Tomotherapy Patient QA using

OmniPro-I’mRT MatriXX- the Marshfield experience

Sugata Tripathi PhDMarshfield Clinic/Saint Joseph’s HospitalMarshfield, WI

(Treatment) Delivery Quality Assurance

• Goal is to verify that the measured dose for a fraction matches the planned dose/fluence map.

• Performed for every Tomotherapy treatment plan.

• Delivery QA Setup & Delivery QA Analysis done in Tomo PS.

• Dose plane computed and extracted to and compared with MatriXX measurement within the OmniPro I’mRT.

OmniPro-I’mRT & MatriXX

• MatriXX – The measurement device: 1020 vented ion chamber array detector in 32 x 32 grid.

• OmniPro-I’mRT – Software that controls the MatriXX and provides analysis tools and storage of measured data.

OmniPro-I’mRT & MatriXX

• MatriXX – 1020 vented ion chamber array detector in 32 x 32 grid, active area 24.4 x 24.4 cm2. • 7.6 cm center to center distance between chambers• 4.5 mm diameter 5 mm depth: 0.08 cm3 chamber volume

• Water equivalent depth from MatriXX surface to effective point of measurement ~ 3.3 mm. MatriXX mass is 10.5 Kg.

• Multicube Phantom: Individual Plastic Water blocks of 5-cm thickness interlock to form phantoms of different sizes.

OmniPro-I’mRT & MatriXX

• Measured current is proportional to dose rate.

• 1020 non-multiplexed ADC channels digitize measured current.

• Measured data are transmitted to a PC via standard Ethernet interface. OmniPro-I’mRT interface.

DQA: Delivery QA Setup

• Delivery QA Setup requires an approved patient plan and DICOM compatible phantom image volume.

Intensity map measurements using I’mRT MatriXX

• Mij and Bij: Obtained from the MatriXX measurement

• NDW(60Co) * Kuni ij : Measured and calculated at production site.

• Kuser : The kuser factor is created on site.• Koff i,j : The Koff i,j is user defined, replaces system

calibration output. Created on site, as needed.• kpT: Measured and calculated by MatriXX system.

The pressure factor is corrected with the offset obtained when comparing the P value as measured by the system and by a reference meter.

More on Kuser

• Specific to the Selected LINAC (dose rate)

• Energy type

• Energy value

•The system calculates kuser using the entered Dose(refernce)

value and the average of the values of four middle MatriXX

chambers. The new factor is displayed in the kuser field.

Criteria for QA Pass/Fail: Gamma Method

If comparing two dose distributions, the simplest methods, e.g. comparing isodoses, or difference, may not be appropriate.

Criteria for QA Pass/Fail: Gamma Method

Misjudgment when checking the agreement of two dose distributions may occur in the following cases:

• The difference between two dose-distributions can be large in high-gradient regions, even if the isodoses are relatively close to each other. • The isodose distance between two dose distributions can be large in regions with a flat dose distributions, though the difference in dose may be quite small.

Criteria for QA Pass/Fail: Gamma Method

The gamma method combines both methods mentioned above.

The user can define dose difference-and isodose distance pass/fail criteria.

If both parameters (dose and isodose distance) are outside their pass/fail criteria, the agreement "fails" according to the gamma method.

If only one parameter is outside the defined pass/fail criteria but the other well inside, the result of the comparison can still pass the calculation.

Criteria for QA Pass/Fail: Gamma Method

General Pass/Fail Criteria used for all IMRT QA in Marshfield:

3%, 3 mm DTA, 95% pass rate.