ESTRO 36 Abstract Book

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S430 ESTRO 36 2017
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technical drawings. Difference of just 3.5 % was obtained
for the experimental and calculated FWHM. Mean energies
calculated from peripheral photon spectra energies
ranged from 0.242 MeV in the mediastinum to 0.171 MeV
in the pelvis. Based on these results, an average energy
correction factor of 10% was applied to TLD readings. A
maximum of 15% difference between calculated and
measured peripheral doses were obtained.
Conclusion
The use of Egspp allowed us to model accurately the
Gamma Knife. The level of detail achieved in the modeled
geometry is essential for the peripheral dose calculation,
since it is dependent on the radiation leakage. The
agreement between simulations and measurements is
good, with higher discrepancies observed in the points
located on the limbs of the phantom. A MC methodology
for peripheral dose characterization has been validated
and therefore, different scenarios regarding patient size
and/or beam geometry can be estimated for future
references.
References
[1] Radiother Oncol 2012;10(5):122-126
[2] JACFMP, vol 14, N2, 2013
[3] Phys. Med. Biol. 57 (2012) 6167–6191
[4] Biomed. Phys. Eng. Express 1 (2015) 045205
PO-0812 Dosimetric impact of using Acuros algorithm
for stereotactic lung and spine treatments
L. Vieillevigne1,2 , T. Younes1,2, A. Tournier 1, P. Graff
Cailleaud1, C. Massabeau 1, J.M. Bachaud1, R. Ferrand1,2
1 Institut Claudius Regaud- Institut Universitaire du
Cancer de Toulouse Oncopole, Radiophysique, Toulouse,
France
2 Centre de Recherche et de Cancérologie de Toulouse
CRCT- UMR1037 INSERM - Université Toulouse 3,
Radiophysique- équipe 15, Toulouse, France
Purpose or Objective
The main aim was to assess the dosimetric impact of
calculating with the Acuros (AXB) algorithm instead of
Anisotropic Analytical Algorithm (AAA) for stereotactic
(SBRT) lung and spine cancer treatments.
Material and Methods
Ten stereotactic lung patients and ten stereotactic spine
patients were selected to investigate the dosimetric
impact of using AXB instead of AAA. Dynamic conformal
arc was used for SBRT lung patients with a prescription of
50 to 55 Gy in 3 or 5 fractions to the 80% isodose. For the
SBRT spine patients, Rapid Arc plans were prepared and
27 Gy was prescribed in 3 fractions to the PTV median
dose. The plans were recalculated with the AXB algorithm
by using the same beam settings and monitor units as the
AAA. Two dose reporting modes of AXB, dose to medium
(Dm) and dose to water (Dw) were studied. Relative dose
differences between algorithms were calculated for PTV
(D98%, D95%, D50% and D2%) and for organs at risk (D2%
and mean dose for the ipsilateral lung, the spinal cord or
the cauda equina).
Results
For the 10 SBRT lung patients, the mean lung density was
around 0.18 g/cm3 which corresponded to a normal lung
tissue. The dosimetric impact on PTV dose (D98%, D95%,
D50%) using AXB instead of AAA was quite small with a
maximum underdosage up to 3.1% for D98%. Larger
differences were obtained on D2% with a maximum
deviation of 7.79%. The average difference on the mean
ipsilateral lung dose was 0.22% and 0.44% for AXBDm and
AXBDw, respectively. AXBDm and AXBDw presented similar
results.
For the 10 SBRT spine patients, large relative dose
disagreement of up to -5.02% for the D98% of the PTV was
observed with AXBDm. On average, for the D50% of the
PTV, AXBDm revealed a relative underdosage of -2.36%,
whereas AXBDw lead to a relative overdosage of +1.64%.
Concerning the spinal cord or the cauda equina, the
average mean dose was reduced up to -6.93% and up to -
4.97% for AXBDm and AXBDw, respectively. For these
organs at risk, differences up to -4.56% and to +3.37% were
found for the D2% for AXBDm and AXBDw, respectively.
Conclusion
The studied lung cases present small mean differences
among all calculation modalities; AXBDm and AXBDw
calculations are similar. The spine cases show strong
difference between AXBDm and AXBDw inside the PTV and
the spinal cord or the cauda equina. Acuros is known to
provide an accurate alternative to Monte Carlo
calculations for heterogeneity management [1] and
reporting dose to medium is the preferred choice [2].
Nevertheless, moving from AAA to AXBDm for SBRT
treatments, in particular for spine or for lung of low
density, has to be carefully evaluated.
[1] A. Fogliata et al. “Dosimetric evaluation of Acuros XB
Advanced Dose Calculation algorithm in heterogeneous
media.,” Radiat. Oncol., vol. 6, no. 1, p. 82, Jan. 2011.
[2] P. Andreo “Dose to ‘water-like’ media or dose to
tissue in MV photons radiotherapy treatment planning: still
a matter of debate.,” Phys. Med. Biol., vol. 60, no. 1, pp.
309–37, Jan. 2015.
Poster: Physics track: Radiation protection, secondary
tumour induction and low dose (incl. imaging)
PO-0813 Cardiac Toxicity after Radiotherapy for
Hodgkin Lymphoma: Impact of Breath Hold and Proton
Therapy
L.A. Rechner1, M.V. Maraldo1, I.R. Vogelius1, P.M.
Petersen1, R.X. Zhu2, B.S. Dabaja3, N.P. Brod in4, L.
Specht1, M.C. Aznar5
1The Finsen Center - Rigshospitalet, Department of
Oncology, Copenhagen, Denmark
2MD Anderson Cancer Center, Radiation Physics,
Houston, USA
3MD Anderson Cancer Center, Radiation Oncology,
Houston, USA
4Albert Einstein College of Medicine, Institute for Onco-
Physics, Bronx, USA
5University of Oxford, Nuffield Department of
Population Health, Oxford, United Kingdom
Purpose or Objective
We undertook this work to quantify the impact of deep
inspiration breath hold (DIBH) and proton therapy, alone
and in combination, relative to treatment in free
breathing (FB) with IMRT with respect to the estimated
risk of cardiac toxicity after radiotherapy for patients with
early-stage mediastinal Hodgkin lymphoma (HL).
Material and Methods
Treatment plans were generated for 22 patients in both
FB and DIBH to 30.6 Gy (Gy(RBE) for proton therapy) in 17
fractions. IMRT plans were created according to the
clinical procedure at the presenting author’s institution.
Proton plans were created with guidance from the authors
with clinical proton therapy expertise. Mean doses to the
heart, heart valves, and left anterior descending coronary
artery (LADCA) were exported and excess relative risks
(ERRs) of radiation-induced myocardial infarction, heart
failure, and valvular disease were estimated. Dose volume
histograms (DVHs) for the heart were extracted and mean
DVHs were created for each treatment technique. The
Friedman test was used to assess statistical significance,
and analysis was performed in Matlab (The MathWorks,
Inc).
Results
The use of both DIBH and proton therapy were found to
reduce the dose as well as the estimated risk to cardiac
structures (Table 1). Mean doses to the heart, valves, and
LADCA, and the ERRs of radiation induced myocardial
infarction, heart failure, and valvular disease were