ESTRO 38 Abstract book

S1205 ESTRO 38

I. Lorenzo 1 , J.F. Calvo-Ortega 1 , S. Moragues Femenia 1 , C. Laosa-Bello 1 , J. Casals 1 1 Hospital Quirónsalud Barcelona, Radiotherapy, Barcelona, Spain Purpose or Objective Skin dose calculations in plans of breast cancer have been performed with and without including the air surrounding the patient external contour. Material and Methods Wang et al have described the effect of considering the air surrounding the external patient contour (“body contour”) in the accuracy of the calculated skin dose [J Appl Clin Med Phys. 2018 Mar;19(2):191-197]. They proved that an air extension of the body contour of at least 1 cm should be done in order to improve the skin dose calculation accuracy. We have included 30 breast plans that were calculated for treatment without air extension of the body contour. Then, these plans were re-calculated for a 2 cm-extension towards the air of the body contour (Fig 1). The Eclipse v. 13.6 treatment planning system with the AAA dose calculation algorithm was used. Photons beams of 6 MV from a Varian Clinac 2100 CD with the Millennium 120 MLC were used. The skin structure was outlined in the axial slices where the PTV was present, and it consisted of a 5 mm thickness layer beneath the default body contour detected by the Eclipse (Fig 1). Skin dose is defined in this study as the mean dose registered in the skin structure as defined above. We compared the skin dose for the two type of plans computed with and without extension of the body contour. Results An average increase of 7% (range: 6% to 8.5%) in the skin dose was observed when a 2 cm-enlarged body contour was used. Conclusion The skin dose increase revealed by the 2 cm-enlarged body contour is considered during the dosimetric evaluation of the breast plans designed to patient treatments. EP-2183 Dosimetric impact of CBCT calibration curve on dose calculated by a radiotherapy TPS C. Laosa 1 , J.F. Calvo-Ortega 1 , S. Moragues Femenia 1 , J. Casals 1 1 Hospital Quirónsalud Barcelona, Radiotherapy, Barcelona, Spain Purpose or Objective A radiotherapy treatment planning system (TPS) requires a relationship between electron density and CT numbers (calibration curve) to account for tissue heterogeneities during dose calculations. In this work we investigated the dosimetric impact of variations of the calibration curve of a CBCT scanner used for online adaptive cranial radiosurgery. Material and Methods A Varian Clinac 2100 CD linac equipped with a kV-CBCT scanner was used. A half-fan protocol (655 projections, 125 kVp, 80 mA and 13 milisec per projection, field of view of 260 mm, 384x384 matrix and 1 mm slice distance) is used in our department for adaptive cranial radiosurgery (Med Dosim. 2013 Autumn;38(3):291-7). A reference CBCT calibration curve was established in the Varian Eclipse TPS by using the CatPhan phantom containing seven inserts (air, PMP,LDPE, polystyrene, acrylic, Delrin and Teflon). After that, the CBCT calibration curve was monitored using the CatPhan phantom in a monthly-basis during a year. CBCT scanner was recalibrated when a variation greater than ±40 HU was noted in any insert, according to the Varian recommendation. For each CBCT scan (obtained before re-calibration if needed), a box plan (6 MV) was planned and the mean dose and CT number were computed on each insert over an inner region of interest.

CT number and dose for each insert were compared to the values obtained for the reference CBCT scan. Results Maximum variations in the CT number and dose respect to the values of the reference CBCT scan were (CT#/dose): 3 HU/0.7%, 34 HU/0.7%, 32 HU/0.6%, 23 HU/0.5%, 34 HU/0.7%, 32 HU/0.6% and 96 HU/0.8% HU for air, PMP, LDPE, polystyrene, acrylic, Delrin and Teflon, respectively. Conclusion Small impact (< 1 %) was observed in the dose quantity even for large absolute variations in the CT number (> 40 HU). Some guidelines (TECDOC-1583; IAEA, Vienna 2008) recommend adjusting the calibration curve in the TPS or tuning the CT scanner when CT numbers vary by more than ±20 HU. We propose to act when absolute dose differences greater than 1% are detected. 1 Seoul National University Bundang Hospital, Department of Radiation Oncology, Seongnam-si, Korea Republic of Purpose or Objective In treatment planning with junction, heterogeneity is considered to be minimized, but if there are adjacent MLCs, dose distribution can be affected and heterogeneity can be increased. The aim of this study was to compare dose of junction between breast and SCL fields in radiation therapy by MLC located at the junction. Material and Methods With 6 MV of 21EX-S equipped with 120-leaf Millennium MLC, treatment plans were designed with 30 patients who underwent radiation therapy using tangential field technique. Plan 1 where the MLC was all used at the junction. In plan 2 and plan 3, MLC was retracted 5 mm from breast and SCL, respectively. Plan 4 with all of MLC retracted at the junction were designed. In all the plans, collimator angle for SCL field was divided into 0° and 270°. To verify junction dose, the dose at 3cm depth of junction was compared with average value by MapCHECK. Results In case of the SCL field with 0° collimator angle, average value of D3cm was 4131.1, 4215.9, 4351.4, and 4423.0 cGy. In case of the SCL field with 270° collimator angle, average value of D3cm was 4044.3, 4246.7, 4291.1, and 4441.2 cGy. In plan 1 and 3, change in average dose depending on collimator angle was changed more significantly than plan 2 and 4. Dose measured at 3cm depth of junction was similar to treatment plan. Conclusion As a result of the study, there was a difference of about 10% between the plan where all the MLC was applied to the junction and the retracted plan. In radiation therapy plan for breast cancer with SCL, retracting MLC from junction between breast and SCL fields will lead to reduce effect of dose of the junction. EP-2185 Study of the seroma volume changes in the patients who underwent Accelerated Partial Breast Irradiation D.H. Kim 1 , S.J. Son 1 , J.G. Mun 1 , S.J. Seo 1 , J.H. Lee 1 1 Seoul National University Hospital, radiadtion oncology, Seoul, Korea Republic of Purpose or Objective By analyzing seroma volume changes in the patients who underwent Partial breast radiation therapy after breast conserving surgery, we try to contribute to the improvement of radiotherapy effect. Material and Methods Enrolled 20 patients who underwent partial breast radiation therapy by ViewRay MRIdian System were subject. EP-2184 A study on dose of the junction in radiotherapy of breast cancer including SCL W. Jung 1