A Comparative Study of AAA Vs PBC Treatment Planning

Algorithms

Rahim Gohar

Medical Physicist

Radiation Oncology,

Dr Ziauddin University Hospital

Study objectives

Introduction to PBC and AAA

Inhomogeneity

comparison

Non-clinical Water phantom

Clinical cases

Head and neck

Importance of body editing

Effects on dose distribution

Limitations of PBC

Validation of AAA and PBC using measurement

OSL, Film Dosimetry, point dose measurement

Dose Calculation Algorithms Broad-beam, Semi-empirical or

Correction-based algorithms MU Calc. (AKUH)

Model- or kernel-basedalgorithms; also referred to as“Fluence-to-dose modeling”

PBC, AAA

Direct Monte Carlo Near future, I hope

Algorithm

“A procedure for solving a mathematical problem in

a finite number of steps that frequently involves repetition of an operation”

OR

“step-by-step procedure for solving a problem or accomplishing some end especially by a computer”

Convolution

A coil or twist, esp. one of many

A thing that is complex and difficult to follow

“Mathematical operation on two functions f and g, producing a third function that is typically viewed as a modified version of one of the original functions”

Where, in the planning system, do we find algorithms?

MU calculations

Isodose distributions

DVH generation

IMRT optimization

DRR generation

Brachytherapy calculations

PBC Dose calculation model

pencil-beam kernels

Convolve

AAA The AAA is a 3D pencil-beam convolution

superposition algorithm

it is modeling for

primary photons, scattered

extra focal photons, and electrons

scattered from the beam-limiting

devices.

Inhomogeneity Correction

Differ in density than H2O This results in

Dose distribution different from that in water Change in absorbed dose Changes in electron Fluence Put limits on location of plan normalization

inhomogeneity correction

Med. Phys., 27, 1266-1274 (2000)

Solid squares: PDD in homogeneous medium

Solid triangles: Calculated using equivalent path length

Solid Line: Measured PDD

inhomogeneity correction

Med. Phys., 27, 1266-1274 (2000)

Solid squares: PDD in homogeneous medium

Solid triangles: Calculated using equivalent path length

Solid Line: Measured PDD

Comparison of PBC and AAA

Experimental validation of AAA and PBC

Comparison of PBC and AAA

PBC over doses, in lower dense medium?????

PBC limitation

Comparison of PBC and AAA

Non-Clinical cases Beam profile using PBC in heterogeneous medium(air)

Non-Clinical cases Beam profile using AAA in heterogeneous medium(air)

Non-Clinical cases Beam profile using PBC in heterogeneous medium(bone)

Non-Clinical cases Beam profile using AAA in heterogeneous medium(bone)

Wedge profile comparison PBC is not reliable for planning a treatment

when using a 60-degree EDW for large field sizes

Importance of wedge profile(EDW)

Measured VS calculated

Wedge profile comparison(From Eclipse(PBC))

Wedge profile comparison(From Eclipse(AAA)

Clinical Cases(PBC)

MR # (246-66-61) Slice#0(with body editing) (without body editing)

Huge difference in iso dose distribution i.e. PBC is not able to model the beamAt interface where there is gradient in terms of tissue density(here bolus and air)

Clinical Cases(PBC)

Maxi. Dose=4918 Maxi. Dose=4801

(with body editing) (without body editing)

Huge difference in iso dose distribution i.e. PBC is not able to model the beamAt interface where there is gradient in terms of tissue density(here bolus and air)

Clinical Cases(PBC)

(with body editing) (without body editing)

Huge difference in iso dose distribution i.e. PBC is not able to model the beamAt interface where there is gradient in terms of tissue density(here bolus and air)

Clinical case(AAA)(with body editing) (without body editing)

Note: as the air gap is not too much so no any major differenceIn iso dose distribution as AAA is taking care of the inhomogeneityIn between bolus and body.

Clinical CasesMR # (241-16-35)

Clinical Cases

MR # (246-26-94)

Clinical Cases

MR#(239-90-25)

Testing of treatment Algorithms

Films dosimetry

Point dose measurements

Conclusion The implementation of AAA represents an

improvement for the Eclipse TPS at the level of dose accuracy Dose calculation in heterogeneous medium

Taking into account the electron contamination

PBC is very weak algorithm for dose calculation where there is sharp gradient in terms of medium density

Mask making and bolus placement are considerable clinical issues

Never place plan normalization at the region of interface

Heterogeneities pose the greatest challenge to predicting accurate dose distributions in patients

Discussion

34

For coplanar beams in transverse plane tissue

heterogeneity usually does not change abruptly

from a given slice to an adjacent slice; for non-

coplanar beams this may not be the case

Thank you for listening meThe whole credit goes to TPS staff

Questions or comments????