A QA Analysis of a Contrast Localized Linac-Based
Technique for Hypo-Fractionated Partial Breast Radiotherapy
Gary
Dillon, Robert Woodburn, Phillip Kulig
Gary Dillon, Robert Woodburn, Phillip Kulig, Methodist
Hospital Merrillville, Indiana, the United States
Correspondence to: Gary Dillon, Methodist Hospital Merrillville,
Indiana, the United States.
Email: gdillon6633@gmail.com
Telephone: +86-18509316339 Fax: +86-0931-4969169
Received: February 23, 2015
Revised: April 6, 2015
Accepted: April 10, 2015
Published online: June 2, 2015
ABSTRACT
AIM: Partial-Breast Radiotherapy following
lumpectomy is an effective treatment for women with early stage breast cancer,
with the technique of either brachytherapy or external beam treatment broadly
being implemented in clinics. However, the current popular brachytherapy
treatments are still limited to treating somewhat regularly shaped targets,
with an additional limitation defined by the skin and rib radiation tolerances.
A linac-based method that utilizes frame fixation, a contrast enhanced
cavity/fiducial location approach, image localization guided treatment
techniques, and special immobilization techniques was demonstrated to be
accurate method to deliver radiotherapy to a site in the breast, a task that
has challenged clinicians for decades.
Methods: A prospective study of ten patients
(six Left Breasts and four Right Breasts) had placed three internal gold-seed
markers inside the surgical bed post-surgery (provided by CIVCO
Corporation)/prior to planning and six external markers (made by Beckley). (Two
sets of three fiducials) were placed in an axial plane above and below the
surgery site) Prior to the Computerized Tomography (CT) scan performed for
planning, contrast is introduced into the cavity to allow localization of the
breast cavity during planning. A simulation was performed in a large-bore 3rd
generation scanner with a patient’s breast affixed using an Electa Body frame
to improve localization and to control breathing variances. The planning was
performed on an Eclipse treatment planning system, and the treatment was
performed using a Varian CD Linear accelerator.
Results: Small volumes
field sizes, sometimes as small as 4 × 4, and Mutli-Leaf Collimators (MLCs)
were required to deliver patient dose verification. This process was performed
prior to treatment using diodes (including planar isodose to evaluate the MLCs
required to deliver compared to the plan verification when using Map Check.
(Diode Array Radiation Dosimeter made by Sun Nuclear Corporation)
Conclusions: A Quality
Analysis (QA) was performed. Five to seven fields were used, indicating good
statistical and clinical agreement in these 10 cases. All fields were imaged
daily prior to treatment using 2
Dimensional (2D) MV portal imaging and correlated and evaluated by the
Radiation Oncologist using the Acculoc patient positioning system.
© 2015 ACT. All
rights reserved.
Key words: Breast Cancer; Partial; Hypo-Fractionated; High Dose
Rate; Brachytherapy; Radiotherapy; Quality Assurance; Megavoltage; Computerized
Tomography; Magnetic Resonance; Planned Target Volume; Clinical Target Volume;
Gross Target Volume; Stereotactic Body Frame; 3 Dimensional; 2 Dimensional;
Defined Field of View; Sterio-Tactic; Intensity Modulated Radiation Therapy;
Digitally Reconstructed Radiographs.
Dillon G, Woodburn
R, Kulig P. A QA Analysis
of a Contrast Localized Linac-Based Technique for Hypo-Fractionated Partial
Breast Radiotherapy. International Journal of Radiology 2015; 2(1): 20-23 Available from: URL:
http://www.ghrnet.org/index.php/ijr/article/view/1083
Introduction
The Center for Disease Control (CDC) reports that “breast cancer is the
most common form of cancer in women”[1]. Breast cancer is the number
one cause of death in Hispanic women and is the second most common cause of
death in other ethnic populations of women. In 2006, it was reported that
191,419 American women were diagnosed with breast cancer, with 40,820 deaths.
It is thought than one in eight women will eventually develop breast cancer.
Accelerator-based partial breast irradiation, either using High-Dose
Rate (HDR) brachytherapy or external beam radiotherapy, continues to gain
popularity as a means of treatment for appropriate breast cancer patients[2].
