1. Out-of-field Dosimetry for Secondary Cancer Studies: Past Experience and Future Needs
David Followill, Ph.D.
Radiological Physics Center
U. T. M. D. Anderson Cancer Center

2. Introduction As we have all known for a long time:
Patients undergoing radiation therapy are exposed to secondary radiation (radiation out of the treatment field).
Secondary radiation is composed of photons, and at high treatment energies (above 8 MV), neutrons, which are produced in the accelerator head.

3. Introduction - Photons
Secondary photon radiation composed of scatter and leakage.
Scatter from within patient and off of collimators is dominant source near the treatment field.
Leakage through the accelerator head is the dominant source away from the treatment field.

4. Introduction - Neutrons
Neutrons are produced primarily by photons striking the primary collimator, jaws, and target.
Neutrons are important because of their high RBE. ????????

5. Why all this Concern?
There are now new treatment techniques and devices being used:
Proton machines
Tomotherapy units
CyberKnife units
IMRT delivery
The new treatment techniques and/or devices are designed to deliver high dose gradients such that the target gets a high dose and the surrounding normal tissues get a lower dose.
The great dose distributions sometimes come at a cost!

6. Why all this Concern?
Amount of secondary radiation is a function of the amount of beam-on time.
Some IMRT treatments may require up to 4 times as many MU’s to deliver as conventional treatments.
For deep treatment sites, low energy treatments typically require more MU’s than high energy treatments.
High energy x-rays and protons produce neutrons
Bottom Line: More MU’s mean more secondary radiation.

7. Why are we Concerned? LNT – BEIR VII
Linear Exponential (Gray 1965, Schneider et al. 2005)
model suggested from human, animal, and in vitro data
Linear Plateau (Ron 1998)
derived from human epidemiological studies of radiation-induced breast, bladder, and stomach cancers
Hall 2006

8. This is what started it ALL
Likelihood of Secondary Fatal Malignancy (%)
Calculated Risk estimates
Followill et al (1997)

9. Where are we Concerned?
Secondary radiation in the “Patient Plane”!!

10. Let’s First Worry about Photons
Early measurements – early 80’s
Ion chambers in large water phantoms
Large volume ion chambers (0.3 – 30 cc)
Scanning tanks

11. More Measurements
Phantoms began to more closely approximate actual patient geometry
Using cylindrical ion chambers

12. More Measurements Solid geometric phantoms also used
Using TLD, diodes and 0.6 cm3 ion chambers
Mutic et al (1998)

13. More Measurements Solid geometric phantoms also used
Using cylindrical small volume ion chambers
Klein et al. (2006)

14. Most Recent Measurements
Anthropomorphic Rando phantom with TLD -100 and 700 depending on x-ray energy at 10 specific organ sites.
3 TLD at each location.
Kry et al (2005)

15. Adult Procedure
Adult Prostate Treatment with TomoTherapy and CyberKnife units
Same prescription for all treatment devices
Common TLD placement in phantom organ locations (2 TLDs)
Starting with the adult prostate treatment..
2 TLD at each organ location
Pinnacle used for IMRT plans, HI-ART TPS was used to develop the tomo plans
Tried to limit variables to come up with the most comparable tx plans:
-Same Rx
-Similar dose objectives
-Same CT and contours

16. Pediatric Procedures
Pediatric TomoTherapy Cranio-Spinal Irradiation (CSI) and CyberKnife GBM treatment
Same prescription for
3D and Tomotherapy treatment plans
IMRT and CyberKnife treatment plans
TLD and EBT film placement in pediatric phantom
Organ doses from TLD-100
EBT film validation of TPS calculations
Because of helical delivery, many organs in primary beam -> need whole organ average -> EBT film used to verify TPS so that I could use the TPS’s DVH tool later for whole organ average
Pinnacle used for 3D
HI-ART TPS used for tomo

17. Photon Measurement Cautions
Biggest issue: low doses = very low rdgs.
Increase in uncertainty of measurements
Long exposure times
Need for multiple rdgs. at each point.
Rdg. location (air vs. phantom) or at what depth?
Point vs. volume measurements.
Neutron component for high X-ray energies

18. Photon Dose Equivalent as a percent of dose at dmax vs. Distance

19. Neutron Measurements Neutron fluence measured with gold foils.
197Au(n,g)198Au
Count the g,b emissions of the foils, convert to neutron fluence by NIST traceable conversion factor:
Gold foils detect thermal neutrons.
Bare gold foils measured the thermal neutron fluence.
Fast neutrons are thermalized by moderators. Gold foils placed in moderators thereby measure the fast neutron fluence.

