can be a key element in developing a plan best

suited for each individual. SM was a case where this element of innovation came to life because

the original plan had to be abandoned last minute in favor of a plan that better fit her anatomy

and treatment needs. During the course of treatment preparation a number of struggles and

things to consider were brought to light. For example, the medical dosimetrist had to consider a

plan that was going to be reproducible on a day to day basis while still maintaining complete

coverage of the breast tissue and a global maximum dose point that was within a tolerable range

for the patient to receive. Another main concern for this particular case was heart dose, and the

question of what can be done to lower that amount as much as possible. The importance of heart

dose when treating a left sided breast, even with the use of breathing monitoring systems, was

highlighted throughout SM’s treatment. Although the use of both photons and electrons created a

hotter plan where the two fields met, the different energy modalities were necessary to reduce the

dose at specific areas. The difference in the shape of an electron and photon isodose curve was

also a point of interest that had to be considered when matching the medial tangent with the

electron field. There was an inevitable area of the breast near the chest wall and anterior of the

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patient that could not covered by 95% dose and an area of maximum dose hot spots due to the

match line. Although the rotated tangential beams treated more lateral normal tissue than

conventional breast tangents, that was outweighed by the need to reduce heart dose in this case.

The act of configuring fields during treatment planning is like fitting together the pieces of

puzzle that’s picture is a dose distribution best suited to each patient’s tumor. Each puzzle’s

image is different, and each piece of the puzzle is necessary to create the bigger picture.

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References

1. I Linares, MI Tovar, M Zurita et al. Hypofractionated breast radiation: shorter scheme,

lower toxicity. Clinical Breast Cancer. 2015;16(4):262-268.

http://dx.doi.org/10.1016/j.clbc.2015.09.012

2. V Rudat, A Nour, M Hammoud, SA Ghaida. Better compliance with hypofractionation vs

conventional fractionation in adjuvant breast cancer radiotherapy. Strahlenther Onkol.

2017. http://dx.doi.org/10.1007//s00066-017-1115-z

3. Qfix. SDX Siprometric Motion Management System. Avondale; Qfix, 2014. Print.

4. Younge, K. Medical Physicist. March 1, 2017.

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Figures

Note: In all figures below, the purple shaded structure is the tumor bed. The 100% isodose line

is expressed with the red line, 95% with the orange line, and 98% with the light blue line.

Figure 4: Photo showing midline markings made on patient during simulation to help daily set-

up

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Figure 5: Photo showing lateral markings made on patient during simulation to help daily set-up

Figure 6: Patient position with breast board, knee fix, and SDX equipment

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Figure 7: Isocenter placement from the AP direction

Figure 8: Isocenter placement in an axial slice. The green crosshair is the user origin and

isocenter

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Figure 9: Isocenter placement from the left lateral direction

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Figure 10; Isocenter placement for the medial tangent

Figure 11: Heart block created by MLCs in the lower corner of the field

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Figure 12: Electron block cutout designed to custom match the medial tangent field. (20x20cm

block)

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Figure 13: Dose distribution of electron field in an axial slice

Figure 14: Dose distribution of electron field in a sagittal slice

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Figure 15: Axial slice of the dose distribution when only the tangential beams are present

Figure 16: Axial slice expressing the dose distribution of the plan sum when both the tangents

and electron beam are present

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Figure 17: Coronal slice expressing the dose distribution of the plan sum when both the tangents

and electron beam are present

Figure 18: Sagittal slice expressing the dose distribution of the plan sum when both the tangents

and electron beam are present

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Figure 19: Axial slice representing the beam orientation used for the boost plan

Figure 20: View of the AP boost field surrounding the tumor bed

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Figure 21: View of the Left Posterior Oblique field used in the boost plan

Figure 22: Dose distribution of the boost plan delivered.