There is a growing interest by clinicians to implementing partial breast
treatment techniques with such irradiation.
Hypo-fractionated dose regimens using a linac-based system are performed
in departments that do not have an HDR unit. In a recent 2009 American Society
of Oncology (ASCO) meeting, Nick Mulcathy (Moderator for ASCO) stated that
“even though partial breast irradiation remains in the investigational phase,
it was found not to diminish survival”[3].
Additionally,
concerns of a changing health care environment suggest that future capital
budget requests for technological upgrades in radiotherapy equipment will be
constrained. Upgrading to new equipment or purchasing new capital equipment to
perform radiotherapy procedures can be costly, so considerations of
retrofitting existing equipment to perform these procedures could be a
cost-saving alternative. It has also been suggested that the software and
hardware to perform these procedures would need to be evaluated prior to
purchase to determine if this equipment meets the recommendations of the
manufacturer, including mechanical/imaging specifications. It is for these
reasons we began to develop a simple, low-cost method of delivering hypo-fractionated
linac-based treatment to deliver radiation to the partial breast (for early
stage disease) and began evaluating the physical results of such treatment.
Furthermore,
there have been significant advances in the localization for breast cancer
patients and the measurements of the positioning of the lumpectomy cavity that
enable targeting a radiotherapy hypo-fractionated regimen with a delivery of a
high degree of accuracy. In these recent studies the use of either breast
surface markers or surgical clips as surrogates for the cavity were reported to
provide improved localization in most patients compared to using bony landmarks
alone as image guidance for improved accuracy of daily radiation treatment[4].
Historically, the breast has always been one of the most challenging sites for
implementing radiotherapy due to the irregularity of the shape of the structure
and to the mobility of the breast itself.
Planning can
also be a challenge when determining the target in external hypofractionated
partial breast irradiation, especially when determining the Planning Target
Volume (PTV). Traditionally, CT was used to determine the PTV[5].
Studies have indicated that introduction of a contrast media can improve
visualization of the surgical cavity during planning. A number of
immobilization devices have been used to duplicate the breast position for
daily treatment, with some improvement in the reproducibility of the breast
position for daily radiation treatment. The following is a schema of this
prospective phase II trial to access the feasibility and clinical results of
this study of employing 3Dimintional (3D)-Conformal techniques and Magnetic
resonance Imaging (MRI)/Computerized Tomography (CT) fusion for Accelerated
Partial Breast Radiotherapy of Breast cancer.
MATERIALS
AND METHODS
Prior to the CT planning session, candidates for this
prospective phase II trial hypo-fractionated partial breast irradiation had
three gold fiducials (1.2 by 3 mm found to be more visible on Megavoltage (MV)
imaging than Platinum with similar cost) by placed into surgical bed
post-surgery (1-2 days); in addition, six markers were placed at the time of
simulation externally on the skin defining two axial planes, one superior and
the other inferior to the surgery site.
Contrast media for localization was placed in the cavity during the time
of fiducial placement. Using similar technique to Vicini[6].The
patients was positioned supine in a stereotactic Electa Sterio-tactic Body
Frame (SBR) body frame utilizing a diaphragmatic plunger to reduce/control
breathing variances and, to some degree, immobilize the breast itself. A large
bore Phillips-Model AcQSim CT 3rd generation scanner was used for all patients.
The technical settings for the CT for simulation were Defined Field of View
(DFOV) 550 mm, 130Kilovoltage ( kV), 250Milliamps ( mA), scan slice
thickness/spacing 2-3 mm (similar protocol for partial breast planning for
HDR), and Computerized Tomography Dose Index (CTDI) Vol: 1394 mGy. An Electa
frame is used to immobilize the breast of interest in addition to vac-lock
system for treatment planning CT. A system of six gold seeds (three in each
plane) above/below the tumor bed of the area centered for treatment. There were
a total of nine seeds used for CT per patient[7] during the
simulation; four points were marked on the patient’s skin surface using the
laser on the SBR frame. An amorphic phantom is used to illustrate the treatment
process in this paper. The planning CT was then sent to a Varian Eclipse
Planning system software version 6.5 with critical structures/target structures
outlined, including the breasts themselves, lungs, heart, cavity, Clinical
Target Volume (CTV), and PTV. The CTV was generated by expanding the expansion
cavity by 15 mm in all directions, with the CTV not to be closer than 7 mm to
the skin surface.
