METHODS AND MATERIALS
Ten pediatric patients (age range, 3 – 17 y.o.) treated with the
VMAT-TBI technique on the Varian TrueBeam from November 2019 to August
2020 were included in this IRB approved study. Patients were treated
with either non-myeloablative (2 Gy in one fraction) or myeloablative
(12 Gy in six fractions) regimens. Corresponding 2D plans replicating
our current clinical setup with lung blocks and chest-wall boost were
created for all ten patients for dosimetric comparison between the 2D
and VMAT techniques.
For VMAT-TBI, patients were simulated head-first supine (HFS) position
in a CIVCO long vac-lok bag positioned on the in-house developed
rotational platform. Patients’ necks were extended resting on the
AccuForm cushion (CIVCO) neck support, arms tightly clasped straight
alongside the body, and the CIVCO knee fix and/or feet fix were placed
under patient’s knees for comfort and leg position reproducibility. The
in-house-developed auto-planning script within Eclipse v15.6 Application
Programming Interface (API) was used for treatment
planning16. The VMAT plans were auto-generated with
three isocenters (head, chest/abdomen, pevis/legs) in HFS position.
Additional AP/PA plans with 1 to 2 isocenters in the feet-first supine
(FFS) position were used if patient height exceeded 115 cm (due to the
longitudinal table limit in HFS position). Upper-body VMAT plans using 6
MV or 10 MV were optimized with all isocenters included in one plan with
at least 2 cm overlap between the fields. The AP/PA Upper Leg plan was
used a baseline dose in the optimization to homogenize the dose
distribution near the matchline area. The auto-feathering optimization
option was turned on to create smooth dose gradients in the
fields’ overlapping areas and to prevent extreme dose heterogeneity in
the event of larger setup variations. The VMAT plans were optimized
and normalized to achieve at least 90% coverage of the whole body PTV
with the prescription dose. The PTV volume was created by cropping 3 mm
from skin and critical normal tissues.
For each VMAT-TBI plan, a simulated 2D plan was developed in Eclipse
based on the VMAT-TBI CT scan. The 2D plan fields were setup for AP/PA
technique at an extended SSD (~608 cm), collimators at
45° and 135°, field sizes set to 40 x 40 and 15 MV energy. Umbilicus was
used as the isocenter. Screen-to-skin distance, separation and off-axis
measurements that are typically acquired during clinical setup were
measured based on the CT scan. The compensator layers were re-created on
the CT scan to homogenize dose distribution based on the following
midline points: head, chin, neck, suprasternal notch (SSN), xiphoid,
umbilicus, hip/pubis, thigh, knee, calf, and ankle. Lung blocks were
generated using the lung contour contracted by 1 cm (lungs-1cm) and 1 cm
below clavicle with the constant thickness of 2.5 cm of Cerrobend for
every patient. The block transmission measurements in the middle of 10
cm thick lung slab sandwiched in-between two 4-cm thick solid water
slabs was 50% for 15 MV beam. Electron chest wall (CW) boost fields
were created for all patients prescribed to 50% of TBI photon
prescription to depth of maximum dose. The accuracy of the dose
calculation was verified using the ion chamber measurement in the
phantom composed of the lung slab sandwiched between the solid water
slabs.
The clinically relevant metrics in this study, including plan global
Dmax, PTV D90%, PTV V110%, lungs and lungs-1cm Dmean, were analyzed
and compared between the VMAT-TBI and simulated 2D plans. For gonadal
sparing comparison, the VMAT-TBI plans were compared to 2D plans for a
girl (Patient 4) and a boy (Patient 3) assuming 5 cm lead shield for
testes/ovaries in front of the patient with 5 mm margin. All dosimetric
comparisons between the VMAT and 2D plans were made with the dose
expressed as a percentage of the prescription dose and the volume
expressed as a percentage of the PTV volume. Paired t-test was used to
compare the dosimetric indices between the VMAT and 2D TBI plans with
p-values less than 0.05 considered to be statistically significant.