1. Task Group 158
Measurement and Calculation of Doses Outside the Treatment Volume from External-beam Radiation Therapy Treatment
Eric E Klein, Ph.D

2. Task Group Members
Stephen F. Kry, co-chair MD Anderson Cancer Center, Houston, TX
Bryan Bednarz, co-chair, University of Wisconsin, Madison, WI
Rebecca M. Howell, MD Anderson Cancer Center, Houston, TX
Larry Dauer, Memorial Sloan-Kettering Cancer Center, New York NY
David Followill, MD Anderson Cancer Center, Houston, TX
Eric Klein, Brown University, Providence, RI
Harald Paganetti, Massachusetts General Hospital and Harvard Medical School; Boston, MA
Brian Wang, University of Louisville, Louisville, KY
Cheng-Shie Wuu, Columbia University, New York, NY
X. George Xu, Rensselaer Polytechnic Institute, Troy NY

3. Status of Report
Measurement and Calculation of Doses Outside the Treatment Volume from External-beam Radiation Therapy Treatment
Med Phys. 2017 Oct;44(10):e391-e429

4. Scope of Report
This report aims to address the following charges as they pertain to non-target radiation:
Highlight major concerns
Provide a rough estimate of doses associated with different treatment approaches in clinical practice
Discuss the uses of dosimeters and phantoms for measuring photon, electron, and neutron exposures
Discuss the use of calculation techniques (including Monte Carlo) for dosimetric evaluations
Highlight techniques that may be considered for reducing non-target doses
Discuss dose reporting
Make recommendations for clinical and research practice

5. Introduction to TG-158 Non-target dose? Out-of-field dose?
Dose outside the PTV (no benefit to patient)
Out-of-field dose?
Dose outside any primary field border
TG-158 addresses non-target dose, but primarily in a low-dose context
In-field non-target dose
Out of field non-target dose
PTV: Target dose
Out of field non-target dose

6. Why do we care? Low doses of radiation can be bad Second cancers
Cardiac toxicity
Fetal damage
Implantable electronic devices
Cataracts
Skin dose (unique considerations not addressed in this report – see TG-176)

7. Where does the dose come from?
Patient scatter
Collimator Scatter
Head leakage
Neutrons
Different properties than in-field radiation

8. So how much dose is there?
Simple square fields
Use TG-36
1995
What about newer
techniques?

9. Chapter 3 of report
IMRT; similar for Tomo, VMAT, FFF, SBRT, electron, proton, brachytherapy, imaging.

10. Caution These doses provide a range that is likely to be encountered
There can be a lot of variability between individual treatments
These are just rough guidelines
You need to determine the dose for your own case
Measurements and/or calculations

11. Techniques to Minimize Non-target Dose
Use details from the report to discuss options and strategies to reduce non-target dose:
Reducing the Target Volume (CTV and PTV)
Treatment Modalities (protons, FFF, electrons)
Treatment Energy (6X vs 18X)
Photon Wedges (hard wedges vs dynamic/universal)
MLC and Collimator Orientation
Coplanar vs Non-coplanar Beams
Jaw Tracking
Patient Shielding
Accelerator Shielding
Imaging Dose Management

12. Peripheral Dose Contributions Characteristics Room scatter <=3%
Close to field collimator contributes 20-40%
Machine leakage is greatest contributor at distances >30 cm from CAX
Characteristics
Increases with increasing field size
Minimal increase with decreasing depth
No significant change with change in SSD

13. Reducing the target volume
Reduce CTV
Hodgkin’s Lymphoma – treat only the involved nodes rather than the whole mantle
Very high impact on dose reduction
Not usually a physics decision
Reduce PTV (reduce margin size)
Requires improved imaging/immobilization
Reduces high and intermediate dose volumes
Often needs additional imaging dose

14. FFF
Out of field dose reduced for both IMRT and SBRT procedures using FFF
Less head leakage, less collimator scatter
Most benefit far from the treatment field, more modulated

15. Collimator orientation
For tertiary MLC, align leaves along patient length (vs across patient) to achieve maximum shielding
Dose reduced by 6-50%
Easily implemented

16. Recommendations
Needs are different between clinical care (e.g., measuring dose to a pacemaker) and a research study to evaluate relative risks between treatment approaches.
Clinical care recommendations
Research focused recommendations
A more stringent set, particularly for neutrons

17. Clinical care rec. 1
Dose assessment should be based on needed precision to yield proper clinical management of the case.
While an overestimate of the dose may be adequate, the magnitude of the error should nevertheless be known so as to ensure proper clinical management.

