ESTRO 2023 - Abstract Book

S1829

Digital Posters

ESTRO 2023

Absorbed dose profiles at 5 cm depth and SSD=95cm of a 10x15 cm^2 Varian Clinac DHX open field of 6 MV and 15 MV were calculated with the AXB and AAA algorithms in the TPS Varian Eclipse v. 16.1. Dose calculations, reported as dose-to-water, were compared with measured profiles, obtained with a PTW LA48 linear ionization chamber array, positioned in the slab PMMA phantom to reproduce exactly the calculated fields. The detector was previously cross-calibrated in dose-to-water against a Farmer ionization chamber for the two considered beam qualities. A schematic view of the setup is reported in Figure.

Results The percentage difference between measured and calculated doses in two off-axis points, namely - 4 cm and + 4 cm from the isocenter in the longitudinal direction, corresponding at points downward titanium and PMMA respectively, were reported in Table. The average accuracy of the dose profiles for 6 and 15 MV respectively was for AAA and MAR -3.8% and -4.0%; for AAA without MAR -1.7% and -2.8%; for AXB and MAR -2.3% and -2.0% and for AXB without MAR -1.4% and -1.7%.

Conclusion Our results show that, limited to the reported materials and irradiation conditions, the MAR option embedded in the GE CT scanners should be used carefully in combination with the AAA calculation algorithm, meanwhile has limited impact on AXB calculations, probably due to the property of the latter of limiting the HU-to-density automatic conversion for the higher density. Although more realistic situations should be investigated, such as real prosthesis or dental implant, our results suggest that the wide use of the MAR option in planning CT images, thanks to the increased image quality, seems justified for dose calculation with AXB but should be carefully considered with AAA.

PO-2054 Implementation of a tangential IMRT solution for breast cancer in Monaco TPS

S. Poeta 1 , D. Rodriguez Garcia 2 , Y. Jourani 1 , A. Desmet 3 , N. Reynaert 2

1 Institut Jules Bordet, Medical Physics , Brussels, Belgium; 2 Institut Jules Bordet, Medical Physics, Brussels, Belgium; 3 Institut Jules Bordet, Radiation Oncology, Brussels, Belgium Purpose or Objective The aim of this work was to implement an efficient tangential solution for breast cancer treatment based on dynamic multi leaf collimator (dMLC) inverse planning in Monaco TPS (v6.0, Elekta AB, Stockholm, Sweden) while preserving the organs at risk (OAR) constraints based on conformal 3D solutions. Materials and Methods Twenty-eight consecutive patients treated were selected: 15 left (LBC) and 13 right (RBC) breast cancers, respectively. Contouring was performed according to ESTRO guidelines. All LBC patients were treated with breath hold technique. The dMLC technique was based on inverse planning in Monaco with 6MV photons. This technique consisted on two opposed tangential beams, as set for conformal 3D techniques, with collimators adapted to be parallel to the chestwall with specific parameters in Monaco to mimic the use of wedge or field in field technique. The main calculation properties and sequencing parameters of the plans are described in table 1. The prescribed dose was 40,05Gy in 15 fractions to the PTV Breast (breast or thoracic wall). Indications of lymph node irradiation were excluded. All plans were normalized to the 95% of the prescribed dose covering 95% of the PTV Breast. Doses to OAR were evaluated against our in-house constraints and two well-known protocols: DBCG HYPO trial and RTOG