the hese radiotherapy methods for pretreatment patient qa€¦ · the analysis of the vmat/imrt qa...

The HESE Radiotherapy Methods for Pretreatment Patient QA

Using Intensity Modulated Techniques

C. MarcelinoMedical Physics ExpertLenicare – Serviço de RadioterapiaHospital do Espírito Santo de Évora, E.P.E.

The HESE Radiotherapy Methods for Pretreatment Patient QALisbon, 28th September 2017

Why we need this pretreatment patient QA?

What can we use?

How we do it? What show our results?

What comes next?


Why we need this pretreatment patient QA?

http://publicdomainarchive.com


In intensity modulated therapies, like IMRT or VMAT, the segmentation of an individual treatment field leads to complex patterns of intensity distributions as well as non-trivial MU settings.

The whole treatment plan must therefore be checked before being applied to the patient.

In addition, patient specific dosimetry should be performed for each individual treatment plan.


Commissioning

Acceptance Tests

Data Acquisition

TPS Algorithm Validation

Quality Control Program

Daily Tests

Monthly Tests

Quartely Tests

Annual Tests

Treatment Protocols

Patient Setup

Planning Targets

Constraints

Objectives

Until now…… never less we still need

Patient QA

Measurement Based

Independent MU/Calculation


IMRT or VMAT delivery techniques have the potential of delivering a highly conformal dose to the target while spreading the dose to the surrounding tissue and thereby sparing organs at risk form receiving high radiation doses.2

The VMAT delivery single arc technique originally investigated by Otto, evaluated for different approaches using multiple coplanar or noncoplanarintensity-modulated arcs for complex treatment.

In VMAT the MLCs are moving dynamically while the dose rate and the gantry speed are also varied throughout the rotation of the gantry.

Potential sources of errors are imprecision of MLC controller that may result in overshooting, undershooting, or dropped segments, as well as inaccuracies of the gantry motion and monitor units MU controller.

Just to get you on the focus…

[2] Korreman S. et al. Dosimetric verification of RapidArc treatment delivery. Acta Oncologica, 2009; 48: 185 - 191


Thomas J. LoSasso, Ph.D.IMRT Delivery System QA

Memorial Sloan-Kettering Cancer CenterNew York, New York

https://medicalphysics.org/documents/IMRT_ch22.pdf


What we can use?

http://pt.freeimages.com/


Ionisation Chambers

▪ Available in different shape and sizes

▪ Dosimetric response is well understood.

▪ Absolute dose measurements

▪ Excellent spatial resolution – small size

▪ Reusable and readily available readout

▪ Versatility in placement

Measurement based for 1D…

Diodes and Mosfet

▪ Finite size

▪ Volume Averaging

▪ Suitable for homogeneous regions

▪ Angular dependency (diodes)

▪ Very dependent on calibration (diodes)

▪ Over-response for scatter (Mosfet)


Detector Arrays (Chambers or Diodes)

▪ Measured dose in several points simultaneously

▪ Long term stability and low dose rate dependence

▪ Excellent spatial resolution (<1mm)

▪ Tissue equivalence (EBT3)

▪ Very weak energy dependence (EBT3)


Films (Radiograhic Radiochromic)

▪ Low spatial resolution

▪ Detector ageing effects (Diodes)

▪ Volume averaging (similar to ion chamber)

▪ For absolute dosimetry requires proper calibration, measurement and processing

▪ Show inter- and intra-batch variation of the film sensitivity


EPID (Electronic Portal Image Device)

Primary fluence prediction

▪ High spatial Resolution

▪ Instantaneous read out

▪ No straighforward correlation with dose delivery

▪ No end-to-end test


Back projection of transit fluence

▪ High spatial Resolution

▪ Instantaneous read out

▪ Direct comparison with dose delivery

▪ End-to-end test


Gel Dosimeters

▪ 3-D information in one irradiation

▪ Energy and dose-rate independent

▪ High sensitivity and linear response.

Measurement based for 3D or 4D…

▪ Sensitive to time, preparation and temp.

