Improved Organ Sparing with VMAT Total Body Irradiation
Nic Ngo1,2, Nataliya Kovalchuk2, PhD, Eric Simiele2, PhD, Erik S. Blomain2, MD, Lawrie Skinner2, PhD, RichardHoppe2, MD, Susan Hiniker2, MD1University of Texas MD Anderson Cancer Center, School of Health Professions, Medical Dosimetry Program,Houston, TX2Department of Radiation Oncology, Stanford University, 875 Blake Wilbur Drive, Stanford, CA
Acknowledgments: We would like to acknowledge our dosimetry, physics, therapy, physician teams and Manny Villegas in successfully establishing VMAT TBI procedure in the clinic.
Many thanks to Dr. Xuejun Gu (UTSW) for consultation and advice on the technique implementation
Contact Information: Nataliya Kovalchuk, [email protected]
Background• Total body irradiation (TBI) is a conditioning
regimen used in bone marrow or stem cell
transplantation during which the whole body is
irradiated with the intention of eliminating
malignant cells and preventing the rejection of
donor cells through immunosuppression.
• TBI is associated with significant pulmonary
toxicity and infertility, which have a paramount
influence on the patient’s quality of life.
Methods: Simulation and Treatment Planning• Simulation: Patients were immobilized in HFS position in a Civco long vac-lok bag on the in-house-made rotational
platform. Patients’ necks were extended resting on the Civco Timo neck support, arms tight close to the body, and the Civco
knee fix and feet fix were placed under patients knees for comfort and leg position reproducibility. For 4 patients with the
height <115cm enabling the treatment in HFS position only, rotational platform was not used. The full body CT scans were
performed on Siemens Biograph PET/CT scanner with 5mm slice thickness and extended field of view to include arms in
the scan.
• Treatment Planning (VMAT): Treatment planning was performed using the in-house created auto-planning script1 with
Eclipse v15.6 Treatment Planning System Application Programming Interface (Varian Medical Systems, Palo Alto) with 6MV
or 10MV energy delivered by TrueBeam linear accelerator (Varian Medical Systems, Palo Alto).
• VMAT plans were generated with 3 isocenters (head, chest/abdomen, pelvis/upper legs) in head first supine (HFS) position
and if needed, with additional AP-PA plans with 1-2 isocenters in feet first supine (FFS) position.
• Lower body AP-PA plans and the VMAT plan were matched on skin with field-in-field generated to improve homogeneity of
the dose distribution. Upper body VMAT plans were optimized with all three isocenters included in one plan with at least
2cm overlap between the fields and using AP-PA Upper Leg plan as a baseline dose to homogenize the dose distribution in
the matchline area. Auto-feathering optimization option was turned on to create smooth dose gradients in the field
overlapping areas and prevent extreme dose heterogeneity in the event of larger setup variations.
• The VMAT plan was optimized to achieve at least 90% of the whole body PTV cropped 3mm from skin and critical normal
structures to be covered by the prescription dose. Table 1 shows the plan objectives for treatment planning.
• Plans for Patients 9-10 were created using an in-house developed Eclipse API VMAT TBI auto-planning script enabling
automatic generation of optimization structures, insertion of treatment fields and optimization.
Objectives/Aims
• To evaluate the dosimetric differences
between VMAT and 2D AP/PA Conventional
TBI Techniques.
Methods
• Ten pediatric patients treated with VMAT TBI
technique on C-arm Linac from November
2019 to August 2020 were included in this
study.
• VMAT TBI plans were generated using the in-
house developed autoplanning script.
• For each VMAT TBI plan a corresponding 2D
AP/PA plan was created replicating institution’s
current clinical setup with the patient
positioned at extended SSD with a
compensator to account for differences in
patient thickness, 50%-transmission daily lung
blocks and electron chest-wall boosts
prescribed to 50% of AP/PA photon
prescription.
• Clinically relevant metrics, global Dmax, PTV
V110%, lungs and lungs-1cm Dmean were
analyzed and compared between VMAT and
2D plans. For patients on non-myeloablative
regimen, the gonads were spared with VMAT
TBI and the dosimetric indices for Dmax and
Dmean were copared between 2D and VMAT
plans.
• All dosimetric comparisons between VMAT and
2D plans were made with the dose expressed
as a percentage of the prescription dose (2Gy
or 12Gy) and the volume expressed as a
percentage of the PTV volume. Paired t-test
was used to compare the dosimetric indices
between VMAT and 2D TBI plans.
