THE PHILOSOPHY OF TG100: WHAT IT IS AND WHAT IT IS NOT

M. Saiful Huq, Ph.D. Department of Radiation Oncology

UPMC Cancer CentersPittsburgh, Pennsylvania

Disclosure

No conflicts of interest

Learning Objectives

• Challenges with the current QA paradigm• New paradigm adopted by AAPM TG 100• Application of the new paradigm to IMRT

To understand

Current Paradigm of QM in Radiation Therapy

• Focus of current QM/QA guidelinesmeasure functional performances of radiotherapy equipments by measurable parametersset tolerances at desirable but achievable values

• QA/QC programs are generic and prescriptive• Example: TG-40 recommendations for

daily, monthly, annual QA of accelerators

The Problem

• Lack of adequate guidance for resource allocation• Lack of adequate personnel• Newer technologies require

More sophisticated testsMore resourcesPhysicists are under pressure to implement new technologies

• Lack of timely guidelines

QA of RTPS, 4D CT, radiographic, fluoroscopic, &

CBCT IGRT, image registration, fusion, US, 4D PET/CT....

Therapist

Dosimetrist

Budget cut

Director of PhysicsPhysicist

Administration

Emerging Technol-

ogies

Physics Resident Physician

Where Are The Resources And Time To Do It All?

What to Do?

• There is a need to develop a systematic QA program that balances patient safety and quality versus available resources and strikes a good balance between prescriptiveness and flexibility

TG 100• AAPM created TG 100 to update TG 40 for new modalities• Two of the original charges of TG 100 were “After the

identification of the broad classes of radiotherapy devices and procedures, develop the details of the QA program” and “create a document that will supersede and compliment TG-40”

• It soon became evident that this was making a hard situation even worse

• The TG decided to take a different approach• The new approach would be based on risk assessment

TG 100 Charge

• Identify a structured systematic QA program approach that balances patient safety and quality versus resources commonly available and strike a good balance between prescriptiveness and flexibility.

• After the identification of the hazard analysis for broad classes of radiotherapy procedures, develop the framework of the QA program

A Method for Evaluating QA Needs in Radiation Therapy

• M. Saiful Huq (Chair)• Peter B. Dunscombe• Benedick A. Fraass• John P. Gibbons• Geoffrey S. Ibbott• Paul M. Medin

TG 100 Members

• Ellen D. Yorke (Vice Chair)• Sasa Mutic• Arno Mundt• Jatinder R. Palta• Bruce R. Thomadsen• Jeffrey F. Williamson

Consultant: Frank Rath

Risk Assessment

• What is Risk?A term which frequently embodies

probability of an event occurring and severity should such an event occur

• Need to quantitate probability and severity• There are techniques for assessing risk, and TG

100 is using them.

Concepts

• Process Trees• FMEA (Failure Mode and Effects

Analysis)• Fault Trees

Process Tree

A process tree helps to understand the temporal and physical relationships between the different steps involved in the process

IMRT Process TreeDiagnosis, Staging,

History and Physical

Patient entered in

data, assigned db

keys, etc

Decision to Tx with

radiation

Scheduling for Planning Process

Immobilization and Positioning

Each Imaging Procedure (CT,

MR, PET…)

Transfer Images

Initial Treatment Planning

Directive (from MD)

RTP Anatomy

MD Review

RTP Planning

Plan Review

Plan Approval

Plan Preparation

IMRT QA

Clinical Plan Preparation

Day 1 imaging verification and

Treatment

Day n Treatment

Weekly Chart Check

Successful Treatment

Processes leading to an

IMRT treatment

Day 1 imaging verification

EPID imagingFor localization

Place patient on table

Align mold marks

Pt in mold

Align allmarks

Make AP imageSetmu

Set gantryMakeexposure Set field size

Set machineMake lateral image

Setmu

Set gantryMakeexposure

Set machineVerify images are adequate

Verify patientsetup

Register EPID andpseudoradiograph

LoadEPID

LoadPseudo-

radiograph

Determine patientShifts and rotations

Reimage if necessary

Approve patient position

a

a

Verify beam outlinesSelect beam in record & verify

Image

SetparametersVerify clearance

and achievabilityRegister beamoutline c plan

Repeat for each beam b

b

Approval to treat

Review setup images

Review beam images

Approve treatmentif good

17

Process Tree

Failure Mode and Effects Analysis (FMEA)

• FMEA looks at each process and at each step asks the questions:

What could possibly go wrong (potential failure mode)How could that happen (i.e., what are the causes that result in a failure mode) What effects would such a failure produce (potential effects of failure)

• TG 100 is performing FMEA for IMRT and HDR brachytherapy

• This is a significant undertaking• The following example will explain why that

is the case

Failure Mode and Effects Analysis (FMEA)

Process Tree

Process #1 Process #17 Process #20

Step #4Step #1 Step #j

Failure Mode #2Failure Mode #1 Failure Mode #k

Cause of Failure #6Cause of Failure #1 Cause of Failure# m

Effects of Failure #4Effects of Failure #1 Effects of Failure #n

IMRT Process Tree

IMRT Process Tree

Step Potential Failure Modes

Potential Causes

of Failure

Potential Effects of Failure

O S D RPN Comments

FMEAFor a given process:

What are O, S and D’s?