Although most of these cases utilized primary CT for contouring, CT/ T1
weighted MRI fusion post-surgery in treatment position could improve the
contouring according to our clinical medical staff. The treatment planning
goals were to prescribe 30 Gy in 6 Gy fractions for five consecutive working
days to the 95% isotope line of the PTV using five to seven high-energy photon
non-coplanar and non-opposed MLC-shaped beams. The PTV included a margin of 3
mm around the CTV utilizing daily stereotactic positioning/targeting of the
breast to compensate for set-up variability, interfractional motion, mobility
of breast tissue, and respiratory excursion7 The dose calculation used tissue
inhomogeneity corrections. The dose limitations include those of normal breast,
heart, and lung. The stated dose limitations are as follows:
1. Uninvolved normal breast. Less than 50% of the whole breast should
receive greater than 50% of the dose and 25% of the whole breast should receive
the prescribed dose.
2. Less than 10% of the lung can receive 30% of the prescribed dose.
3. Contra-lateral breast should receive less than 3% of the prescribed
dose.
4. Contra-lateral lung: Less than 10% of the lung should receive 5% of
the prescribed dose.
5. Heart: less than 5% of the heart should receive 5% of the prescribed
dose.
Treatment including image-guided protocol was performed by a Varian
2100EX linear accelerator ser #2090 using 6 MV photons that was calibrated
using the Task Group (TG)-51 protocol. A Varian MV portal imager utilizing an
imaging review system called Acculoc version MT-NW-425-109 was used to evaluate
accuracy of the treatment. Imaging treatment verification was performed daily
for each beam daily, initially, an orthogonal pair was checked and adjustments
were made, followed by each beam being reviewed and corrected clinically daily
by the radiation oncologist prior to treatment.
Dose verification was accomplished by comparing the verification plan
dose profile and the coronal beam profiles (verification plan generated by
Eclipse) and comparing those plan results to measurements made in a phantom at
a 6cm depth equipped with a diode array system MapCHECKTM Model1175.
This N-type diode array has 445 diodes in a 22×22 cm2 2-D array with
variable spacing. The entire array of diodes allow measurements of absolute
dose in a clinic[8].
Silicon Diodes, and new detectors such as metal oxide silicon
field-effect transistors
Metal-oxide-semiconductor field effect transistor(MOSFETs) are currently
available for immediate in-vivo dosimetry and have similar relative dose
accuracy compared to TLD and are very easily implemented into a clinical
setting[9,10]. Because there is a need to deliver a significant
number of daily machine setting relative to a linear accelerator (MUs) due to
the small field sizes, 95% Normalization to PTV, and 6 Gy daily prescription linearity
of dose by semiconductor diode (diode) is a challenge for dose/profile analysis
due to the linearity of dose in this range. (To address this issue, we measured
each beam separately in the linear range) and then used the internal MapCheck
program to determine the total dose/profile values). Although there is a small
risk that there could be some increased integral dose to the patient due to
leakage radiation when increasing MUS (small field sizes vs. standard large
field sizes) comparing the out-of field dose to the 3D conformal dose to the
patient to be considered[11], The use of 6 MV beams reduces the risk
of neutron production or production of second primaries compared to beams of
higher energies[12].
DISCUSSION/RESULTS
Although the emphasis of this study was on QA Analysis, the accuracy of
treatment was based off the planning, rigid positioning of the breast with
breath control, and radiation dose/imaging verification prior to every day
treatment with MV imaging and Image Guided Radiation Therapy (IGRT).
