ESTRO 36 Abstract Book

S789 ESTRO 36 2017 _______________________________________________________________________________________________

3 Liverpool School of Medicine, Department of Biostatistics, Liverpool, United Kingdom

GLAaS, %2, 2mm PDIP, %1, 1mm GLAaS, %1, 1mm

98.95

1.68

Purpose or Objective Using flattenning filter free (FFF) beams shortens the treatment time especially for stereotactic treatment techniques when the high dose rate is used. It is not possible to do quality assurance (QA) with all types of amorphous silicon (aS) detectors because of their saturation limit. In this study, verifications of volumetric modulated arc therapy (VMAT) stereotactic treatment plans were evaluated with PDIP and GLAaS algorithms by using a new unsaturated aS detector and the results were compared. Material and Methods aS1200 image detection unit (IDU) which has aS detector integrated on a Varian TrueBeam STx linac was used (Table 1). Portal Dosimetry (PD) (v.13.0.26) with PDIP algorithm (Varian Medical Systems, Palo Alto) and Epiqa (v.4.0.11) with GLAaS algorithm (Epidos s.r.o., Bratislava) QA tools were configured at SDD: 100 cm with 2400 MU/min dose rate through the detector without saturation issue to use for pre-treatment verification.10 MV FFF VMAT treatment plans (2400 Dose Rate) of 35 patients with in total 72 arcs were calculated by Anisotropic Analytical Algorithm (AAA, ver.13.0.26) in Eclipse Treatment Planning System. The verification plans were irradiated on the aS1200 imager. The evaluations for both QA tools were done with the technique of gamma analysis (GA). The GA criterias for Distance to Agreement (DTA) and Dose Difference (DD) were defined as 3%/3mm, 2%/2mm and 1%/1mm and applied for "field" (defined with jaws) and "field+1cm" areas. The results were analysed with 2 sample T-test.

97.03

2.08

<0.05

85.03

7.78

Conclusion We could able to detect more errors with the hardest criterias (DD:%1, DTA:1mm) as expected. Epiqa had better performance for detecting the errors. It could be the result of the differences in workflow. Epiqa compares the dose calculated with clinical algorithm and irradiated image, but PD compares the calculated dose with PDIP and irradiated image. PD and Epiqa can be used for stereotactic VMAT plans with aS1200 detector without saturation problem at SSD: 100cm reliably. If the speed is important for the clinics have high workload, PD could be prefered through being an internal software. EP-1492 Influence of induced accelerator’ errors on dosimetric verification result and DVH of treatment plan M. Kruszyna 1 , K. Matuszewski 1 1 Greater Poland Cancer Centre, Medical Physics Department, Poznan, Poland Purpose or Objective The commonly used gamma criteria of 3% dose difference (global method) and 3 mm distance to agreement could mask clinically relevant errors. The aim of this work was to evaluate the influence of induced accelerator’s errors on 3D gamma method results with the varies criteria and on the patient’ dose distribution (DVHs). Material and Methods In the treatment prostate plan with VMAT high- fractionated (2x7.5Gy), FFF technique the errors of dose (differences ±1%; 2%; 3%; 5% 7%, 10%), collimator angle (rotations in both directions: 0.5; 1.0; 1.5; 2.0; 2.5; 3.0) and MLC shifts were introduced. For each modified plan, the pre-treatment verification plan was created and measured with 2D-arrays: 729 and SRS 1000 with rotational phantom Octavius® 4D and Verisoft 6.1 software with DVH option (PTW, Freiburg, Germany). Measured (with errors) and calculated (reference plan) dose distributions were analyzed with 3D gamma evaluation method for various tolerance parameters DTA [mm] and DD [%] 1.0; 1.5; 2.0; 2.5; 3.0, by global and local dose methods with a 5% threshold. To detect errors, the achieved score should be less than the assumed tolerance of 95%. Additional the DVHs from error-induced and reference plan were analyzed for CTV D 50 , D 98, D 2 , and D 25 , D 50 for OARs. Results For 12 error-induced plan with dose discrepancies, proper detection for 729 and SRS 1000 were obtained as follows: 3/12 and 6/12 (G3%/3mm); 8/12 and 6/12 (L3%/3mm); 8/12 and 7/12 (G2%/2mm); 8/12 and 8/12 (L2%/2mm). The rotations of collimator were detected >3° for 729 and >2° for SRS 1000. The MLC errors were discovered for plans with 1 leaf (MLC1) and 1 pair of leaves (MLC2) blocked, for all leaves shifted about 0.05cm (MLC3) misalignment weren’t indicated so obvious. The clinical relevance of plan with MLC errors and chosen discrepancies for collimator rotation (3°) and dose differences (+5%) were presented in the table 1.

Table 1. Specifications of aS1200 IDU Maximum active field size 43cm x 43cm Maximum readout speed 20 frame/second Matrix 1280 x1280 Resolution 0.0336 cm Results

Epiqa had dramatically low GA (gamma<1.0) results; less than 95% with the criterias DD:%1, DTA:1mm for field (Average: 85,03, SD:±7,8) and field+1cm (Average: 93,30, SD: ±2,4) comparison areas. There is no significant difference (p>0,05) between PD and Epiqa for GA results of DD:%3, DTA:3mm criterias. Table 2. Average and standard deviation (SD) results of each method Evaluated field Method, DD, DTA Average value Standard deviation p value

Field+1cm PDIP, %3, 3mm

99.95

0.14

>0.06

GLAaS, %3, 3mm PDIP, %2, 2mm GLAaS, %2, 2mm PDIP, %1, 1mm GLAaS, %1, 1mm PDIP, %3, 3mm GLAaS, %3, 3mm PDIP, %2, 2mm

99.98

0.05

99.87

0.18

<0.05

99.72

0.30

99.14

0.75

<0.05

93.30

2.44

Field

99.92

0.16

>0.707

99.90

0.31

99.59

0.50

<0.05