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S826
ESTRO 36 2017
_______________________________________________________________________________________________
Bologna, Italy
5
Policlinico Universitario "A. Gemelli"- Università
Cattolica del Sacro Cuore, Radiation Oncology
Department, Roma, Italy
Purpose or Objective
The aim of this study was to assess the feasibility in the
delivery of highly heterogeneous doses to symptomatic
large tumor using VMAT technique and simultaneous
integrated boost during a short course palliative
accelerated radiotherapy.
Material and Methods
For this dosimetric analysis we selected a patient with a
large symptomatic sarcoma. A Planning Target Volume
(PTV) and a Boost Target Volume (BTV) were defined as
the GTV plus and minus 1cm, respectively. Two different
doses were simultaneously delivered to the PTV and BTV
according to a dose-escalation protocol in 4 fractions. Five
dose levels were planned: Level 1 (basal plan: PTV:
20Gy/5Gy), Level 2 (PTV: 20Gy/5Gy; BTV: 25Gy/6.25Gy),
Level 3 (PTV: 20Gy/5Gy; BTV: 30Gy/7.5Gy), Level 4 (PTV:
20Gy/5Gy; BTV: 35Gy/8.75Gy) and Level 5 (PTV:
20Gy/5Gy; BTV: 40Gy/10Gy). The aim was to irradiate the
central part of the tumor up to 10Gy/fraction while
maintaining the border area of the tumor and the
surrounding healthy tissues with <5Gy/fraction. SIB-VMAT
plans were generated using Oncentra Masterplan TPS, in
the dual-arc modality. The mean dose, D98%, D95% and
D2% doses were scored for each target. A conformity
index, PTV_CI, defined as the volume encompassed by the
PTV 95% isodose divided by the PTV volume, was
calculated. A dose contrast index (DCI) was defined as the
mean dose to the BTV divided by the mean dose to the
PTV (excluding BTV). For healthy tissue, an integral dose,
Dint, was defined as the product of mean dose and volume
of normal tissue, excluding the PTV. This was reported
together with the irradiated volumes at the dose levels of
5, 10, 15 and 20Gy (V5, V10, V15 and V20).
Results
Overall results are reported in Table 1. When BTV dose
escalated up to 200% of PTV prescription, the PTV_CI
increase was <8% (from 1.11 to 1.20), proving that SIB
strategy was able to reduce the dose to the BTV
surrounding volume despite the major dose escalation.
Similarly the percentage increase of ID to normal tissues
was 11%. The increase in healthy tissues receiving more
than 5, 10 ,15 and 20 Gy was about 2%. Deviation from the
ideal contrast dose slightly increased with increased BTV
dose.
Conclusion
We quantified the capability of SIB-VMAT to deliver highly
heterogeneous doses in the treatment of large tumors.
Despite the major dose escalation in the BTV, the dose
conformity to PTV and the integral dose to the normal
tissue minimally increased, with a slow increase of dose
spillage from PTV to normal tissue. The safe delivery of
ablative dose in the central part of the tumor has the
potential to greatly improve the palliative effect.
EP-1555 Improving inter-planner variability in head
and neck (H&N) VMAT
H. James
1
, C. Scrase
2
, K. Yip
2
1
Suffolk Oncology Centre The Ipswich Hospital,
Radiotherapy Physics, Ipswich Suffolk, United Kingdom
2
Suffolk Oncology Centre The Ipswich Hospital, Clinical
Oncology, Ipswich Suffolk, United Kingdom
Purpose or Objective
Inverse plan optimisation for H&N
VMAT is resource intensive. Variation exists between
planners when determining optimal solutions. Re-
optimisation of sub-optimal plans impacts upon the
patient pathway. In our institution class solutions and dose
assessment criteria improve consistency in prostate VMAT
planning. In this study inter-planner variability was
assessed for H&N VMAT. Analysis of plans and shared
learning informed the development of optimisation
templates to improve consistency and meet clinician
expectations.
Material and Methods
VMAT plans for a
radical tonsil treatment were created by 10 individuals
with varying levels of experience. Plans were expected to
meet or exceed pre-defined PTV and PORV dose
requirements. Planners used their own judgement to
determine optimisation objectives and priorities, define
dummy structures and assess whether an optimal plan had
been produced. Quantitative dosimetric analysis of the
plans covered 3 areas – PTV coverage (conformity index
(CI), homogeneity index (HI)), PORV doses and dose spill.
Parameters were scored against a gold standard and
ranked. Plans were independently reviewed by 2 clinicians
specialising in H&N RT and their assessments were
compared with quantitative analysis. Optimisation
objectives and priorities and the use of dummy organs
were analysed in conjunction with the dose
distributions.
Results
Each plan met the pre-defined PTV
and PORV doses. There was no significant variation in HI
(1.05-1.09) regardless of the priorities applied in the
optimisation. A larger variation in CI (1.07-1.18) was
attributed to use of the Normal Tissue Objective function.
There were variations in dose to normal tissues as planners
applied varying dose constraints to keep doses as low as
reasonably practicable without compromising PTV
coverage. Clinician reviews picked up more subtle but
potentially clinically relevant variations between plans –
high dose spill, dose spread across mid-line and over-
zealous sparing of normal tissues. The plans scored most
highly by the clinicians were created by the most