18. Clinical care rec. 2
Treatment planning systems show large errors as close as 3 cm from the field edge and doses calculated by the planning system should not be relied on beyond 3 cm from the field edge (or below the 5% isodose line).
The mean organ dose can generally be trusted for cases where the majority of the organ is within the 5% isodose line.

19. Clinical care rec. 3
Photon measurements can be conducted with a wide range of dosimeters.
Physicists must be aware of the nature of the radiation field and the response of the detector so as to at least know the magnitude of the sometimes large corrections that may be necessary.

20. Clinical Care rec 4.
Because neutron measurements are prone to very large errors, TG-158 recommends that for clinical scenarios, data from the literature should be used to estimate patient doses rather than performing patient specific measurements.
Surface doses from TG-158 can be used as conservative estimates of patient dose.

21. Clinical care rec. 5 and 6
For structures of interest within the IGRT imaging field, the imaging dose should be included in the total out-of-field dose.
When indicated or possible, the non-target dose to the patient should be minimized following the approaches outlined in report.

22. Research recommendations
Additional recommendations are presented on
Neutron dosimetry
Monte Carlo methods
Dose reporting

23. Research Recommendations for Neutron Dosimetry
Optimal neutron dosimetry consists of spectra measurements in air using, for example, Bonner spheres. Other measurement systems can provide reasonable and acceptable results.
Monte Carlo is well suited for in-patient/phantom neutron dose calculations, after the model has been well validated against in-air neutron measurements.

24. Research Recommendations for Neutron Dosimetry
Neutron measurements in-patient/phantom,, require that spatial variations in the neutron energy spectrum be accounted for in the response and calibration of the detector.
Low-energy neutron detectors (e.g., standard Bonner spheres, track etch detectors, bubble detectors) should not be used in proton beams. Appropriate detectors include extended-range Bonner sphere systems, SWENDI detectors, and TEPCs.

25. Neutron Survey Instruments
Neutron Survey Measurements Rem-Meter
Detector assembly is a polyethylene cylinder, containing a BF3 proportional counter.
At high Dose rates:
Pulse pile-up and dead time occurs Only for use outside the field (≥ 1m)
Bubble detectors for Neutron head leakage Note: too much pile-up if use Rem-meter for head leakage. Note: in many states, this is not necessary measurement because state will accept manufacturer documentation of neutron head leakage.
Un-irradiated detector
Bubbles form after neutron exposure

26. Monte Carlo Simulations
For photon models, full modeling of the head is necessary beyond 15 cm from the field edge
For situations with neutrons, head shielding and major structural components must be included
Validation of the model must rely on out of field measurements (and/or neutrons), not in-field characteristics
Phantom should match patient if possible, height is the most important parameter

27. Dose Reporting
Relative merit of point dose, integral dose, effective dose, etc. for dose assessment
How to manage neutron RBE (Q, wR, ambient dose equivalent, etc.)
Effective dose should not be used when doses >2Gy are involved.
Fun technical reading.

28. Use Diode Dosimetry Silicon semiconductor diodes Real-time measurement
High sensitivity
Good spatial resolution
Small size
Simple instrumentation
No bias voltage
Ruggedness
Independence from temperature/air pressure changes

29. Or OSLD 7mm diameter x 0.2 mm thick disk
Dose is nearly proportional to OSL signal
Linear until ~ 3 Gy
Some energy dependence
Z ≈ 11.4
-Image courtesy of Landauer
Some research as to could this be done during Jursinic (2007)
-Figure from Jursinic (2007)
-Figure from Jursinic (2007)

30. Do’s and Don’ts Do design techniques to reduce exposure
Use tables from TG-158
Confer with Clinicians
Do NOT use TP data !!