Commissioning of the Delta 4 Discover An investigation of its influence on the beam quality and of its error detection capabilities Vikren Sarkar,

Transcription

1 Commissioning of the Delta 4 Discover An investigation of its influence on the beam quality and of its error detection capabilities Vikren Sarkar, Adam Paxton, Bill Salter

2 Questions we aimed to answer 2 main categories Does the device influence my treatment beam? How sensitive is the device in its error detection?

3 How does it influence my beam? What we checked Influence on PDDs/Profiles Attenuation Superficial Dose Accounting for it in the TPS End to end testing

4 Effect on PDDs and Profiles Measured on a Truebeam using 3DS water tank Measurements with and without Discover in place 10 x 10 cm 2 and 30 x 30 cm 2 fields All available photon energies 6X, 6FFF, 10X, 10FFF, 15X, 18X PDDs taken along CAX Profiles (crossbeam) taken at d max and 10 cm

5 Effect on PDDs and Profiles Across all energies: PDDs did not change by more than 0.4% (past 5 mm depth) PDD 10cm and PDD 20cm values did not change by more than 0.1% Depth of maximum dose did not change by more than 1 mm Maximum detected field size change 0.2 mm Maximum change in penumbra < 0.2 mm Maximum change in symmetry = 0.2% Maximum change in flatness = 0.1%

6 Example PDD and Profile (6X) w/o 10x10 w/o 30x30 w/ 10x10 w/ 30x30 30x30 1.5cm w/o Discover w/ Discover

7 Attenuation Measured in solid water using a Farmer chamber 90 SSD, 10 cm depth 10 x 10 cm 2 and 30 x 30 cm 2 fields Energy Average* Attenuation (%) 6X 1.3 6FFF X FFF X X 0.8 * Averaged over 2 field sizes

8 Superficial Dose Measured by delivering same plan to Delta 4 Phantom+ with and without Discover OSLDs placed on surface of phantom 89 cm SSD Plans covering several treatment sites, energy and prescriptions Bladder/Pelvis 10X, 180 cgy/fraction Head and Neck 6X, 180 cgy/fraction Prostate SBRT 10FFF, 725 cgy/fraction Spine SBRT SIB 6FFF, 2400/1600 cgy/fraction Pituitary Fossa 6X, 180 cgy/fraction Scalp 6X, 250 cgy/fraction Pelvis SIB 6X, 180/240 cgy/fraction

9 Superficial Dose

10 IMRT QA results with and without Discover Results from plans delivered to the Phantom+ as previously described.

11 Conclusions regarding beam influence The presence of the Discover does not affect the beam spectrum to any clinically significant level. The beam attenuation from the Discover is very small and can be accounted for within the TPS (through a traylike accessory). There is a very small increase in skin dose on the order of 1% from the presence of the Discover.

12 On the ability to catch mistakes 2 ways to use the Discover: By itself from the beginning We get leaf position information (with gantry angle for arc deliveries) but it is not possible to evaluate the dosimetric effect of any leaf position discrepancy In conjunction with the Phantom+ for at least one fraction In that case, fluence measured per control point by the Discover can be correlated to dose measured for the same control point in the Phantom+ so that further fluence measurements can be correlated to dose in the Phantom+ (or the patient using additional software)

13 On the ability to catch mistakes MLC leaves were moved by known amounts from a base plan and each resulting plan was measured with the Discover. 2 types of plans considered Single 3D field Single angle IMRT field with 130 control points Leaves moved in multiple geometries by 0.25 mm, 0.5 mm, 1 mm, 2 mm and 5 mm.

14 Leaf motions introduced Baseline Shifted Opened Random Shift Central Leaf Only

15 Results from single 3D beam - Gamma

16 Single 3D field Other mistakes introduced

17 Single 3D field Other mistakes introduced

18 IMRT Field - Shifts Baseline

19 IMRT Field - Shifts Baseline

20 IMRT Field Other introduced mistakes

21 Conclusions on error detection The Discover detected errors on the order of 1 mm easily. The results confirm that the Discover should be used in conjunction with the Phantom+ to take full advantage of its error detection capabilities (wrong energy, wrong jaw position etc) but, by itself, there are still several errors it can catch.

22 Further Investigation A few aspects to be systematically investigated soon: 1. Step-and-shoot vs Dynamic MLC deliveries 2. Arc deliveries and the MLC/Gantry gamma functionality 3. An investigation of how changes in gamma pass rates & leaf position correlate with patient dosimetry.

23 Thank you!

24 Results from single 3D beam - Gamma