Planning
volumes were greatly assisted by the Radiation Oncologist by fiducial
placement; contrast introduced into the cavity at the time of ultrasound
–guided placement, and Planning CT/Post surgery T1 MRI fusion supine in
treatment position. (Only done in two of the ten cases evaluated) Gross Target
Volumes (GTV)’s, CTV’s, and PTV’s were defined post surgically. Due to fact
that 3D-conformal techniques were employed in this case inverse planning dose
volume constraints for organs at risk /evaluation of conformality index were
not used. The decision on plan approval was based on the isodose distribution,
Dose volume histogram, and the compliance to five-point dose limitations of the
protocol. All ten approved for treatment cases met the criteria of the
protocol. Conformality index (CI) was not available to evaluate in this version
of software.
The imaging
protocol on based off 3mm accuracy of defined CTV to PTV was very dependent on
the stereotactic-like daily set-up reproducibility and careful evaluation of
pre-treatment MV imaging. Gold markers placed after surgery was preferred over
Titanium due to the fact they were better visualized with MV imaging. (Even
though the cost of Gold markers for different vendors may be slightly higher
than titanium ones). The department procedure for this protocol requires daily
approval by the Radiation Oncologist MV imaging accuracy orthogonally prior to
treatment. As quoted by Radiation Oncologist Robert Woodburn III (Medical Director)
stated the following “Gold seed fidutials are visualized well than Titanium on
Digitally Reconstructed Radiographs of Megavoltage Portal Imaging in contrast
to visualizing anatomical structures”
The dose
verification protocol show a good agreement between the planned vs. measured
central axis doses in the ten patients studied, with the mean dose difference
being 2.7%. For the ten cases
evaluated. (due the low values of
the means values presented in these 10 cases standard deviation was insignificant)
In addition to
the planned vs. measured dose comparison at central axis, a profile analysis
was performed on all patients using the same standard Gamma guidelines
recommended for Intensity Modulated Radiation Therapy (IMRT) profile analysis,
for planned vs. Measured i.e., 3% variation per 3 mm, as calculated using the
previously used formula[13]: % Dose Difference=Planned
Dose/Delivered Dose×100
A comparison
was also performed for the total number of MUs delivered vs dose (lowest
to highest) to consider increased leakage radiation to patient due use
increased number of small field sizes. .and increased patient’s daily integral
daily dose to 600 cagy (vs. standard 160-300cGy), which may. This leakage issue
will increase with the ratio of the number of MUs used vs. dose.
Past research
comparison of the Mu ratio vs. the integral dose has indicated that Conformal
RT had a lower ratio than IMRT and a significantly lower ratio than the ratio
of cyberknife (radiosurgery)[14].
The Left and
Right Breast Statistical Analysis highlighted the structures of the cavity,
PTV, Heart, Breasts, Right Lung, Left Lung, and spinal cord met dose/isodose,
dose volume histogram criteria, and protocol 5 point dose limitation constraint
of the protocol in all ten cases. The results are presented in Tables 1-6
CONCLUSIONS
The results of patient specific QA for this treatment
were within expectation (average dose variation of 2.7% mean dose different
between measured vs. planned radiation dose. The Mu ratio to radiation dose was
f 2.05). It is common in IMRT analysis because of the greater number of dose
points sampled to compare the fluency map created vs. the small field size in
hypo-fractionated radiotherapy (maximum sampled=223 of possible 445).
Conformality seems to be generally achieved for the five to seven beam
arrangements, dose prescription/ limitations were met in all ten cases a,
although clinical evaluation of this technique is still being investigated.
The Statistical Analysis for Both Right and Left Breasts indicated
impressive dose delivery and constraints in the areas of heart, lung, and
targeting of the PTV. Each field was filmed every day of the radiation
treatment, which required radiation oncologist involvement. The Radiation
Oncologist must be present for each daily hypofractionated treatment to
evaluate the DRR accuracy prior to treatment.
Currently, there is no feedback mechanism in place to monitor changes of
table movement or positional changes for the analysis of intra- and
inter-fractional changes except for the film records. Improvements in the
delivery of this technique include using real-time correction of the IGRT. This
study is pending the outcome of the clinical results. This technique can be
implemented with existing equipment and little additional capital equipment
investment, along with some clinical training.
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Peer reviewer: Luciano M Feggi, MD,
Medico Nucleare, Medico Radiologo,
Direttore- Dipartimento di Diagnostica per Immagini e Medicina di
Laboratorio, Segreteria.
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