20. Neutron Measurements
Determining neutron dose equiv. comprises several steps
Obtain NIST traceable calibration
Measure neutron fluence
Calculate neutron dose equivalent at dmax
Calculate neutron dose equivalent at depth

21. Neutron Measurements
Bonner sphere system to measure the fluence from which the neutron spectrum is deconvolved.
Howell et al (2006)

22. Neutron Measurements
Bubble detectors or neutron meters

23. Neutron Contribution (%) to Secondary Dose
Data from S. Kry

24. Neutron Fluence Fast neutron fluence measured on CAX and out of field.
Fast neutron fluence out of field varied by less than the uncertainty in the dosimeter. Fast neutron fluence assumed constant out of field.
For each distance from central axis, the neutron fluence was broken down into 12 components to account for energy and geometry.
Neutron fluence was examined at the same 10 points as where the photon dose was measured.

25. Neutron Measurement Cautions
Gold foil activation – not for everyone.
NIST traceability
Still the “gold” standard
Low doses = very low rdgs.
Increase in uncertainty of measurements
Long exposure times
Need for multiple rdgs. Along patient plane.
Measurement variability among the different neutron dosimeters
Difficulty measuring the neutron dose at depth in a patient

26. Dose Equivalent per Complete Prostate Treatment (photon and neutron)
Data from Kry et al, S. Lazar, M. Bellon

27. Risk (%) of Secondary Cancer per Complete Prostate Treatment (photon and neutron)
Data from Kry et al, S. Lazar, M. Bellon

28. Dose Equivalent to Edge of Stomach

29. Dose Equivalent and Risk (%) per Complete Pediatric GBM Treatment
New optimization and tuning tools as well dose homogeneity

30. Now let’s include Proton Treatments
Seems that every other day there is a new proton facility being built

31. Is there any Secondary Radiation to worry about?
YES!
Proton beams generate neutrons by interacting with the scattering systems, range modulator wheel, collimators and even the patient

32. How significant can this be?
Slide coutesy of J Fontenot
Hall (2006)

33. Neutron Measurements Measured and calculated data is sparse
Results are not consistent
Measurement techniques are not consistent
Many different factors affect neutron production
Machine type (synchrotron vs cyclotron)
Proton energy
Range modulation
Field size
Lateral scattering technique

34. Bonner Sphere Extension
The BSS and BSE response function from thermal to MeV is verified and corrected using AmBe & Cf-252 source at Georgia Tech.
The response function from 15 MeV up to 800 MeV is corrected using the 800 MeV neutron beam at LANSCE
Slide courtesy of Rebecca Howell

35. Experimental Setup
Head
Hip
23.5 cm
23 cm
Shoulder

36. Neutron Measurement Cautions
Neutron energies much higher than observed around electron accelerators
Is your neutron calibration technique calibrated appropriately for these “neutrons”?
Long exposure times
Moderators used on electron accelerators are not adequate.

37. Comparison of the doses in the pediatric CranioSpinal case treated by 3D-conventional, tomotherapy, and proton therapy
Organ site
3D 6 MV (cGy)
TomoTherapy (cGy)
Proton ( MeV) (cSv)
Thyroid
2797 (12)
362 (12) 22
Lt. Breast Bud
152 (9)
437 (6)
Heart center
2957 (41)
865 (16)
21.8
Heart edge
2345 (18)
438 (16)
Lt. Lung center
226 (13)
907 (43)
Lt. Lung Edge
242 (33)
446 (9)
Liver Center
2583 (18)
1107 (124)
Liver edge
217 (14)
545 (16)
Lt. Kidney
221 (6)
748 (83)
Bladder
195 (11)
77 (2)
17.7
Pelvic
86 (4)
528 (30)
Lt. Ovary
322 (60)
135 (22)
Data from S. Lazar and Z. Wang, MDACC

38. Summary
No consensus as to the best measurement technique. Calculations will play a larger role in the future.
Increases in the secondary dose is highly dependent on the number of MUs and photon energy.
Measurement of the secondary dose requires an established NIST traceable technique and characterization of the dosimeters at the appropriate energies.
New treatment planning software with better optimization routines have reduced the number of MUs per treatment reducing the secondary doses.
Neutron dose may play less of a role than previously thought.

39. I believe this risk to be small regardless of the treatment technique
Take Home Message
Measurement techniques are maturing.
Patient models and dose calculations are more sophisticated and accurate.
Biggest uncertainty is the RISK estimate
There are many factors to be considered in the treatment of a patient and the risk of a secondary cancer is only one of many.
I believe this risk to be small regardless of the treatment technique