Abstract Book

S1114

ESTRO 37

to obtain minimum CTV dose (D99%) of 95 % for more than 90 % of population with conventional- and hypo- fractionation, respectively.

-d ref

)}, where d ref

is the reference

R=mean{abs(d i

amplitude

from

the

planning

CT.

S=mean{sd(d i }. Differences between inter and intra-fraction in reproducibility and stability were analysed (paired t- test). CWE was calculated as a)the reference amplitude in the planning CT, and b)the mean DIBH amplitude with respect to the free-breathing baseline immediately before the DIBH(upper Fig.2). Correlation tests were made between CWEs and reproducibility and stability. We also studied the correlation between CWE (definition a , CWE a ) and OAR doses (heart, lung, liver and contralateral breast,CB) obtained from dose-volume histograms of the treatment plan. Results Fig.1 shows the intra and inter-fractions results with both definitions. There are significant differences between both definitions, and between intra and inter fraction in Cerviño’s definition. CWE has significant different values between a and b definitions (Fig.2). CWE doesn’t show significant correlation with reproducibility and stability, nor CWE a with OAR doses.

Conclusion Use of electromagnetic tracking during prostate external beam RT reduces intra-fraction geometric uncertainty. However, incompatibility with MRI increases registration uncertainties, as registration is based on soft-tissue contrast, rather than seeds, between CT and MR images in a dual modality workflow. This results to increased registration uncertainty between the two modalities which propagates to increased systematic spatial uncertainty. Consequently, use of Calypso system for prostate EBRT decreases total geometric accuracy for EBRT of prostate cancer when MR image is used for target delineation. For hypo-fractionated treatments, and longer treatment times, use of Calypso becomes more relevant. EP-2039 Reproducibility and stability of DIBH in intra and inter fraction in RT of left-sided breast cancer M. Lizondo 1 , P. Carrasco 1 , A. Latorre-Musoll 1 , T. Eudaldo 1 , A. Ruiz-Martinez 1 , C. Cases 1 , N. Jornet 1 , P. Delgado- Tapia 1 , I. Valverde 1 , M. Ribas 1 1 Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain Purpose or Objective The aim of this study was to determine the inter and intra fraction stability and reproducibility of Deep Inspiration Breath-Hold (DIBH). We also analysed whether the chest wall excursion (CWE) is related to reproducibility of DIBH, stability of DIBH or OAR doses obtained in the treatment plan. Material and Methods Left-sided breast cancer patients are currently treated in our centre with DIBH (using the Real-time Position Management™, Varian) and visual coaching. A DIBH gating window of 0.5 cm is used. The treatment fractionation is about 25-33 fractions and 4-8 treatment fields are used in each fraction. 728 breathing patterns (57 patients) from May 2016 were recorded. Each DIBH in these fractions were automatically analysed by an in-house routine (R DevelopmentCoreTeam) to study stability (S) and reproducibility (R) both intra and inter fraction in each patient treatment. We used the following definitions:

LUNG

LIVER

CB

HEART

p- value

V25(%) D mean

(Gy) V5(%) D mean

(Gy) D mean

(Gy) V5(%)

CWE 0,138 0,160

0,650

0,921

0,618 0,533

1)Cerviño et al.[1] R=max{d i }-min{d i

}, where d i

is the average amplitude of

each

DIBH.

S=max{|m i and are, respectively, the slope of the linear fit (amplitude respiratory signal vs time) and the duration of each DIBH. 2)Stock et al.[2] |Δt i }, where m i