ESTRO 2020 Abstract book

S780 ESTRO 2020

PO‐1379 Quantifying the errors associated with the AAA in the presence of high‐density implants A. Fischer 1 , R. Wills 1 1 Mount Vernon Cancer Centre, Radiotherapy, London, United Kingdom Purpose or Objective Our aging population means the number of patients with bilateral hip prostheses is increasing. Such high-density implants present multiple challenges when planning a radiotherapy treatment. The majority of commercial dose calculation algorithms are unable to correctly model electron scatter at the implant-tissue interface. The aim of this work is to quantify these errors for Eclipse’s analytic anisotropic algorithm (AAA) and to use the results to inform a safe and efficient approach to treatment planning. Material and Methods Following a literature search, we could not find data comparing the AAA with measurements downstream of a metallic inhomogeneity for a 6MV beam. We irradiated a hip prosthesis of mean density 3.5g/cc inside a water tank using a 10cmx10cm beam and took measurements using a Farmer chamber and gafchromic film. The water tank containing the prosthesis was scanned using CT, so that the measurements could be compared to doses calculated using the AAA. Results The doses calculated using AAA were consistently below the Farmer chamber measurements, with an error of -3.1% 1.3cm downstream of the prosthesis, reducing to -0.1% 9.3cm downstream of the prosthesis. The measurements and calculated doses approach each other as the distance from the prosthesis increases. Similar behaviour is seen for the film measurements. This is in contrast to previously published data for 18MV, where the AAA was compared to film measurements when irradiating a metal slab in water [1]. The different behaviour could be due to the different geometries, particularly if the slab had a cross sectional area larger than the field size. Our film measurements at the upstream prosthesis-water interface indicate a large error of -15.6% in line with the existing literature. Conclusion The large backscatter peak at the upstream tissue- prosthesis interface not modelled by the AAA should be borne in mind when planning treatments involving nodes, for which the PTV may directly abut the prosthesis. In our department we have decided to keep 105% hotspots at least 1cm from the prosthesis. Furthermore the relatively low errors several cm downstream of the prosthesis suggest it is not necessary to completely prevent the beam from entering the PTV through the prosthesis. We use a VMAT planning approach with strongly weighted low dose objectives on the hip PRVs to minimise the proportion of the beam that enters the PTV through the prosthesis. This minimises the errors associated with the AAA calculation whilst covering the target. Allowing a small fraction of the dose to enter the PTV through the prosthesis has allowed us to achieve V95%=99.4% (average over 5 test plans) compared to V95%=96.2% for our previous fixed field IMRT approach, where no dose was permitted to enter through the prosthesis. References [1] Lloyd, S. A. M. & Ansbacher, W., 2013. Evaluation of an analytic linear Boltzmann transport equation solver for high-density inhomogeneities. Medical Physics, 40(1), p. 011707 PO‐1380 Validation of the PRIMO Monte Carlo software for stereotactic radiosurgery plans M. Hermida Lopez 1 , J.F. Calvo-Ortega 2 , S. Moragues- Femenía 2 , C. Laosa Bello 2 1 Hospital Universitario Vall d'Hebron, Servei de Fisica i Protecció Radiològica, Barcelona, Spain ; 2 Hospital

data. The correct position of the Farmer chamber at the isocentre was validated using the megavoltage imager controller at two orthogonal gantry angles. The full PPS was in place to simulate the treatment situation. The 5x5 cm 2 beams were delivered automatically in the step-and- shot mode with 100 MU in each segment,at 214 gantry angles in step of 1º in the range 90º to 270º and in 5º steps outside this range. The measurements were repeated after rotating the phantom 180º around its cylindrical axis to cancel out asymmetry in the Farmer response.

Results The data were normalised to gantry 90º and the effect of the cryostat was removed by dividing the data with the cryostat characterisation measurements. A minor sinusoidal effect from residual misalignment was extracted from the measured doses using D’ =( D + a ×sin ϕ )/ k with a and k being fitting parameters to ensure that D’ (90º)= D’ (270º) and ϕ is the gantry angle. Eight measurements were carried out on the UAS Linac and two at the OUH Linac and the mean attenuation is shown in Figure 2. The differences in attenuation between the two PPSs are generally within 0.5%. The largest differences are 1% and can be seen around angles 115º and 245º where the attenuation pattern is very complex.

Conclusion This work shows that the two MR-Linac PPSs have only minor differences in attenuation. Differences of around 1% was observed where the attenuation pattern is complex. The peak attenuation is at gantry angles at 122º and 238º degrees. The strong gradients in the attenuation may contribute to dose delivery uncertainties in the situation of a patient misalignment. An evaluation of the TPS against the measured PPS attenuation is the next step to quantify the uncertainties in dose deliveries to patients irradiated through the PPS.