ESTRO 38 Abstract book

S930 ESTRO 38

receiving radiation therapy after mastectomy. CT images were taken with bolus. CTVT and PVT were delineated according to national (SBRG) guidelines. 3DCRT plans were generated in Eclipse TPS using AAA and re-calculated with the same MU by Acuros XB and a Monte Carlo (MC) system based on EGSnrc. Calculations were performed on original CT scans and modified scans with bolus excluded. Dose distributions were obtained for three groups, (10 patients in each group), with selected chest wall thicknesses; below 15 mm (group 1), between 20 - 25 mm (group 2) and between 30 - 35 mm (group 3). Target coverage was evaluated in terms of DVH parameters; CTVT mean dose, CTVT_V95%, PTV_D98%, PTV_V95% and PTV_V105%. Results The distribution of chest wall thickness for the 160 patients is shown in the Table. The bolus effect was most pronounced for patients in group 1. About 10-15% higher CTVT_V95 and 10-20% higher PTV_D98 values were obtained in case of bolus (Figure). For the patients in group 3, CTVT_V95 differed by 4% and PTV_D98 by 5-15% (Figure). CTVT mean dose for group 1 was 2% higher as estimated by Acuros XB and MC (4.5% for AAA) and 1% lower for groups 2 and 3 (negative bolus vs no-bolus difference). The increase of PTV_V95% in absolute volume due to presence of bolus was comparable for the three groups: about 20 cm3; 15 cm3 and 10 cm3 as estimated by AAA; Acuros XB and MC, respectively. PTV_V105% decreased in case of bolus by 5 - 10 cm3 (group 1) and 20 - 40 cm3 (group 3). The presence of bolus enhanced the agreement between the different algorithms and reduced the variation of the estimation within the group (max-min intervals in the figure). Table . Chest wall thickness near CTVT for 160 patients undergoing post-mastectomy radiation therapy

Esteya® is an electronic brachytherapy device with a 69.5 kV X-ray source enabling treatments up to 4 mm depth. In general the applied dose rate is 3.3 Gy/min and the maximum aperture is 30 mm. Esteya® QA consists of two parts: mandatory daily tests on radiation days carried out with the QA tool provided by Elekta and periodic QA tests. During the monthly tests reference dosimetry, energy, symmetry, flatness and penumbra size are checked for the 30 mm applicator. Annually the other applicator diameters are checked as well. The reference dosimetry in air is performed with an ionization chamber (34013, PTW-Freiburg GmbH, Germany). Three fractions of 6 Gy prescribed at 0 mm depth are delivered for verification. The energy consistency is determined by measuring at three depths. A Plastic Water Low Range (PWLR, Cirs inc., USA) slab with a cavity for the ionization chamber is placed on top of 60 mm of PWRL backscatter material. As a reference a measurement without an additional slab is used. Besides we perform measurements with additional slabs of 5 and 10 mm PWLR. Flatness, symmetry and penumbra size are determined using Gafchomic EBT3 film (Ashland inc., USA) irradiated with a 5 Gy fraction. The film is placed on top of 60 mm PWRL and covered with PWLR slabs with a total thickness of 3 mm. The film is scanned with an Epson Perfection V750 Pro transmission scanner (Seiko Epson Corp., Japan) and analyzed using FilmQA Pro software (Ashland inc., USA). The penumbra size is determined as the distance between points representing 20% and 80% of the central axis dose. Results During acceptance measurements, the characteristics of Esteya® have been determined. Since acceptance neither the X-ray source, nor other major items have been replaced. The results presented here are based on the monthly checks. The mean value of the reference dosimetry is 5.96 Gy (SD 0.03 Gy). All measurements are in tolerance of ±0.12 Gy (±2%). The mean value for the percentage depth dose at 5 mm is 72.8% (SD 0.7%) and at 10 mm 53.7% (SD 0.3%). This is well within ±2% compared to the reference values of 73.1% and 53.9% at respectively 5 and 10 mm depth. The larger SD found at 5 mm depth is due to a difference in thickness of the slabs. All measurements of the symmetry and flatness are within the tolerance of ±3%. The mean penumbra size is 0.3 mm (SD 0.1 mm). Conclusion All measurements are within the set tolerance limits. The physics characteristics of Esteya® have proven very stable and consistent. Hence, in combination with the sharp penumbra (mean size 0.3 mm) and steep dose fall off in depth, Esteya® enables a reliable and precise treatment of non-melanoma skin cancer. EP-1726 Utilization of bolus in post-mastectomy radiation therapy R. Chakarova 1 , D. Lundstedt 2 , H. Svensson 1 , M. Gustafsson 1 , M. Hällje 1 , P. Karlsson 2 1 Sahlgrenska University Hospital, Medical Physics and Biomedical Engineering, Göteborg, Sweden ; 2 Sahlgrenska Academy, Dept. of Oncology- Institute of Clinical Sciences, Göteborg, Sweden Purpose or Objective Women receiving radiotherapy to the chest wall after mastectomy due to breast cancer usually have a bolus over the scar. The aim of this work is to evaluate the bolus effect on target coverage and in particular to determine if bolus recommendation should be related to the chest wall thickness and if different dose calculation algorithms estimate target coverage differently. Material and Methods The smallest thickness of the thoracic wall in the CTVT region was estimated for 160 consecutive patients

Figure . CTVT_V95% and PTV_D98% for group 1 and 3 estimated by: AAA, Acuros XB dose to water and medium (AXBw, AXBm), Monte Carlo (MCw, MCm). Blue bars: bolus; red bars – no-bolus. Error bars indicate min-max interval.