ESTRO 37 Abstract book

S983

ESTRO 37

EP-1823 Relation between uncertainty and dose from small fields for a 2D pre-treatment dose prediction model C.J.A. Wolfs 1 , S.M.J.J.G. Nijsten 1 , F. Verhaegen 1 1 Department of Radiation Oncology MAASTRO, GROW – School for Oncology and Developmental Biology- Maastricht University Medical Centre, Maastricht, The Netherlands Purpose or Objective To evaluate the relationship between the uncertainty of a 2D pre-treatment dose prediction model and the proportion of dose coming from small fields for a range of clinical treatment plans. Material and Methods The model predicts 2D portal dose images (PDIs) at the level of the EPID using fitted parameters, and an RTPLAN as input. For the uncertainty analysis, 100 measurements sets for fitting the model were simulated. These were within the uncertainty of the measurement set originally used for fitting. The fitting process was repeated for each of the simulated measurement sets, resulting in 100 alternative parameter sets. The uncertainty was evaluated in two ways. First, the maximum deviating simulated measurement set was used to fit parameters and predict a PDI using these parameters. This PDI was compared with the original PDI0, providing dose deviations per pixel. The median of the dose deviations in the in-field region of the PDI was considered the maximum uncertainty. Second, each of the simulated parameter sets was used to predict a PDI, leading to 100 PDIs. These were all compared with the original PDI0. The median of the dose deviations in the in-field region of each PDI was calculated. The overall uncertainty was taken as the mean of these 100 median dose deviations. The proportion of dose coming from small fields was determined by changing the input RTPLAN to only contain small fields (< 4x4 cm). For each segment of the VMAT arcs, the equivalent field size was determined. If this was larger than 4x4 cm, the segment was removed, so that only the small field segments remained. This changed plan was then used to predict a PDI with only small fields. By dividing this PDI(small fields) by PDI0, for each pixel the proportion of dose that came from small fields was calculated. The median of these values in the in-field region was the overall measure of the proportion of dose from small fields. The maximum and overall uncertainty and proportion of dose from small fields were calculated for 46 VMAT arcs from 4 head-and-neck, 7 prostate and 17 lung plans. All PDIs were time-integrated over the complete VMAT arc. Results Figure 1 shows the proportion of dose from small fields and the uncertainty of the PDIs predicted for VMAT arcs using the Varian TrueBeam High Definition MLC and beam energy 6 MV. From these preliminary results it can be seen that there is a correlation for the maximum uncertainty, but not for the overall uncertainty.

booklet 7 for the IMRT treatments. When tighter spatial criteria were considered for γ analysis (delta distance=1mm), a passing rate decrease is observed: its entity is correlated to the complexity of the analysed plan. The γ analysis results on the homogeneous phantom show the reliability of the developed dosimetric protocol. Conclusion The Montecarlo algorithm implemented in the MRIdian TPS accurately calculates the dose distribution in presence of 0.35 T magnetic field at the tissue-lung interface, allowing to elaborate reliable treatment plans also in presence of such interfaces. EP-1822 Surface dose variations by protective dressings during radiotherapy M.M. Aspradakis 1 , T. Götzfried 1 , P. Logaritsch 1 , M. Bazzana 1 , T. Collen 1 1 Luzerner Kantonsspital, Institut für Radio-Onkologie, Luzern, Switzerland Purpose or Objective Silicone dressings are used in radiotherapy for the protection from further trauma to the sub-lethally damaged basal skin layer [1, 2]. The purpose of this work was to investigate the dosimetric effect of such dressings when present during treatment. Material and Methods The changes to dose in the build-up region when using Safetac dressings MepitelÒ Film and MepitelÒ Lite were investigated in megavolt photon beams (6, 10, 18 MV, 6FFF and 10FFF). Relative doses in solid water-equivalent plastic (Plastic WaterÒ DT, CIRS) were measured with the PTW Advanced Markus® plane-parallel ionization chamber and with radiochromic film (GAFCHROMIC TM EBT3, Ashland) for field sizes 4, 10 and 20 cm at a source to surface distance of 90 cm on a Varian TrueBeam linear accelerator. Ionization readings were corrected for ion recombination, polarity and perturbation effects. Films were scanned with an Epson Expression 10000XL (Epson America, Inc.) flat-bed scanner and were analysed using the FilmQA Pro TM software by Ashland. Results There is an increase in surface dose with the dressings. At the lowest beam quality (6FFF), differences in relative surface dose with and without the Mepitel Film range between 20 % to 35% for the field sizes examined and these reduce below 0.5 % at a depth of 3 mm. Mepitel Lite has a higher build-up effect, with changes in relative surface dose for 6FFF at the surface ranging between 30% to 45%. Relative dose differences decrease below 2 % at the depth of 3 mm. The bolus effect is in general smaller at the higher beam energies. The figure below summarises the evaluation of the bolus effect (or the water equivalent thickness) of these two dressings. Conclusion Mepitel Film affects less the dose to and below the skin than Mepitel Lite. The use of this during radiotherapy treatment causes negligible increase in skin dose.

References 1.

Glover, D. and V. Harmer, Radiotherapy- induced skin reactions: assessment and management. Br J Nurs, 2014. 23(4): p. S28, S30-5. Herst, P.M., et al., Prophylactic use of Mepitel Film prevents radiation-induced moist desquamation in an intra-patient randomised controlled clinical trial of 78 breast cancer patients. Radiother Oncol, 2014. 110(1): p. 137-43.

2.