Abstract Book

S1120

ESTRO 37

Purpose or Objective Daily shape changes and swelling of breast may occur during radiotherapy (RT) treatment course. The aim of this work was to quantify the anatomical changes of breast surface to determine the extent of in-air outer PTV margin needed in breast cancer (BCa) RT. Material and Methods The anatomical changes of breast surface during the course of RT were investigated on 40 BCa patients. Patients underwent breast conserving surgery with lumpectomy only ( n =4), with sentinel node biopsy ( n =21) or with radical axillary lymph node dissection ( n =15). Patients were imaged with CT in supine position and arms up. Right sided patients ( n =20) were imaged in free breathing and left sided ( n =20) in deep inspiration breath hold (DIBH) with a guidance of an optical system (C-RAD). Mean age was 63 years (range 49-78 yr) and the PTV volume was 1330 ± 373 cm 3 (mean±SD). All patients were treated with a VMAT technique, 20 with hypo- (40.05Gy/15fr) and 20 with conventional fractionation (50Gy/25fr), with daily CBCT image guidance (Elekta Infinity/XVI). Maximum mutual information (MMI) registration (Mosaiq v2.62, Elekta AB) was performed in 6D with a box shaped region of interest between 800 CBCT and the respective planning CT images. Registrations were first computed for the whole PTV (Fig 1A) and second only for the surface area of the breast (Fig 1B). The difference in orthogonal shifts and rotations was calculated between the two registrations. The maximum value of right/left or anterior/posterior shift was calculated from each fraction to attain the maximum breast surface expansion (MBE).

Likelihood”, has on the setup, the data was analysed both with and without the offline corrections applied. Thus, including the correction strategy, LBS under the best conditions possible was compared to SBS. Results For LBS, excluding all offline corrections, the amount of fractions where setup deviation exceeded the clinical setup deviation threshold for online correction of 4 mm was 44% / 31% / 36% in the vertical/longitudinal/lateral (vrt/lng/lat) direction, respectively. The corresponding result for SBS was 7.5% / 7.5% / 27% in vrt/lng/lat, respectively. Including offline corrections showed improvement in the setup for LBS, but SBS still provided a higher setup accuracy (see Figure 1). The amount of fractions where the setup deviation exceeded 4 mm when the offline corrections were included in the data was 22% / 21% / 28% for LBS and 7.5% / 6.5% / 20% for SBS in vrt/lng/lat, respectively.

Fig1. Planning CT (magenta) and CBCT (green) image registrations were performed (A) first based on the mutual information of the whole target volume and (B) second only on the surface areas. Results The MBE was on average 2.4 ± 2.1 mm during treatment course. With 25 patients (62.5%) the MBE was ≥ 5 mm at least once during the treatment course. The largest MBE was 12 mm and in 107 fractions (13.4%) the MBE was ≥ 5 mm (Fig 2). An outer margin of 8 mm would have been required to cover the whole breast surface in 95% of the fractions. PTV volume or patient age did not correlate with the MBE. Instead, a weak correlation was observed with the extent of surgery ( r =0.36). The MBEs were also greater with left sided patients (L: 2.6±2.3 mm, R: 2.1±1.9 mm, p <0.01) and the difference was also significant in coronal (L: 1.4±3.0 O , R: -0.8±2.3 O , p <0.01) and transversal (L: - 1.8±2.2 O , R: -0.4±2.5 O , p <0.01) rotations.

Conclusion The results obtained show that SBS is a better method of positioning patients than LBS. The amount of setup deviations exceeding 4 mm was decreased in all directions for SBS compared to LBS. Although the offline correction strategy improved the positioning for LBS, SBS still showed a higher setup accuracy. EP-2048 Surface changes during the course of breast cancer radiotherapy: effect on the in-air outer margin. J. Seppälä 1 , A. Pandey 2 , A. Al-Gburi 2 , J.T.J. Honkanen 1 , T. Virén 1 , J. Heikkilä 1 , T. Koivumäki 3 , K. Vuolukka 1 1 Kuopio University Hospital, Cancer Center, Kuopio, Finland 2 University of Eastern Finland, Department of Applied Physics, Kuopio, Finland 3 Central Finland Central Hospital, Department of Medical Physics, Jyväskylä, Finland