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TG-142: Quality Assurance of Medical Accelerators

Presentation Introducing TG-142, comparison with TG-40 and discussion questions

James Kerns

on 16 February 2011

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Transcript of TG-142: Quality Assurance of Medical Accelerators

Quality Assurance
of Medical Accelerators James Kerns
Mentor: Mohammed Salehpour
2/16/11 Purpose Recommendations Discussion Replace Table II of TG-40 New technologies "The recommendations of this task group are not intended to be used as regulations. [They] are guidelines for QMPs to use and appropriately interpret for their individual institution and clinical setting." Quality Assurance Keeping the patient safe

ICRU reccomendation of delivering +/- 5% of prescribed dose with spatial uncertainty of +/- 5mm.

Unexpected changes in machine performance from malfunction, failure, or replacement. Responsibility Contributors to machine QA: Therapists, Engineers, Physicists (Clinical, Service Chief, Deputy Chief)

"The overall responsibility for a linear accelerator QA program be assigned to one individual: The qualified medical physicist (QMP)." MLC, IMRT

asymmetric jaws

dynamic wedges


CBCT, static kV

respitory gating

SRS, SBRT Daily TG-40 Recommendations Thanks: Mohammed Salehpour

Jennifer Johnson

Rajat Kudchadker Action Levels The machine fails, now what? If a parameter exceeds baseline or tabulated values, action is required.

"These actions should be set by the MP in terms of the level of action (inspection, scheduled, immediate stoppage). The actions should be well known by all personnel involved in the QA process." Level 1: Inspection Action
Measurement that suddenly changes, but is not out of tolerance (personnel, setup, maintenance)
Treatments continue, cause investigated during routine QA

Level 2: Scheduled Action
Consecutive measurements at or near tolerance level
Treatment may continue, but mitigation of the cause should be scheduled within one to two working days

Level 3: Immediate Action
Excessive dosimetry error, non-functional safety mechanisms
Treatment should be stopped immediately until problem is resolved Normally performed by therapists

"...who should be trained by a quaified medical physicist with a well defined policy and procedure to follow if any of the tests are found to be out of tolerance."

Equipment should be easy to setup, robust, and reproducible

All tests are to be documented and reviewed by the QMP at least once a month. Always performed by QMP, with involvment of other QA team members

Measurements should be done using commissioning-quality equipment, defined in TG-106 (water tank, ion chambers)

Output should be calibrated as per TG-51 with NIST-traceable calibrated ion chamber

Upon calibration, all secondary QA devices & procedures (i.e. daily, monthly) should be cross-checked.

After completion of annual QA, a report should be generated, divided into sections including: 1) dosimetry, 2) mechanical, 3) safety, 4) imaging, 5) special devices/procedures.

Is anything missing? Differences between TG-40 & 142:
More comphrehensive tests
Tighter tolerances
Tables separated by frequency
Tolerances separated by machine type (non-IMRT, IMRT, SRS/SBRT)
Added accessory tolerances Monthly Performed by QMP or individuals supervised by QMP

Although some tests overlap with daily/annual, equipment used should differ as an independent check of QA devices. However, "the full extent of monthly independence from the daily measurements is decided upon by the QMP." Annual Wedges Baseline What is it?

"Note that the tolerance value is not absolute in that should not be interpreted as a comparison to the machine specification; instead it is a tolerance value from the baseline"

Is it accurate?

Is there any other method of evaluation? MLC Imaging So what's the difference? Accessories Summary of Recommendations 1) Develop QA team
Policies, Procedures, Roles, Responsibility, Equipment, Result Documentation

2) Develop institution-specific baselines & absolute reference values for all QA procedures
Ensure machine performance, determine dose deviations from TPS

3) QMP leads the QA team
Education of other members, set Action Levels & procedures

4) Daily QA is done by therapist
Use easy-to-use equipment, document all tests, checked by physicist once a month

5) Monthly QA is done with QMP present
Independent equipment, validate daily equipment

6) Annual QA is done by QMP with other members
Most extensive machine tests, possible city or state regulations

7) Start-to-finish check when new procedures introduced
Run sample patient, perform point dose measurements, ensure consistency

8) Machine output performed following TG-51
NIST-traceable chamber, cross-check daily & monthly systems

Lastly, generate an annual report of the tests with significant findings, including 1) dosimetry, 2) mechanical, 3) safety, 4) imaging, 5) special devices/procedures The physicist does a monthly QA test QA aspects
Safety Frequency
Annual TG-40 Table II Case 1:
Performing Annual QA

For the first 3 years coincidence of radiation & mechanical isocenters was 1.2mm. Last year it was 1.7mm.

This year the coincidence is 2.1mm.

Is this acceptable?
TG-40: 2mm
TG-142: +/-2mm from baseline Case 2:
Performing Monthly QA

105mm ODI is showing 1.2mm further out than MDI.

Is this acceptable? Responsibility of the linac rests on the QMP

Other QA members are trained or advised by the QMP

Daily, monthly, & annual QA measurements should be independent & adjusted at time of annual

Tests should be simple, rapid, and reproducible
Measurement systems should be repeatable such
that two standard deviations for 3 consecutive
measurements be within the tolerance value

"The recommendations of this task group are not intended to be used as regulations. [They] are guidelines for QMPs to use and appropriately interpret for their individual institution and clinical setting."

What would a prudent MP do, considering time, efficiency, & equipment? Things to Remember Newer, better machines & devices "[T]he quality of linear accelerators in terms of accuracy and precision has improved in recent years, allowing for procedures such as SRS, SBRT, and IMRT."

"The expansion of tests is also justifiable due to the fact that since TG-40 and post-IMRT the selection of available QA tools makes annual testing less burdensome; thes tools range from 3D water scanning tanks to large area detector arrays."

"[T]he QA program should be flexible enough to take into account quality, costs, equipment condition, available test equipment, and institutional needs." TG-40 2% TG-40 2% "Some cause happiness wherever they go, others whenever they go." -Oscar Wilde
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