DISCUSSION
To our knowledge, this is the first treatment planning study of
craniospinal irradiation using the RefleXion X1 system. We have
previously reported on treatment planning comparison between RefleXion
X1 and Eclipse VMAT for 42 patients across 6 cancer sites [25]. In
this study, we tested the feasibility of CSI using RefleXion X1. We have
successfully generated clinically acceptable RefleXion CSI plans for 5
pediatric medulloblastoma patients with target length less than 50 cm.
Dosimetric indices were comparable between RefleXion X1 and Eclipse VMAT
modalities except for statistically significantly improved bowel sparing
with RefleXion X1.
Due to 2 cm field size and long PTV CSI targets, the average beam-on
time was approximately 4.5 times greater using RefleXion X1 compared to
Eclipse VMAT. For VMAT CSI delivery using 2-isocenter plans and treated
on Varian C-arm linear accelerator, treatment times for the first
fraction from start of pre-treatment imaging to the end of treatment
session had a wide range of 16.3 – 43.5 min (mean, 29.2 min) signifying
existing challenges in imaging and aligning each isocenter separately.
RefleXion X1 has a capability of overcoming this challenge with imaging
a long extent of the patient (up to 90 cm), localizing, and delivering
the whole treatment using 1 isocenter in axial mode moving the couch in
the cranio-caudal direction with 2.1 mm increments. This may potentially
reduce beam matching and shifting errors which may arise from
multi-isocenter delivery. In addition, X1 was recently upgraded to
enable 1000 MU/min dose rate from the initial dose rate of 850 MU/min.
This upgrade promises to improve the beam-on time but needs to be
validated.
While there are currently no studies comparing RefleXion CSI and VMAT
CSI, literature discussing TomotherapyTM delivering
the treatment using 2.5 cm jaws in helical fashion may be useful as a
comparison due to its similar delivery approach to X1. A study in 2019
by Sun et al. [26] comparing VMAT, IMRT, and Tomotherapy plans found
that the Tomotherapy plans offered superior PTV homogeneity, conformity,
and brainstem, optic chiasm, and optic nerve sparing compared to VMAT
plans. IMRT was superior to VMAT and Tomotherapy in terms of OAR sparing
in the mid body region (esophagus and heart). Results of this study by
Sun et al. differed from the results of the current RFX study, which
found difference in Dmean to the bowel bag as the only statistically
significant dosimetric parameter. However, just as the average beam-on
time for RFX plans were estimated to be longer than average beam-on time
for VMAT plans in our study, delivery time of HT was found to be longer
than the delivery time of VMAT for Sun et al.
Another study by Herdian et al. [27] found that differences in oral
cavity Dmean, kidneys Dmean, and mean D2% to the spinal PTV were
statistically significant between IMRT and Tomotherapy plans.
Differences in oral cavity Dmean, kidneys Dmean, mean D2% to the
cranial PTV, and mean D2% to the spinal PTV were also statistically
significant between 3D-CRT plans and HT plans. Additionally, Tomotherapy
plans resulted in longer mean beam-on time than both IMRT and 3D-CRT
[27].
One limitation of this study is the small sample size (n=5) due to the
maximum target length threshold of 50 cm. The vendor is planning to
upgrade the system to enable treating targets greater than 50 cm in
their next clinical release. This will enable us to expand the patient
selection. Another limitation is that this study only focuses on
comparison between VMAT and RefleXion X1 plans. It would be interesting
to include Tomotherapy plans in the testing cohort. Based on our
communication with the vendor, RefleXion Medical Inc will lift the
maximum target length limit in the future software releases of X1
treatment planning system. This will enable us to include larger target
sizes in the investigation and collect and analyze the treatment
delivery times. This work shows the feasibility of CSI planning using
RefleXion X1 paving the way to potentially use RefleXion X1 for CSI
treatments. There is a potential benefit of single-isocenter treatment
with near-diagnostic image-quality kVCT guidance enabling long scanning
length of 90 cm. This can simplify the IGRT workflow and streamline the
treatment delivery for CSI patients, especially important for pediatric
patients treated under anesthesia.