Conclusions• Superior lung sparing with the superior
target coverage and similar global Dmax
were observed with the VMAT plans as
compared to 2D plans.
• In addition, VMAT TBI plans provided great
dose reductions in gonads, kidneys, brain
and thyroid.
1Simiele E, Skinner L, Yang Y, Blomain ES, Hoppe RT, Hiniker SM, Kovalchuk N. A Step Toward Making VMAT TBI More Prevalent: Automating the Treatment Planning Process. Pract Radiat Oncol. 2021 Mar 10:S1879-8500(21)00061-8.2 Hiniker SM, Bush K, Fowler T, et al. Initial clinical outcomes of audiovisual-assisted therapeutic ambience in radiation therapy (AVATAR). Pract Radiat Oncol. 7(5), 311-318 (2017).
Results
• For Patient 1, Figure 3 shows the DVH
comparison between VMAT TBI and 2D
Conventional TBI plans. The testes were
spared to the maximum dose of 71.9 cGy
and mean dose of 44.7 cGy, For Patient 2,
the ovaries were spared to the maximum
dose of 87.8 cGy and mean dose of 64.8
cGy with VMAT plan (compared to 2D plan
of 147 cGy and 150 cGy, respectively, brain
was spared to mean dose of 152.6 cGy.
Global Dmax was 232.9 cGy (116.5%).
Figure 1. VMAT TBI at Stanford (Patient 2).
Results• All VMAT TBI plans achieved D90% ≥ 100% of prescription. PTV coverage, D90%, was reduced significantly (-6.2%± 2.4%,
p < 0.001) with 2D plans, whereas no significant differences were observed between the 2D and VMAT global Dmax (p <
0.226) and PTV V110% (p < 0.444), Table 2.
• Compared to 2D plans, VMAT TBI plans produced significant decrease in the Dmean to the lungs and lungs-1cm volumes of
-25.6% ± 11.5% (p < 0.001) and -34.1% ± 10.1% (p < 0.001), respectively. In addition to lungs, VMAT TBI technique
provided sparing to other organs: for 12 Gy prescription, kidneys Dmean of 64.7% ± 3.3%; for 2 Gy prescription,
testes/ovaries Dmean of 31.6% ± 10.7%, brain Dmean of 74.8% ± 1.6% and thyroid Dmean of 72.5 ± 3.5%.
Table 2. Dosimetric comparion between 2D Conventional and VMAT TBI plans for all ten
patients.
Figure 3. DVH comparison between 2D Conventional and VMAT TBI
plans for Patient 1.
Table 1. Plan objectives for VMAT TBI.
• Treatment Planning (2D): Conventional AP/PA technique plans
were generated using 15 MV beams at ~608 cm SSD with the
compensator to homogenize the dose distribution along 7
positions on CAX, 50% transmission lung blocks, and electron
chest-wall boosts prescribed to 50% of AP/PA photon
prescription normalized to the depth of maximum dose.
• For gonadal sparing comparison, the VMAT TBI plans were
compared to 2D plans assuming 5 cm lead shield for
testes/ovaries with 5 mm margin and 1 cm water bolus (to
decrease back scatter).
Figure 2. TBI dose distribution on coronal view for Patient 2 with
gonadal sparing with 2D plan (left) and VMAT (right).
Figure 4. Dose distribution on axial slices for 2D conventional
plan (left) and VMAT plan (right) for Patient 1.
Treatment• Since the auto-feathering optimization
option was turned on, the plan robustness
testing resulted in only 2.5% global Dmax
increase for VMAT TBI plan for Patient 1
when the isocenters were shifted ±5 mm.
• The dose measurements based on Optically
stimulated luminescent dosimeters (OSLDs)
placed on the match-line and testes were
within 5% of the planned dose.
• Beam-on time for 10 patients ranged from
25.1 to 57.5 min. Beam on time was 18.8
min for Patient 1 and 15.0 min for Patient
2. Patient 1 patient was watching a movie
during treatment using the AVATAR
system2, Patient 2 patient was under
anesthesia during treatment.
Figure 5. Cone-beam CT was acquired in the chest area to
verify the positioning of Patient 1 before treatment (left).
Dosimetric shifts were applied to shift the patient to
consequent isocenter positions, MV ports were acquired to
verify the match after each shift (right)
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