• O : Probability that a specific cause will result in a failure mode

• S: Severity of the effects resulting from a specific failure modeD: Probability that the failure mode resulting from the specific cause will go undetected

• Risk Probability Number RPN = O*S*D• For O, S and D one assigns values from 1 to 10. In industry

RPN values below 125 carry little concern; The challenge is to assign values of RPN that should be of concern in medicine

Probability that a Specific Cause will Result in a Failure Mode (O)

Qualitative Review Ranking Frequency of Occurrence

Failure is unlikely 1 1/10,000 2 2/10,000

Relatively few failures 3 5/10,000 4 1/1000 5 <0.2%

Occasional failures 6 <0.5%7 <1.0%

Repeated failures 8 <2.0%9 <5.0%

Failures are common 10 >5.0%

Severity of the Effects Resulting from a Specific Failure Mode (S)

Not noticeable, no effect on the patient or on the department

1

Inconvenience 2-3Minor dosimetric error 4

Limited toxicity (may not require medical attention) or minor under-dose

to PTV5-6

Potentially serious toxicity or injury (may require medical attention) or

major under-dose to PTV7-8

Possible serious toxicities (requires medical attention)

9

Catastrophic 10

Probability that a Failure Mode Resulting from a Specific Cause will go Undetected (D)

Detection Ability of FailureMode in %

Probability that failure mode goes undetected in %

Ranking

99.99 0.01 199.80 0.20 299.50 0.50 399.00 1.00 498.00 2.00 595.00 5.00 690.00 10.00 785.00 15.00 880.00 20.00 9

Extreme likelihood >20.00 10

Examples of FMEA

Step Potential Failure Modes

Potential Cause of Failure

Potential Effects of Failure

O S D RPN Comments

Import images into RTP system data base

Wrong patient’s images

MiscommunicationUser error

Wrong dose distributionWrong volume

3 9 5 135

Wrong imaging study (correct patient)Viz.; wrong phase of 4D CT selected for planning; wrong MR for target volume delineation

Ignorance of available imaging studiesAmbiguous labeling of image setsInadequate training MiscommunicationUser error

Wrong dose distributionWrong volume

7 8 7 392

File(s) corrupted Network problem Lost imagesWrong dose distributionWrong volume

43

39

24

24108

File probably would not open

Process: RTP Planning

Hazard

• Going through the exercise makes one wonder how we ever get a case right.

• It also takes a long time• But it helps direct resources to the greatest

hazard

An Example of Paradigms: Annual Calibration and QA

• The annual calibration takes several days to complete• If everything checks out, the effort was mostly wasted

that is it could have been spent checking things with a higher likelihood of failure

• If some problem was found, how long had it been wrong and shouldn’t it have been found earlier?

What to Do ?Risk based approach

• Identify the aspects of linac performance that are most important to patient safety

• Determine their likely failure modes

• Determine the frequency with which these functions should be checked so that a failure would be recoverable

• The less critical functions can be checked at a lower frequency: determine this frequency

• Develop a program to perform these checks accordingly

TG 100’s Task

• Generate a process tree for representative RT procedureIMRTHDR Brachytherapy

• Develop an FMEA table for the procedure and estimate O, S, D (and RPN) for each failure mode

• Provide a template or generic guidelines for particular procedures and for general QM for small clinics.

• Small clinics may want to adopt this template or modify it to suit their own experiences

Challenges Facing TG100 in Developing the New QA Paradigm

• Performing the FMEA analyses require skills gained from experience in such analysis

• Large institutions may have the resources to go through this, but a small, community hospital will not

• That being said, the greatest advantage is to analyze your situation

Next Speakers

• TG 100 Methodology applied to IMRT process: Sasa Mutic, Ph.D.

• Using risk analysis to develop QM procedures for high dose rate brachytherapy: Bruce Thomadsen, Ph.D.

• Beyond FMEA: Summary and future developments : Jeffrey Williamson, Ph.D.