ESTRO 2021 Abstract Book

S1356

ESTRO 2021

Po-1634 Transit Dosimetry With Portal Images For Ultrahypofractionated Radiation Therapy For Breast Cancer D. Hernandez 1 , P. Castro 1 , M. Roch 2 , R. Fayos-Sola 3 , P. Botella 3 , A. Valiente 4 , M. Escobar 5 , M.S. Talaya 4 , M.T. Murillo 5 1 h.U. La Princesa, Medical Physics, Madrid, Spain; 2 h.U La Princesa, Medical Physics, Madrid, Spain; 3 h.U.La Princesa, Medical Physics, Madrid, Spain; 4 h.U. La Princesa, Radiation Oncology, Madrid, Spain; 5 h.U.La Princesa, Radiation Oncology, Madrid, Spain Few years ago, several studies have been published showing that treatment schedules with a smaller numbers of fractions combine to higher dose per fraction are at least as safe and effective as the normofractionated treatments. The COVID-19 situation has highlighted the need to minimize the length of stay of patients in hospitals. The aim of this work is to analyze the transit dosimetry in patients treated with extreme fast-forward hypofractionation at Hospital Universitario de La Princesa. Materials and Methods 50 integrated images have been analyzed of 10 patients diagnosed with breast carcinoma without nodal irradiation criteria. The patients were simulated with a CIVCO radiotherapy posiboard inclined plane if they were positioned supine and with the BIONIX radiotherapy PBRT plane if they were in prone position. The fast forward treatment prescription is 5 fractions at 5.2Gy per fraction in the PTV. There is no boost irradiation. The treatments have been delivered on TrueBeam 2.7 (Varian, Fl) with an amorphous silicon portal imaging system "MV Imager" model as1200. The flat panel has dimensions of 43x43 cm 2 with 1280 x 1280 pixels. The treatment planning has been 3D conformal photon radiotherapy. Integrated imaging is performed in all fields during treatment delivery. Pre-treatment delivery, 2.5MV imaging is acquired to verify the correct positioning of the patient. The displacements must be less than 5 mm to be able to deliver the treatment. Post- treatment, the integrated images are analyzed through the Perfraction, a module of the SunCheck platform (SunNuclear, Fl). A gamma analysis of the 50 integrated images was performed. The gamma criterion consists of establishing a dose difference test in percentage and a distance-to-agreement test (DTA) in millimeters, in which the points must not exceed values established by the user. In the gamma analysis of this work, the gamma criteria 3% 3mm, 5% 5mm and 7% 7mm have been analyzed with a threshold value of 10%. Results Table 1 shows the average results of the 50 transit dosimetry images performed. The average number of points passing the 3% 3mm gamma criterion is 91.1 ± 7.6 %. If the gamma test is changed to 5% 5mm, on average, 96.7 ± 3.6 % of the points approve the criterion. If the tolerances are set at 7% and 7mm, the average number of points passing the test is increased up to 98.7 ± 1.8 %. Purpose or Objective Breast cancer treatment is in a permanent evolution.

Conclusion Performing transit dosimetry is very useful to verify correct treatment delivery. The 5% 5mm gamma test is a good criterion for fast forward breast treatments.

PO-1635 Dosimetric characterization of patient-specific three-dimensional tissue-equivalent bolus. V. Lewitzki 1 , S. Wegener 1 , A. Toussaint 2 , M. Flentje 2 , S. Pollmann 3 1 Julius-Maximilians University , Radiation Oncology, Würzburg, Germany; 2 Julius-Maximilians University, Radiation Oncology, Würzburg, Germany; 3 ulius-Maximilians University, Radiation Oncology, Würzburg, Germany Purpose or Objective In adjuvant radiochemotherapy of HNSCC patients the post-operative scar region from neck dissection represents a target volume at risk. In order to generate a sufficient surface dose silicone bolus is used in clinical routine for the dose build-up. The use of standard silicone boluses may result in extensive skin coverage and unintended dose elevation in non-risk skin areas increasing radiodermatitis. Contemporary 3D printing techniques allow the fabrication of individually customized boluses. One anticipates more precise dose build-up in the target volume, while sparing skin unintended to be treated. We compared customized and standard boluses in terms of equivalence, and dose distribution in the scar and spared region. Materials and Methods Dose build-up using a customized silicone bolus was compared with standard one-size silicon bolus using a slab and a human phantom. Next, individually customized silicone boluses for the scar area were fabricated for 25 patients. A 5 mm thick 3D bolus contour on the patient surface around the surgical scars with 2 cm safety margin in each direction was generated from the planning CT data set. This 3D structure was printed as a mold and poured with silicone. Dose measurements were made with radiochromic films at three locations including the high-dose area (scar)