ESTRO 2021 Abstract Book

S1511

ESTRO 2021

Results The LARFAC associated with a prescribed dose of 50 Gy to target volume in 25 fractions was in the approximate range of 1 to 3%. The risk was higher for young and female patients. The largest contributing organ to this risk were the lungs by far. Using the LNT model to calculate risk at therapeutic doses overestimated the risk up to tenfold. Conclusion The LARFAC to patient from thymoma radiotherapy was quantified taking into consideration the inapplicability of the LNT model at therapeutic doses. The risk is not negligible; the information may be relevant to patients and clinicians. PO-1785 Non-target dose reduction at phantom study for prostate radiotherapy using TrueBeam and CyberKnife M. Kruszyna-Mochalska 1,2 , A. Skrobala 2 , P. Romanski 3 , A. Ryczkowski 3,2 , W. Suchorska 4,2 , K. Kulcenty 4 , I. Piotrowski 4,2 , N. Matuszak 2 , J. Malicki 2,3 1 Greater Poland Cancer Centre, Medical Physics, Poznan, Poland; 2 University of Medical Sciences, Electroradiology Department, Poznan, Poland; 3 Greater Poland Cancer Centre, Medical Physics Department, Poznan, Poland; 4 Greater Poland Cancer Centre, Radiobiology Laboratories, Poznan, Poland Purpose or Objective Despite the availability of highly precise advanced irradiation techniques, low dose radiation outside of the treatment field, such as non-target doses in prostate gland radiotherapy, remains a concern. The aim of this study was to determine and reduce out-of-field doses in selected points of a phantom for prostate radiotherapy delivered with the TrueBeam (Varian, Medical Systems Inc, USA) and CyberKnife (Accuray Inc., USA) linacs. Materials and Methods A quasi-anthropomorphic phantom to measure out-of-field doses in evaluated points located both close to (< 20 cm) and more distant from (> 20 cm) the target volume (simulating the prostate) was constructed. Multiple treatment plans were created. To find a method to reduce non-target dose, the measurements with an ionization chamber (Semiflex, PTW, Freiburg) for various VMAT plans for TrueBeam: energies with and without of flattening filter (6 FFF and 6 FF), with jaw tracking (JT) and for CyberKnifee and TrueBeam: modulation degree and number of MUs were compared. For the absolute dose measurements at specific points for TrueBeam (6FFF and 6FF) and CyberKnife (6FFF), the Gafchromic EBT3 films (Ashland, Inc. USA) were used. Based on these measurements, non-target doses for different treatment strategies were estimated. Results The phantom measurements show that the out-of-field doses (>20 cm) did not exceed the value of 0.11% (TB) and 0.18% (CK) of the target dose. A reduction of non-target doses for the irradiated prostate region is possible. For example, doses in the points located 35 cm away from the target were reduced: up to 18% due to the use of a flattening filter and up to 10% for using jaw tracking option, up to 15% (TB) and 14% (CK) for a smaller number of MUs. Closer to target (15 cm), a dose reduction was also achieved with JT and 6 FFF, but a higher dose was obtained with less modulation, especially for non-coplanar CK techniques. For different treatment strategies, non-target doses were estimated: for SBRT 36.25 Gy in 5 fractions (CyberKnife and TrueBeam 6 FFF) using a conventional fractionation schedule: 74 Gy in 36 fractions and 60 Gy in 20 fractions (Fig 1).

Figure 1. Non-target dose estimation for SBRT 36.25 Gy in 5 fractions (CK and TB 6FFF) delivered in a conventional fractionation scheme: 74 Gy in 36 fractions and 60 Gy in 20 fractions.

Conclusion