Acosta, O.

Blay, C.

Dardelet, E.

De Crevoisier, R.

Dubray, B.

Gibon, D.

Haigron, P.

Simon, A.

Viard, R.

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S832                                                                                                                                                  ESTRO 35 2016 
_____________________________________________________________________________________________________ 
heterogeneous tumors. The insert has an outer low-uptake 
volume encompassing a high-uptake inner volume. SUV ratio 
of 1:2 was intended. The second phantom accommodates 
applicators that can hold Farmer ion chamber in a location 
matching the center of the inner volume and in four locations 
matching the outer volume. 4D PET/CT scans of the phantom 
were acquired with three breathing wave forms of ideal 
sinusoid and two patient-specific breathing patterns fed to 
the moving platform. Patient-specific wavefronts were 
selected to represent a regular and an irregular breather. 
Two scenarios were investigated for image reconstruction, 
planning and delivery: a gate 30-70 window, and no gating. 
ITVs were delineated on the obtained 4D PET/CT scans and 
21 VMAT-SIB treatment plans were generated with two 
fractionation regimens: 
· Conventional fractionation: 2 Gy/fx to outer ITV, 2.4Gy/fx 
to high SUV inner ITV, 30 fx.  
· Hypo-fractionation delivered in both flattening filter and 
flattening filter free (FFF) modes: 8 Gy/fx to outer ITV, 9 
Gy/fx to inner ITV,5 fx. Treatment plans were delivered in 
two gating scenarios: no gating and gate 30-70. Two ion 
chamber readings for the inner ITV, and two readings for one 
arbitrarily selected outer ITV were acquired. Measured doses 
in the inner ITV and the outer ITV were compared to planned 
doses. 
 
Results: For both fractionation regimens and both delivery 
modes, measured doses in outer and inner ITV were between 
93 and 99% of planned doses. Measured dose as compared to 
planned dose demonstrated independence from breathing 
pattern or gating window. In particular, measured doses in 
FFF mode were consistent with measured doses in filtered 
beam mode, 94-96% of planned dose. 
 
Conclusion: The phantom has been validated for end-to-end 
use from 4D PET/CT scanning and radiotherapy planning, to 
dosimetric verification. Measured doses for SIB plans were in 
reasonable agreement for all three breathing patterns and 
for both gating windows and delivery modes. 
 
Electronic Poster: Physics track: Inter-fraction motion 
management (excl. adaptive radiotherapy)  
 
 
EP-1775  
CBCT based prostate IGRT accuracy and PTV margins 
C. Blay
1Centre Eugène Marquis, Radiotherapy, Rennes, France 
1, A. Simon2, E. Dardelet2, R. Viard3, D. Gibon3, O. 
Acosta2, P. Haigron2, B. Dubray4, R. De Crevoisier1 
2Rennes University 1, Campus de Beaulieu- LTSI, Rennes, 
France 
3Aquilab, Aquilab, Lille, France 
4Centre Henri Becquerel, Radiotherapy, Rouen, France 
 
Purpose or Objective: Purpose: Image guided radiotherapy 
(IGRT) is the standard treatment of prostate cancer, widely 
based on Cone Beam CT (CBCT). The accuracy of CBCT based 
prostate registration is however not well established, 
conditioning the choice of the Planning Target Volume (PTV) 
margins. The goal of the study was to quantify the 
uncertainty of this registration and propose therefore 
appropriate margins. 
 
Material and Methods: Materials and methods: A total of 306 
prostate CT to CBCT alignments were analyzed in 28 prostate 
cancer patients treated by IGRT. The prostate was manually 
delineated on all the CBCT. Three prostate alignment 
modalities were afterwards simulated and compared, based 
on skin marks, on CBCT registration performed by the 
technologist at the fraction (IGRTt) and on the prostate 
contours. The IGRT uncertainty (IU) was defined as the 
difference between the contour based and the CBCT 
alignments, in each space direction. Dice index (DI) were 
calculated. Margins were calculated, based on the IU and the 
Van Herk formula. 
 
Results: Results: The mean (min;max) absolute values of the 
IU were, in mm: 1.5 (0;10), 0.7 (0;12) and 0.9 (0;7), in 
antero-posterior (A/P), cranio-spinal (CS) and lateral 
directions, respectively. After IGRTt alignment, 25 prostate 
(11% of cases) still projected partially out of the PTV, 
corresponding to an average prostate volume (min; max) of 
2.3 cc (0.0;12.6). The mean + standard deviation of the DI 
were 0.84 + 0.08, 0.90 + 0.07 and 0.93 + 0.03 for the skin 
marks, CBCTt and contours registration, respectively. For at 
least 95% of the IGRT registrations covering 100% of the 
prostate, the required A/P, CS and lateral PTV margins (mm) 
should be at least 4.5, 2.0 and 3.0, respectively. The Van 
Herk PTV margins (mm) were 5.5, 4.1 and 3.0 in the A/P, CS 
and lateral directions, respectively. 
 
Conclusion: Conclusions: CBCT based prostate registration 
presents uncertainties requiring at least 3 to 5 mm PTV 
margins. 
 
EP-1776  
Assessment of setup uncertainties in modulated 
treatments for various tumour sites 
E.S. Sandrini
1Grupo COI, Física Médica, Rio de Janeiro, Brazil 
1, L.R. Fairbanks1, S.M. Carvalho1, L.R. Belatini1, 
H.A. Salmon1, G.A. Pavan1, L.P. Ribeiro1 
 
Purpose or Objective: The aim of this study was to analise 
the patients setup errors for various tumor sites based on 
clinical data from modulated treatments using cone beam 
computed tomography (CBCT) imagine guidance and portal 
imaging for breast site. It was also calculated the planning 
target volume (PTV) margins of all disease sites and 
stipulated action level for online correction. 
 
Material and Methods: The patients analyzed in this study 
were treated in our institution between January 2012 and 
December 2014 with VMAT and IMRT via flash technique for 
breast cancer. The various tumor sites were divides into six 
categories; 175 breast (1173 fractions); 53 thorax (475 
fractions); 60 prostate (585 fractions); 100 H&N (858 
fractions); 100 SNC (789 fractions) and 77 pelvis (620 
fractions).  
For every treatment fraction, it were acquired KV-CBCT 
images using the on-board imager (OBI) (Varian Medical 
Systems), and for breast cancer it were acquired MV portal 
images using the Electronic Portal Imaging Device (EPID) 
(Siemens AG) in the first week and twice per week. The 
registration procedure was performed for all treatments sites 
according to the tumor localization. For prostate site, it was 
also analyzed the physiological state of bladder and rectum.  
It were calculated the systematic (Σ) and random (σ) errors 
of couch shift obtained, and PTV margin (2,5Σ + 0,7σ). 
 
Results: The Σ and σ for all treatment sites are summarized 
in table 1 as well PTV margins.  
Table 1. The systematic and random errors and PTV margins  
 


EP-1775: CBCT based prostate IGRT accuracy and PTV margins 

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EP-1775: CBCT based prostate IGRT accuracy and PTV margins

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