▪ Relative dosimeter - require cross-calibration

▪ Cost

3D Array or Rotational Cylinder


▪ Peforms the treatment plan recalculation

▪ Act like different TPS, verifying mainly the UM calculation

▪ Capable of volumetric calculation

▪ Allow several analysis between prior treatment, independent prior treatment and really measured

Independent MU Calculation…


How do we do it?

https://pixabay.com/


▪ 2 Linear Accelerators Varian Clinac DHX, equiped with Multileaf Collimator 120 leafs

▪ 1 with the VMAT (*) capability (RapidArc®)

▪ 2 Treatment Planning Systems Varian Eclipse® EX

▪ including Analytical Anisotropic Algorithm (AAA) and Electron Monte Carlo (EMC) algorithms

▪ 1 Computer Tomography GE BrightSpeed 16

▪ Oncology Information system network ARIA® v13.5

Our department…

VMAT - Volumetric Modulated Arc Therapy

varian.com


The treatments plans for VMAT delivery…


Going on the validation process…

▪ Analyze the 3D distribution and select the most suitable 2D plans for validation:

▪ Targets doses, organs at risk evolved or dose gradients near organs at risk

▪ Validation plans are generated and recalculated on the CT-image of PTW Octavius II comparison between planned and measured doses.


Going on the validation process…

1D Ion Chamber

Absolute dosePoint Dose

Arc-by-Arc and Plan dose

2D Ion Chamber Array

Absolute Dose

729 Point Doses (Máx.)

Gamma Analysis

EBT3 Films

Absolute Dose

Relative Dose

“Thousands” of pixels

Gamma Analysis

EPID

Relative Dose

“Thousands” of pixels

Gamma Analysis


Índice Gammma ɣ▪ Compare the measured dose with the TPS calculated dose

Dose Difference

Distance to Agreement

Índice ɣ

• Dose difference suitable for low dose gradient regions• Distance-to-agreement more suitable for high gradient regions• Both gradients exist on VMAT plans• Gamma index combine both variables

Low et al, Med Phys 25 656, 1998 Depuydt et al, R&O 62(3) 309, 2002 Ju et al, Med Phys 35 879, 2008

Gamma Index γ…


Identify that regions where dose difference (D) and distance-to-agreement (d) fail simultaneously

▪ Describes a orthogonal space defined by DTA dimension and dose difference

▪ Combines both difference concepts together by a abstract metric similar to one distance

▪ Gamma index is determined by variation of r, to find the minimal value of T and is calculated for all distribution points

Gamma Index γ…


Analysis examples…

Film DosimetryRIT Sofware

2D Ion Chamber ArrayPTW Verisoft

EPID DosimetryPTW seven29


Interpolation

▪ Slice width in the planning CT

▪ Grid selection on your optimizer

▪ Grid selection on your dose algorithm

Resolution

Independent of your equipment choice be aware about interpolation and resolution…

▪ Resolution of the measuring equipment

▪ Resolution on the data analysis software

▪ Evaluation criteria


EPID

EBT2

PTW seven29


Eclipse PTW Verisoft PTW seven29

One case of our…


What show our results?

http://pt.freeimages.com/


▪ ICRU 83 without consensus about aceitability criteria for gamma index

Which criteria should we apply…

ESTRO Booklet Nº 9 (2008) A. Mans et al. Phys. Med. Biol. 61 (2016) 7221–7235


The Global statistics…


With 1D Ion Chamber…

▪ 445 Patients

▪ Absolute Dosimetry


With 2D Ion Array …▪ 1357 Patients, Absolute Dosimetry, γ criteria: 3% / 3mm (tr. 20%)

Mean = 99,46 %

SD = 1,2%

Counts = 2693


With 2D Film …▪ 1052 Patients, Absolute Dosimetry, γ criteria: 3% / 3mm (tr. 20%)

Mean = 97,7 %

SD = 2,60 %

Counts = 2110


With 2D Film …▪ 1092 Patients, Relative Dosimetry, γ criteria: 3% / 3mm (tr. 20%)

Mean = 98,7 %

SD = 2,00 %

Counts = 2116


The analysis of the VMAT/IMRT QA process performed with different methods was very satisfactory

All methods showed distinct advantages and drawbacks.

The most important drawbacks are the calibration process for film absolute dosimetry and understand the deviations causes between predicted and measured doses.

Conclusions…


What Comes next?


Improving themachine QA,

particular the MLC

ImplementingClass Solutions

(Prostate, H&N, Lung)

Implementingindependent

MU calculation

Participation atexternal audits


Thanks for your attention!

the hese radiotherapy methods for pretreatment patient qa€¦ · the analysis of the vmat/imrt qa...

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