Out-of-field Dosimetry for Secondary Cancer Studies: Past Experience and Future Needs

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)

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

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

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? Proton beams generate neutrons by interacting with the scattering systems, range modulator wheel, collimators and even the patient YES!

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

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 15 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 cm 23.5 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 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. 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