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Brittany Check
Case Study 1
February 23, 2015
SBRT to Right Lung of Previously Treated Patient
History of Present Illness: Patient MH is a 65 year-old male who presented with shortness of
breath. He underwent a chest x-ray which revealed a right pneumothorax. Increasing shortness of
breath caused him to be admitted into the hospital and given a chest tube. A chest CT was
reviewed which showed a suspicious pulmonary nodule in the upper lobe of his right lung. He
had a PET/CT scan which revealed uptake by the right upper lobe pulmonary nodule as well as
lymph nodes in the mediastinum and right hilum. The pathology from a CT-guided biopsy of the
upper right lobe nodule revealed a poorly differentiated adenocarcinoma. A bronchoscopy with
endobronchial ultrasound (EBUS) was negative for malignant cells in the suspicious lymph
nodes and it was felt he had stage I disease. It was felt that the nodes had increased
fluorodeoxyglucose (FDG) uptake due to inflammation following his pneumothorax and chest
tube placement.
During his stay in the hospital, the patient also had an evaluation of a systolic murmur with
cardiac catheterization which revealed severe aortic valve stenosis and two-vessel coronary
artery disease. He received an aortic valve replacement and bypass grafts. An occlusion occurred
in one of the grafts and he had a myocardial infarction. He underwent angioplasty and received a
drug-eluting stent.
After seeing the surgical oncologist regarding his lung malignancy, MH was informed that he
could receive a lobectomy but he would need to wait at least a month because of his recent
medical events. The patient was unwilling to wait a month for surgery. After education and
discussing acute and late toxicities with the radiation oncologist, the patient wished to pursue
stereotactic body radiation therapy (SBRT).
Past Medical History: MH has a past history of stage IVa (T3 N2b M0) squamous cell
carcinoma of the retromolar trigone treated with chemoradiotherapy. He completed his treatment
in May 2011 has since been without evidence of disease. He also has a past medical history of
severe coronary artery disease, temporomandibular joint disorder, tobacco abuse, myocardial
infarction, hyponatremia, hospital-acquired pneumonia, hyperlipidemia, underweight,
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pneumothorax, gastroesophageal reflux disease, chest pain, hypothyroidism, smoking addiction,
and neuropathy.
Social History: MH is a retired farmer. He is widowed, has four children, and no pets. He is
active but not on a regular exercise regimen. His father passed away at 87 years and his mother
passed away at 79 years. The cause of death is not stated. He has one sister and two brothers that
are alive. MH is a former smoker, smoking a quarter of a pack per day of cigarettes for 40 years
and quit December 2014. Patient denies smokeless tobacco and drug use. He drinks five to six
drinks per week.
Medications: The medications MH is currently using are aspirin, atorvastatin, hydrocodoneacetaminophen, levothyroxine, metoprolol-XL, nitroglycerin, omeprazole, senna-docusate, and
clopidogrel.
Diagnostic Imaging: MH had a PET/CT scan done on December 4, 2014. This study showed a
small lobulated and hypermetabolic nodule in the right apex. The lesion measured 13 mm. The
nodule was sampled with a core needle biopsy and revealed a high grade poorly differentiated
non-small cell adenocarcinoma. The disease was stage IA (T1b N0 M0). Additionally, there were
several small mildly hypermetabolic lymph nodes in the right hilum, superior mediastinum, right
paratracheal region, subaortic region, and subcarinal region that were suspicious for metastatic
disease. The patient underwent a bronchoscopy with EBUS to biopsy the nodes, which came
back negative for malignancy.
Radiation Oncologist Recommendations: MH refused a lobectomy and state that he was
unwilling to wait a month to treat his lung cancer. Consequently, the radiation oncologist
recommended the use of SBRT to treat the lung nodule. For stage I lung cancer, SBRT is shown
to have excellent local control and survival.1
The Plan (prescription): The radiation oncologist initially pursued a volumetric modulated arc
therapy (VMAT) radiation therapy plan. The desired prescription was 50-55 Gy in five equal
fractions with a curative intent. Because of his previous head and neck treatment, an intensity
modulated radiation therapy (IMRT) radiation therapy plan was used with 55 Gy to the planning
target volume (PTV) and 45 Gy to the gross tumor volume (GTV) in five equal fractions.
