ESTRO 38 Abstract book

S1100 ESTRO 38

the vertical direction for FB and DIBH, but larger for the longitudinal direction when used in combination with DIBH. Otherwise, margins agreed within 1 mm for both techniques.

complexity of ATS compared to ATP, further evaluation of the dosimetric and clinical benefit is ongoing. EP-2008 Positioning errors in free breathing and DIBH breast cancer radiotherapy: SGRT vs. skin markers A.M. Acosta Roa 1 , S.G. Mikalsen 1 , T.P. Hellebust 1 1 Oslo University Hospital, Department of Medical Physics, Oslo, Norway Purpose or Objective The purpose of this study was twofold: 1) To compare positioning errors in patient alignment for surface guided radiotherapy (SGRT) and skin markers (SM), both for free- breathing (FB) and deep inspiration breath hold (DIBH) breast cancer radiotherapy (BCRT). 2) To calculate the required PTV margins due to setup uncertainties in the same patient groups, if no corrections are applied prior to treatment. Material and Methods Data from 83 whole breast or chest wall BCRT patients was retrospectively analyzed. Due to different equipment available at two different sites in our department the procedure for patient alignment and pre-treatment imaging is different: SGRT with cone beam CT (CBCT) or SM with extended no-action limit protocol (eNAL) from portal images. Thereby, four patient groups were identified: SM DIBH, SGRT DIBH, SM FB, and SGRT FB with 25, 14, 19, and 25 included patients, respectively. Varian’s RPM system was used for DIBH at the SM site while AlignRT (VisionRT) was used at the SGRT site. Mean positioning shifts (MPS) in the vertical (Vrt), longitudinal (Lng), and lateral (Lat) directions were found from the pre-treatment image registration (matched to planning CT following bony anatomy), and the vector length |r 2 | of the shifts was calculated. Data from every treatment fraction was analyzed for patients undergoing the CBCT imaging protocol, whereas only the 3 first fractions were included for those following the eNAL protocol. Minimum PTV margins for 95% dose coverage to the CTV in 90% of the patients were calculated using van Herk’s equation. Results The MPS from initial setup in the Vrt, Lng, and Lat directions together with the mean displacement vector |r 2 | for all patient groups are shown in Figure 1. A tendency towards larger positioning errors was observed in |r 2 | for patients aligned with SGRT in comparison to SM, for both DIBH and FB (0.45 to 0.41 cm, and 0.49 to 0.45 cm, respectively). These differences are not statistically significant (p > 0.05) according to a two-sample t-test assuming unequal variances.

Conclusion Patient alignment by SM seems to be generally more accurate but less precise than SGRT. Larger SDs in the MPS lead to increased PTV margins. A wider spread in SD was however expected for the SM patient group, as fewer image registrations could be included due to use of eNAL protocol. Calculated margins account only for setup uncertainties, and are required if no correction is effectuated prior to treatment. EP-2009 Inter-fractional Motion of Intact Cervical Cancer Treated On A MR-Guided Radiation Therapy System D. Asher 1 , K. Padgett 1 , R. Llorente 1 , E. Mellon 1 , A. Wolfson 1 , B. Farnia 1 , G. Simpson 1 , N. Dogan 1 , L. Portelance 1 1 University of Miami, Radiation Oncology, Miami, USA Purpose or Objective Treatment verification has improved significantly over the past decade, with on-board MRI-guidance now available. MRI provides superior soft tissue contrast compared to CT- based imaging, allowing for identification of the tumor on daily imaging. Consensus contouring guidelines for IMRT of cervical cancer advise including the whole uterus in the target volume and adding large planning volume margins to account for motion. The primary objective of our analysis was to assess the inter-fractional GTV motion for those with an intact cervix using a MRI-guided-Radiation- Therapy (MRgRT) system and investigate the benefit of reducing the primary PTV to the GTV with a margin for microscopic disease and target motion. Material and Methods We analyzed 125 daily set-up MRIs from five patients with cervical cancer who received MRgRT. The GTV, bladder, uterus, and rectum were contoured on all 125 MRIs, then the positional and volume changes of the OARs were calculated to assess their effect upon the displacement of the tumor. Using these data, margin calculations were performed to account for the daily inter-fraction motion and compared to the consensus guidelines for margin expansions. Results The GTV decreased in size during the course of treatment for all patients, 34.0%-85.2%. The median GTV displacement range was 0.69cm-1.04cm. The margins calculated were: 0.78cm Left-Right, 1.31cm Anterior- Posterior and 1.38cm for the Superior-Inferior directions. The formalism presented here reduces the PTV by 36% compared to consensus guidelines sparing both the sigmoid and bowel. Conclusion Including the whole uterus in the target and adding large margin expansions, to account for inter-fractional motion, is the standard practice. However, by utilizing daily on- board MRI-guidance the GTV becomes readily visualized allowing for a reduction of margins and potentially excluding a portion of the uterine fundus from the PTV with the ultimate goal of decreasing treatment toxicity while maintaining tumor control.

The results from calculation of the PTV margins for 95% dose coverage of the CTV in 90% of the patients are shown in Table 1. It can be observed that margins do not necessarily correlate to MPS, since margin calculation depend strongly on the size of the standard deviation (SD), and not on the average. SGRT yielded smaller margins in