Tann Scottish Meeting 2012 - Scottish+ Radiotherapy Physics ...

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Tann Scottish Meeting 2012 - Scottish+ Radiotherapy Physics ...

TPS and EPID:

Connecting the dots

Yun Inn Tan 1,2,3 , Martin Glegg 1 , Mohamed Metwaly 1 , Alex Elliott 2

1 Radiotherapy Physics, Department of Clinical Physics & Bioengineering

2 University of Glasgow

3 National University Cancer Institute, Singapore


Aim

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 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 Commissioning

• Position solid water phantom at isocentre

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

• Reference beam: 6MV, 100MU, SAD 100cm, FS20cm 2

• Image captured with Elekta and Varian EPID

M0 M


TPS Commissioning

• Create virtual phantom in Eclipse and Xio

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

• Reference beam: 6MV, 100MU, SAD 100cm, FS20cm 2

• Export dose planes M0 and M

T

SAD100cm

M0

Iso

M


Quadratic 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 TPS

EPID TPS

Common parameter T, Physical Thickness


(M/M0) as a function of T

EPID Physical Thickness (T)

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


Predict MEPID EPID

(M/M0)

EPID

(M/M0)

EPID

(M/M0)

EPID

(M/M0)

EPID

(M/M0) EPID for an

object

EPL TPS matrix

of an object

calculated from

TPS M and M0

M0

M

9


Predict MEPID EPID

(M/M0)

(M/M0)

EPID

EPID

MEPID (OBJECT) = * M0EPID (M/M0)

(M/M0)

•Compare M EPID (OBJECT) with EPID measured to verify

accuracy of delivery

EPID

EPID


(M/M0) as a function of T

TPS Physical Thickness (T)

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


Predict M TPS,exit

(M/M0)

TPS

(M/M0)

TPS

(M/M0)

TPS

(M/M0)

TPS

(M/M0) TPS for an

object

EPL EPID matrix

of an object

calculated from

EPID fluence

12


Predict M (dose at Exit plane)

TPS TPS

TPS

MTPS (OBJECT) = * M0TPS •Compare M TPS (OBJECT) with TPS calculated dose at exit

level to verify accuracy of delivery

(M/M0)

(M/M0)

TPS

(M/M0)

(M/M0)

TPS


Results: M EPID

Varian, Th0-35cm, Th0 35cm, FS20cm 2


Results: M EPID

Elekta, Th0-35cm, Th0 35cm, FS20cm 2


Results: M EPID

Elekta, Th5cm, Th5cm,

FS3-15cm FS3

15cm 2


Conclusion

• 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

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