VMAT Treatment Planning Experience for
Glioblastoma Patients
Lizzie Findlay, Geraldine McClure, Andrew Bromiley, Rafael Moleron
Aberdeen Royal Infirmary, AB25 2ZD, Scotland, UK
Use of Half arcs: Reduction of Normal Brain Doses
The use of volumetric modulated arc therapy (VMAT) makes it possible to
obtain dose distributions conforming very closely to the region we aim to treat,
the planning target volume (PTV). Surrounding organs at risk (OARs) can
therefore be spared much more effectively than was the case for 3D conformal
radiotherapy. For patients with glioblastomas (GBMs), VMAT is particularly
useful given the numerous OARs surrounding the PTV. The close proximity of
the OARs, which sometimes overlap with the PTV, makes it challenging to create
optimised VMAT treatment plans for these patients. Furthermore, PTV volumes
tend to be large and it is important to consider how best to spare surrounding
normal brain. A retrospective planning study was carried out with the overall
aim of refining the planning approach taken for GBM patients.
Example GBM Cases
Examples illustrating the main planning challenges encountered for GBM patients:
PTV
Normal
brain
PTV overlapping
with brainstem
Multiple overlapping
regions
Effect of arc choice on normal
The use of half arcs rather than
brain V18Gy
full arcs reduced normal brain
Full arcs with avoidance sector
(defined as the volume
Half arcs: 2 coplanar + 1 non-coplanar
70
1 cm away from the PTV) V18Gy
60
by on average 29.7%. V36Gy,
50
40
V30Gy and mean dose also
30
improved.
20
10
The use of half arcs could
0
therefore help reduce the risk of
1
2
3
4
5
6
Patient no.
neuro-cognitive side effects.
Use of Half Arcs: Improved Dose Compactness
Normal brain V18Gy (%)
Introduction
Dose compactness was assessed using the 50% conformity index (CI) defined as
follows, with lower values indicating better dose conformity to the PTV:
Volume of 50% isodose
CI 50 
PTV volume
When using full arcs, CI50 was on average 2.54. With half arcs, CI50 reduced by
on average 0.46 to 2.08. A reduction was observed for all patients, with a range
of 0.31 to 0.56. This appears to be due to the use of a non-coplanar arc:
Multiple overlaps + large PTV volume
Treatment Planning Study
The study involved investigating how best to optimise PTV coverage and dose
conformity, spare surrounding normal brain and OARs and address overlapping
structures.
Eleven GBM patients treated using VMAT at Aberdeen Royal
Infirmary were included in the study. For each patient, a number of VMAT
treatment plans were created using different planning/delivery approaches. The
prescription was 60Gy/30# and plans were optimised against a set of cliniciandefined dose constraints. DVH data for each was used in subsequent analysis.
Investigation into Arc Combinations
For each patient, treatment plans were created using different arc combinations:
• Two full arcs with an avoidance sector across the eyes
• Two coplanar half arcs and one non-coplanar half arc delivered with the couch
at 90°
For patients with a lateralised tumour not extending over midline (7 of the 11
investigated), the use of the half arc combination rather than full arcs resulted
in significant improvements in normal brain doses and dose compactness:
7
Half arcs:
2 coplanar
1 noncoplanar
Full arcs:
2 full with
avoidance
sectors
CI50 = 2.24
CI50 = 2.77
Combination:
2 full with
avoidance
sectors
1 noncoplanar half
CI50 = 2.31
Comparison of CI50 values indicates that the addition of a non-coplanar
arc is what improves dose compactness.
Overlapping Regions
The VMAT calculation algorithm does not work efficiently if there are regions for
which conflicting dose constraints are specified by the planner e.g. where the PTV
overlaps with an OAR. To avoid this, ‘optimisation volumes’ are created by
cropping overlapping structures away from one another; the algorithm then
optimises dose to these volumes. When overlapping regions are large, however,
this approach has the potential to over-compromise PTV coverage. Creating
optimisation volumes of the overlapping region was found to help in these
situations:
Overlap volume: Treating this region to the
PTV
brainstem’s tolerance dose (54Gy) helps
maximise PTV coverage. Compared to simply
cropping PTV back from the brainstem PRV,
the use of an overlap volume makes it easier
to control dose in this region.
overlapping with
Brainstem PRV
Example GBM Treatment Plan
Dose volume
histogram (DVH)
comparison:
Volume (%)
Full arcs
with
avoidance
sector
Shown below is an example GBM treatment plan created using two coplanar half
arcs and a non-coplanar half arc:
Half arcs:
2 coplanar and
1 non-coplanar
Brainstem:
improves with half
arcs
PTV:
unchanged
54Gy: dose conforms to
overlap volume
30Gy: CI50 = 2.13
8Gy: Right side of brain and
lenses receive very little dose
Conclusions
Normal
Brain:
improves
with half
arcs
Dose (Gy)
A retrospective planning study carried out to investigate VMAT planning techniques
for GBM patients showed that, for patients with a lateralised tumour, the use of half
arcs gives significant reductions in normal brain doses. The addition of a non-coplanar
arc helps improve dose compactness. The often significant overlapping regions seen
for these patients can be dealt with using overlap optimisation volumes.