SRS/SBRT Errors and Causes

Ryan Foster, Ph.D.Assistant Professor

Director of Clinical Medical PhysicsDepartment of Radiation OncologyUT Southwestern Medical Center

Dallas, TX

Disclosures

• I receive research funding from the Cancer Prevention and Research Institute of Texas.

Outline of Presentation

• Introduction• Summary of accidents and misadministrations• Resources and guidance• Conclusions

Learning Objectives

• To learn from previous accidents and misadministrations during SRS/SBRT

• To understand the types of errors that can occur• To understand how to prevent these errors from

happening to you!

What are the sources of errors?

WHORadiotherapy Risk Profile 2008

According to the WHO Radiotherapy Risk Profile, what are the two most common sources of actual adverse events in radiation therapy?

4%4%87%2%3% 1. Positioning/immobilization and commissioning

2. Simulation/imaging and treatment planning3. Commissioning and treatment planning4. Planning and treatment information transfer5. Commissioning and treatment information

transfer

Answer

WHORadiotherapy Risk Profile 2008

3. Commissioning and treatment planning

Two Major Categories in SRS/SBRT Accidents

• Commissioning – Small field measurements– Absolute calibration

• Treatment parameter transfer– SRS Cones– Wrong side/site treatments

SUMMARY OF INCIDENTS

Small field commissioning

Small field commissioning

This happened in France in 2007 and was reported in 2008!

Small field commissioning

No communication

How many patients in Missouri could have received the correct treatment if the incident in France had been more widely reported?

Small field measurement issues persist

According to TG101, an appropriate measurement device for SRS/SBRT small

fields would be a

62%31%6%1%0% 1. 0.6 cc Farmer chamber

2. CC13 ion chamber3. Parallel plate chamber4. 0.015 cc pinpoint ion chamber5. Stereotactic diode

Answer

5. Stereotactic diode

Wellhofer

ExradinPTW

PTW

IBA SFDPTW

Sun Nuclear

Much more detail coming in tomorrow’s presentations!

Small Field Output Factors

Das, Ding and Ahnesjo. Med Phys Vol. 35, No.1, 2008.

A 1x1 cm2 6 MV output factor measured with a 0.6cc Farmer chamber

1%6%1%0%92%1. Would be 40% smaller than the true output factor

2. Would be the true output factor3. Would be 5% smaller than the true factor4. Would be 50% larger than the true factor5. Would be 10% larger than the true factor

Answer1. Would be 40% smaller than the true output

factor.

Das, Ding and Ahnesjo. Med Phys Vol. 35, No.1, 2008.

How do you know if your data is good?Compare with Other Institutions / Machines

Followill et al. JACMP 2012 and Erratum, JACMP Vol 15, No 2, 2014

Miscalibration of SRS Linac

Independent Check of Calibration

Improper Jaw Size During SRS

In 2004, physicist told therapist to set a “40x40” for cone SRS treatment; therapist set 40x40 cm2

Some normal tissue received more dose than the target; developed “fibrosis and oeso-tracheal fistula” requiring surgery; patient died from “brutal haemorrhage” a few days after surgery

LESSONS FROM RECENT ACCIDENTS IN RADIATION THERAPY IN FRANCE, S. Derreumaux*, C. Etard, C. Huet, et al.Institut de Radioprotection et de Suˆ rete´ Nucle´aire, Direction de la Radioprotection de l’Homme, IRSN, BP 17, F-92262 Fontenay-aux-Roses Cedex, France Radiation Protection Dosimetry (2008), Vol. 131, No. 1, pp. 130–135

Improper Jaw Size During SRS

This occurred in France in 2004!

SRS Cone Left Out

State of Texas Response

Checklists!

• Appendix of Solberg et al. PRO 2011 provides excellent examples of checklists for SRS, SBRT, simulation and treatment planning

GammaKnife Misadministrations

GammaKnife Misadministrations

Analysis of GammaKnife Errors

Goetsch IJROBP Vol. 71, No. 1, Supplement, S118-S121, 2008.

Beam data acquisition for SRS / SBRT is challenging

and time consuming•Small fields•Sharp gradients•Detector position-orientation effects

•Loss of lateral electron equilibrium

•Must get this right!•Commissioning errors affect all patients treated with the device – not just a select few!

Dosimetric commissioning: Do your calculations agree with measurement?

Dosimetric commissioning: Do your calculation agree with measurement?

Patient Specific QA

What About Localization Accuracy?

Must perform end to end tests!

RPC Lung Phantom

RPC Spine Phantom

RPC H&N Phantom

Planning

R/V

Tx Unit

Are your electronic systems configured correctly?

Do all of your commissioning in clinical mode and through your R/V system

AAPM/ASTRO Resources

• AAPM Task Group 101• Target Safely – IMRT Safety White Paper• Target Safely – SBRT/SRS Safety White Paper• ASTRO – Safety is no accident – A framework

for quality radiation oncology and care

SRS/SBRT White Paper• Solberg et al. Quality and safety considerations

in stereotactic radiosurgery and stereotactic body radiation therapy. PRO 2012.

SRS/SBRT White Paper Key Points• Focus on personnel qualifications and

technology requirements• Commissioning/credentialing/QA• SRS and SBRT are SPECIALIZED procedures

and should be treated as such!

International Resources

Specific Lessons Learned from Accidents and Overexposures

• Get an independent check of machine calibration and commissioning

• Perform end to end commissioning tests, including the R&V system

• Use an independent method to check MU/time calculations

• Evaluate changes in TPS, R&V and other software thoroughly before implementation

• Carefully plan your program

Summary | Conclusion• SRS and SBRT are ABLATIVE treatments!• Care must be taken during commissioning• Plan your program carefully!

Thank you!