Therefore, the PTV received a fractional dose of 11 Gy while the GTV received a fractional dose
of 9 Gy. The change in technique and prescription was done to avoid excessive dose in
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previously irradiated critical structures. Each fraction is scheduled two to five days apart. The
noncoplanar IMRT plan resulted in seven total beams.
Patient Setup/Immobilization: MH underwent a CT simulation on February 9, 2014 on a GE
LightSpeed 16-slice CT scanner in 4-D phase gated acquisition mode followed by a standard CT
simulation. After reviewing the scans, the radiation oncologist decided not to gate the treatment
because there was not a significant amount of movement in the apex of the right lung. For the CT
simulation, the patient was immobilized using a total body fix bag (Figure 1). The patient was
positioned supine with his arms at his sides. The patient was simulated and is treated under 80
psi of compression to reduce motion from breathing. A reference isocenter was placed by
marking the patient with radiopaque markers. For treatments, patient is placed in simulation
position and aligned using a cone-beam CT (CBCT) and kV orthogonal images. During
treatment, multiple snaps are taken using a set of kV orthogonal images with ExacTrac to ensure
there is no patient movement.
Anatomical Contouring: The CT scan was transferred to Varian Eclipse treatment planning
system. Anatomical structures were delineated by the radiation oncologist and dosimetrist.
Contours include the spinal cord, esophagus, brachial plexus, right and left lungs, and GTV right
upper lobe nodule with an expansion to PTV.
Beam Isocenter/Arrangement: The patient was treated on a Varian Clinac 21EX linear
accelerator with 6 MV photons. The dosimetrist placed the isocenter centrally in the patient to
avoid clearance issues when rotating to several gantry angles. To avoid previously treated critical
structures, beams were dispersed around the patient at 205˚, 231˚, 27˚, 56˚, and 127˚ to avoid
entering and exiting through the brachial plexus or spinal cord (Figure 2). Since the goals set by
the radiation oncologist could not be achieved with a coplanar plan, the dosimetrist used a couch
kick to use more beams that did not traverse the brachial plexus or spinal cord. The AIO beam
utilized a 270˚ degree couch rotation and a 20˚ gantry angle to treat the nodule while traveling
inferiorly and posteriorly to the brachial plexus (Figure 3). Similarly, the RASO beam utilized a
352˚ couch rotation and a 305˚ gantry angle to reach the nodule without entering or exiting
through the brachial plexus or the cord (Figure 4). Using as many beams as possible without
treating through these critical structures made the dose distribution as conformal as possible.
Field sizes and MLC apertures were set to include the PTV volume when possible while
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blocking out the brachial plexus and cord. The modulation by the MLC was determined by the
IMRT plan derived from the treatment planning system.
Treatment Planning: The CT scan from the head and neck treatment to 70 Gy that patient
received in 2011 was fused in MIM with the CT scan for treatment of upper right lung nodule.
The previous dose distribution was used for organ limiting dose criteria in the plan. The radiation
oncologist specified goals for avoiding normal tissue and treating the tumor. The initial
prescription was 55 Gy in 11 fractions or if this could not be achieved, 50 Gy in ten fractions.
The prescribed isodose line should cover 95% of the PTV while 90% of the prescribed dose
should cover 99% of the PTV. Initial dosimetry goals were based off of Radiation Therapy
Oncology Group (RTOG) study 0813.3 Conformality of high and low isodose lines were
monitored and intended to meet constraints set by RTOG 0813 (Figure 5). The dose delivered to
critical structures was minimized to meet RTOG 0813 constraints (Figures 6-7). However, since
the patient was previously treated, additional constraints for the spinal cord and brachial plexus
were set. The radiation oncologist specified that the spinal cord should receive no more than 500
Gy to 0.03 cc. The brachial plexus should receive no more than 100 cGy to 0.03 cc. Additionally,
the esophagus should receive no more than 1200 cGy to 0.03 cc. The dosimetrist used the Acuros
XB algorithm to calculate the dose distribution because of its superior accuracy in lung
treatments.2,3 A VMAT plan was attempted initially as the radiation oncologist had instructed,
however, the low dose region spread around the lesion was too high for the cord and brachial
plexus. Next, a hybrid plan was attempted utilizing an AP/PA arrangement to create a column of
dose that did not go through the critical structures with an IMRT plan. The radiation oncologist
did not approve of the conformality of the dose. The approved plan was an IMRT plan that
utilized couch and gantry angles that allowed seven beams to enter and exit without contributing
significant dose to the cord and brachial plexus (Figure 8). In order to achieve reasonable doses
of 750 cGy to the brachial plexus and 600 cGy to the spinal cord, a dose of 55 Gy was delivered
to the GTV and 45 Gy was delivered to the PTV. With this alteration, most RTOG 0813
constraints could be met as demonstrated in the dose volume histogram (DVH) (Figure 9), with
the exception of the low dose spillage evaluation because beams intersected since entrance
angles were limited (Figure 8).
