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ESTRO 36
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Poster: RTT track: Treatment planning and dose
calculation / QC and QA
PO-1007 The effect of VMAT on tumor coverage and
organs at risk for head and neck cancer patients
M. Kertevig
1
1
Rigshospitalet, Department of Oncology- Section for
Radiotherapy, Copenhagen, Denmark
Purpose or Objective
Throughout the course of radiotherapy in head and neck
patients tumor shrinkage occurs. This may influence the
dose to organs at risk (OAR) around the tumor area, as
tumor shrinkage can lead to different dose distribution in
the patient than originally calculated. As Volumetric
Modulated Arc Therapy (VMAT) technique is frequently
used for head and neck patients, it is relevant to study the
impact of VMAT treatment on tumor coverage and OAR,
during a course of radiotherapy, and the necessity to
adaptive plans during the course of treatment.
Material and Methods
A retrospective study has been carried out on 13
consecutive patients who have been treated with VMAT for
head and neck cancer. The Cone-beam Computed
Tomography (CBCT) from the first treatment fraction was
compared with the CBCT from the last fraction. Dose and
volume comparison was performed for planning target
volume (PTV), spinal cord, brainstem and both parotid
glands. A paired t-test was used to test for significance
and
p
-values <0.05 were considered statistically
significant
Results
The mean volume of PTV on the CBCT from the first
treatment fraction was 283.98 cm
3
compared with the
CBCT from the last fraction which was 270.33 cm
3
. The
mean volume of the PTV decreased significantly with 5%
(
p =
0.003), due to tumor shrinkage during the course of
treatment. The mean D
95
to the PTV decreased by less than
1% from 62.34 Gy to 61.88 Gy. The mean D
max
to the spinal
cord increased by 1% from 41.33 Gy to 41.78 Gy and to the
brainstem by 3.8% from 32.11 Gy to 33.33 Gy. The mean
dose to the left parotid gland decreased with less than 1%
from 22.08 Gy to 22.06 Gy. In contrast, the mean dose to
the right parotid gland was significantly increased by 6.5%
(
p
= 0.033) (table 1). There were no significant differences
in the mean dose to either PTV (
p
= 0.12) spinal cord (
p
=
0.27), brainstem (
p =
0.22) and left parotid gland (
p
=
0.98), which means that treatment with VMAT, had
negative effect on dose to spinal cord, brainstem and left
parotid gland with a 95% probability for this patient
cohort. Even though the dose to the right parotid gland
increased significantly, the dose to all OAR remained
within the defined constraints. In addition, the tumor
coverage remained sufficient throughout the treatment.
This need to be studied further with larger sample sizes
together with a dose study for all the OAR in the head and
neck region to fully determine the necessity to adapt the
patients plan, especially since it might be possible to
reduce the dose to the parotid glands for patients
suffering from xerostomia.
Conclusion
This study showed that VMAT treatment plans were
relatively robust during the treatment course. In this
patient cohort small changes in dose to OAR were not
significant, despite a reduction in PTV.
PO-1008 Feasibility of stereotactic ablative
radiotherapy for locally-advanced non-small cell lung
cancer
K. Woodford
1
, V. Panettieri
1
, T. Tran Le
1
, S. Senthi
1
1
The Alfred Hospital, Alfred Health Radiation Oncology,
Melbourne, Australia
Purpose or Objective
Stereotactic ablative radiotherapy (SABR) has enabled a
curative treatment for elderly patients or those with
significant comorbidities diagnosed with early-stage non-
small cell lung carcinoma (NSCLC) who would have
otherwise gone untreated. As a result population-based
survival has improved. If SABR could be utilized in the
treatment of locally-advanced NSCLC in the same way, the
public health impact would be greater, as twice as many
patients are diagnosed with advanced disease. We
assessed the feasibility of SABR for locally-advanced
NSCLC.
Material and Methods
Twenty three patients with N2 and/or N3 locally-advanced
lung cancer were retrospectively replanned. Targets and
organs-at-risk (OAR) were delineated using 4DCT and
replanned with RapidArc delivery (AcurosXB Vn13.6).
Three planning approaches were assessed; conventional
approach (1.0cm ITV to PTV expansion, prescribed to
100%); SABR approach (0.5cm ITV to PTV expansion,
prescribed to 80%) and a hybrid approach (0.5cm ITV to
PTV expansion, prescribed to 100%). We assessed the
feasibility of three dose regimes, with PTV doses all having
a biologic equivalence of 60Gy in 30 fractions (α/β=10).
The planning aim was to determine the least number of
fractions to deliver an acceptable plan. Acceptable was
defined as ≥95% target coverage by the prescribed dose
whilst maintaining the OAR tolerances below. Marginally
acceptable was defined as 90-95% target coverage with
lung V20 <30% and other OAR tolerances met. Descriptive
statistics were used. We assessed doses to the PRVs (2mm
expansion) of each OAR to determine the IGRT
requirements for each strategy.
Results
Fourteen patients had N2 involvement whilst nine had N3
involvement. Mean ITV size was 207.7cc (range 31-
706.1cc). The hybrid approach generated acceptable
plans in 48% of patients (11/23), while the conventional
and SABR approaches achieved 26% (6/23) and 4% (1/23)
respectively. If acceptable was defined by >90% target
coverage by the required dose and lung V20 was less than
30%, 70% (16/23) of patients had acceptable plans with
the hybrid approach. Those that failed the hybrid