ESTRO 37 Abstract book

S1116

ESTRO 37

Results (1) The chest motion measured with Breathe Well and the vertical AlignRT motion are correlated (p<0.001, r= 0.63) and both systems measure a similar reproducibility and stability (Table 1). Breathe Well-guided BHs stay well within the clinical AlignRT tolerance of 1cm. (2) The BH reproducibility is significantly (p<0.0001) improved with visual feedback (Table 1). The stability of the guided and unguided BHs was similar, but the unguided BHs were substantially shorter, which has potentially an impact on the results.

Results Based on the van Herk’s formalism of required CTV to PTV margins, estimated lateral vertical and longitudinal CTV to PTV margins were 7.1 mm, 7.5 mm and 8 mm with Calypso system and 5.5 mm, 6 mm and 6.5 mm when using standard implantable fiducials. According to the computer simulations, uniform margin of (w/o vs. with Calypso) 6 mm vs. 8 mm and 6 mm vs. 6 mm was needed to obtain minimum CTV dose (D99%) of 95 % for more than 90 % of population with conventional- and hypo- fractionation, respectively.

Conclusion Breathe Well’s ability for a reproducible and stable BH guidance was verified with the AlignRT system. Visual feedback improved BH reproducibility compared to unguided BHs. EP-2038 Impact of Calypso system to treatment margin for prostate EBRT with MR-CT fusion-based workflow R. Kemppainen 1 , T. Seppälä 1 , P. Arponen-Esteves 1 , M. Myllykangas 1 , T. Kiljunen 2 , M. Tenhunen 1 1 Helsinki University Hospital, Department of Oncology, Helsinki, Finland 2 Docrates Cancer Center, Helsinki, Finland Purpose or Objective Recently introduced electromagnetic Calypso tracking devices promise to reduce geometric uncertainties of external beam radiation therapy (EBRT) of prostate cancer by allowing real-time monitoring of the treatment target. However, the implanted beacons are incompatible with magnetic resonance imaging (MRI). As MRI enables more accurate contouring of prostate compared to CT, contemporary state of the art prostate treatments are based on delineation using T2-weighted MR-images and propagation of the contours to the planning CT for treatment planning. Consequently, use of Calypso system affects total geometric uncertainty by reducing intra-fraction uncertainty and increasing registration uncertainty between CT and MRI. In this work, we evaluate the overall impact of using Calypso system for prostate EBRT with MR-based target contouring. Material and Methods The evaluation was based on published and measured estimates of spatial uncertainties in EBRT of prostate cancer. We evaluated the total geometric uncertainties for two alternative workflows at our institution by comparing the reduction of intra-fraction spatial uncertainties and increase of registration uncertainties in target definition when using Calypso system. Intra- fraction motion was assessed by using tracking recordings of 17 patients from a previously published study. CT to MR registration uncertainty was estimated from literature and by performing manual registrations for 4 patients by 3 experts. Required CTV to PTV margin was estimated for both workflows using the van Herk’s formalism and for different fractionations using Monte Carlo simulations.

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