Quality Assurance/Physics Check: To perform a check of the dose, the physicist measured the
dose in a phantom. The seven field plan was delivered to a 30 x 30 cm2 solid water phantom. A
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diode array was located at 5 cm depth. At 100 cm SAD, 4 cm inferior to the central axis, a point
dose measurement was made at a region of high uniform dose. The calculated dose was 365.93
cGy, and the measured dose was 367.10 cGy. A comparison was made of the composite
calculated dose to the diode array in the coronal plane. There were 168 diode measurements
made and 99% agreed within 3% or 3 mm. The maximum points were at 5% or 4 mm. A diode
array was also exposed at 5 cm depth for each segmented field to compare alignment. All fields
passed so the plan was deemed ready for treatment.
Conclusion: This case was especially challenging to plan because of the very tight constraints
placed on critical structures that were previously treated. Although VMAT treatments can be
very conformal and spare critical structures of high dose, the low dose is spread out around the
lesion. In this case, this low dose was still too high for these structures so different techniques
were utilized. The idea of a hybrid plan using an AP/PA arrangement with IMRT would help to
keep most of the dose entering where there were not critical structures. However, the low isodose
lines would not be very conformal. In this situation, noncoplanar beams and tighter margins had
to be utilized to meet constraints. Although the dose delivered to the PTV was 45 Gy rather than
the initial 50-55 Gy that the radiation oncologist had intended, sparing critical structures that had
already been treated may help prevent the patient from experiencing additional problems. In
complicated cases like this, compromises may need to be made to ensure the best outcome for
the patient.
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References
1. Videtic GMM, Stephans K, Reddy C, et al. Intensity-modulated radiotherapy-based
stereotactic body radiotherapy for medically inoperable early-stage lung cancer: excellent
local control. Int J Radiat Oncol Biol Phys. 2010;77(2):344-349.
doi:10.1016/j.ijrobp.2009.05.004
2. Kroon PS, Hol S, Essers M. Dosimetric Accuracy and clinical quality of Acuros XB and
AAA dose calculation algorithm for stereotactic and conventional lung volumetric modulated
arc therapy plans. Radiat Oncol. 2013;8(1):1-8. doi:10.1186/1748-717X-8-149
3. Rana S, Roger K, Pokharel S, Cheng C. Evaluation of Acuros XB algorithm based on RTOG
0813 dosimetric criteria for SBRT lung treatment with RapidArc. J Appl Clinc Medic Phys.
2014;15(1):118-129. doi:10.1120/jacmp.v15i1.4474
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Figures
Figure 1. Patient position and immobilization using body fix system for simulation and
treatment.
Figure 2. Isocenter and beam arrangement shown in axial, coronal, and sagittal views.
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Figure 3. Beam’s eye view for AIO beam.
Figure 4. Beam’s eye view for RASO beam.
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Figure 5. Conformality of prescribed dose for calculations based on deposition of photon beam
energy in heterogeneous tissue table from RTOG 0813.
Figure 6. Tissue constraint table from RTOG 0813.
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Figure 7. Tissue constraint table from RTOG 0813.
Figure 8. Isodose lines shown in axial, coronal, and sagittal view. Red isodose line represents 55
Gy and yellow isodose line represents 45 Gy.
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Figure 9. Dose volume histogram for final SBRT plan.