Assessment of Very High Energy Electron therapy as a clinical modality for external beam therapy

Abstract

Very high energy electron (VHEE) radiotherapy
is being investigated as a potential replacement of photon therapy. VHEE pencil
beams have a small penumbra and strong depth dependence for radiotherapy when
compared to photons. This allows a lower dose to healthy tissue. Generating
these high electron energy beams could be achieved by using laser accelerators.
These accelerators allow the equipment to be smaller than currently possible
and make it possible to fit them in a standard radiotherapy treatment bunker.
This makes VHEE a potential middle ground between photon and proton therapy in
relation to equipment costs and treatment quality. The purpose of this project
is to compare photon based Intensity Modulated Ra­diation Therapy (IMRT) and
VHEE treatment plans for treatment of prostate cancer.  For 10 prostate cancer patients IMRT
treatment plans were generated with 23 beams. These plans were optimized using
Erasmus MC in-house developed Erasmus-iCycle automated treatment plan
optimization tool. The dependence of VHEE plan quality on beam energy (200, 300
and 400 MeV) and number of equi-angular beams (9, 18 and 36) was investigated. The
treatment plans were optimized using the multi-criterial optimizer
Erasmus-iCycle. For each patient VHEE pencil beam dose distributions were
pre-calculated using TOPAS MC, a Monte Carlo simulation program based on
Geant4.  Results: VHEE treatment plans show
either a reduced or similar mean OAR dose when compared to IMRT treatment plans
except for the 9 beam 200 MeV plan which was worse compared to IMRT. All
treatment plans were normalized to a PTV coverage of 99 % V57Gy.  It is found that the mean rectum dose reduces
from 13.5 Gy for IMRT to between 9.2 and 11.9 (p=0.002-0.004) for the VHEE
plans. For the anus dose a reduction in mean dose was found for all VHEE
treatment plans except for the 9 beam 200 MeV and the 18 beam 200 MeV VHEE
treatment plans. The mean anus dose reduced from 12.4 Gy for IMRT to 7.0 - 10.6
Gy (p=0.002-0.014) for the VHEE treatment plans. The bladder dose reduced from
20.1 Gy to between 15.2 and 18.2 Gy (p=0.002). 
Increasing the number of VHEE beams in a treatment plan reduces the OAR
dose. Comparing 9, 18 and 36 beam treatment plans with 300 MeV. The mean rectum
dose reduces from 14.0 Gy for the 9 beam plan to 11.1 Gy (p=0.002) and 9.9 Gy
(p=0.002) for the 18 and 36 beam plan, respectively. The same pattern is found
for the anus and bladder.  Treatment
plans with a higher beam energy reduces the dose to OAR. The mean rectum dose
reduces from 13.4 Gy for the 200 MeV plan to 11.1 (p=0.002) and 10.0 (p=0.002)
Gy for the 300 and 400 MeV plans. The same pattern is found in the mean bladder
dose with 18.2 Gy, 16.8 Gy (p=0.002) and 15.9 Gy (p=0.002). The right femoral
head maximum dose increases from 28.0 Gy for 200 MeV to 29.4 Gy(p=0.002) and
29.6 Gy (p=0.049) for the 300 and 400 MeV treatment plans.  VHEE is a potential replacement for IMRT due
to the reduced dose to healthy tissue while maintaining similar target coverage
compared to IMRT. By increasing the number of beams and/or the electron beam
energy we can further reduce doses to the healthy tissue.