TPS and EPID: TPS and EPID: Connecting the dotsConnecting the dots

Yun Inn Tan1,2,3, Martin Glegg1, Mohamed Metwaly1, Alex Elliott2

1 Radiotherapy Physics, Department of Clinical Physics & Bioengineering 2 University of Glasgow

3 National University Cancer Institute, Singapore

AimAim

Prove of concept for a new EPID dosimetry model

•Transit method

•Generalised model

•Predict dose at EPID plane and at phantom/patient exit plane

Quadratic EquationQuadratic Equation

• (i,j) is the coordinate of pixel

• M is exit dose fluence

• M0 is entrance dose fluence

• T is thickness

• A and B are fitting coefficients

EPID CommissioningEPID Commissioning• Position solid water phantom at isocentre

(T=0,5,10,15,20,25,30,35cm)

• Reference beam: 6MV, 100MU, SAD 100cm, FS20cm2

• Image captured with Elekta and Varian EPID

M0 M

TPS CommissioningTPS Commissioning

IsoT

M0

M

SAD100cm

• Create virtual phantom in Eclipse and Xio

(T=0,5,10,15,20,25,30,35cm)

• Reference beam: 6MV, 100MU, SAD 100cm, FS20cm2

• Export dose planes M0 and M

Quadratic EquationQuadratic Equation

• (i,j) is the coordinate of pixel

• M is exit dose fluence

• M0 is entrance dose fluence

• T is thickness

• A and B are fitting coefficients

Relating EPID and TPSRelating EPID and TPS

EPID TPS

Common parameter T, Physical Thickness

(M/M0)(M/M0)EPIDEPID as a function of Tas a function of T

Physical Thickness (T)

0cm 5cm 10cm 15cm 20cm 25cm 30cm 35cm

Predict Predict MMEPIDEPID

9

EPLTPS matrix of an object calculated from TPS M and M0

(M/M0)EPID for an object

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

M0

M

MEPID (OBJECT) = * M0EPID

•Compare MEPID (OBJECT) with EPID measured to verify accuracy of delivery

Predict Predict MMEPIDEPID

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

(M/M0)EPID

(M/M0)(M/M0)TPSTPS as a function of Tas a function of T

Physical Thickness (T)

0cm 5cm 10cm 15cm 20cm 25cm 30cm 35cm

Predict MPredict MTPS,exitTPS,exit

12

EPLEPID matrix of an object calculated from EPID fluence

(M/M0)TPS for an object

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

Predict MPredict MTPSTPS (dose at Exit plane)(dose at Exit plane)

MTPS (OBJECT) = * M0TPS

•Compare MTPS (OBJECT) with TPS calculated dose at exit level to verify accuracy of delivery

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

(M/M0)TPS

Results: MResults: MEPIDEPID

Varian, Varian, Th0Th0--35cm, FS20cm35cm, FS20cm22

Results: MResults: MEPIDEPID

Elekta, Th0Elekta, Th0--35cm, FS20cm35cm, FS20cm22

Results: MResults: MEPIDEPID

Elekta, Elekta, Th5cmTh5cm, , FS3FS3--15cm15cm22

ConclusionConclusion

• Proved that a newly proposed method of EPID

dosimetry is valid

• Excellent results for homogeneous phantoms with non-modulated beams

• Further validation work on non-homogeneous

objects with modulated beams

Thank you Thank you