Post on 04-Oct-2021
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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.