Neutron dose evaluation in radiotherapy Francesco d Errico University of Pisa, Italy Yale University, USA
Radiation therapy with a linear accelerator (LINAC)
Photoneutron production in accelerator head Photoneutrons produced by interaction of photon beam with accelerator components Produced mainly in the target, primary collimator, flattener and jaws/collimators Typical materials are copper,tungsten, gold, lead and iron Neutron production in electron mode is lower than in photon mode Direct production of neutrons by electrons is at least 2 orders of magnitude lower Lower electron current Nisy E. Ipe, 2007 AAPM Summer School
Photoneutron production in accelerator head Photon must have energy greater than binding energy of nucleus in atom S n =Separation Energy Neutron production in primary laminated barrier Lead has lower S n than Iron Iron Pb-207 (22.1%): S n =6.74 MeV (NCRP 79) Pb-208 (52.4%) : S n = 7.37 MeV Fe-57 (2.1%): S n = 7.65 MeV Fe-56 (91.7%): S n =11.19 MeV Lead has a higher neutron yield than iron Steel is a better choice for reducing neutron production Nisy E. Ipe, 2007 AAPM Summer School
Schematic of Varian accelerator head
Photoneutron production in accelerator head
Photoneutron production Photoneutron spectrum from accelerator head resembles a fission spectrum Spectrum changes after penetration through head shielding Concrete room scattered neutrons will further soften the spectrum Spectrum outside the concrete shielding resembles that of a heavily shielded fission spectrum Nisy E. Ipe, 2007 AAPM Summer School
Photoneutron production Two Processes Direct Emission Average energy of direct neutrons is ~ few MeV Spectra peak at energies > 2 MeV Have a sin 2 J angular distribution, therefore forward peaked Contributes about 10 to 20% of neutron yield for bremsstrahlung with upper energies of 15 to 30 MV Nisy E. Ipe, 2007 AAPM Summer School
Photoneutron production Two Processes Evaporation Neutrons Dominant process in heavy nuclei Emitted isotropically Spectral distribution is independent of photon energy for energies that are a few MeV above neutron production threshold Average energy is ~ 1-2 MeV Spectra peak at ~ 200-700 kev Nisy E. Ipe, 2007 AAPM Summer School
Recent treatment modalities IMRT SmartBeam IMRT "paints" a dose to the tumor with pinpoint precision, while sparing healthy normal tissue. Minimizes hot spots Improves target inhomogeneity Provides detailed dose painting to the target Sculpts dose around critical structures more effectively Allows treatments to occur in conventional 10-15 minute time slots Dose resolution, with up to 500 segments per field Spatial resolution of 2.5-5mm
Recent treatment modalities VMAT RapidArc RapidArc uses a unique algorithm that provides excellent treatment delivery control. Its single gantry rotation speeds treatment delivery so clinicians can develop treatments that take one-half to one-eighth the time of conventional IMRT treatments just two minutes in many cases. A RapidArc treatment may also result in less radiation leakage and scatter, so peripheral tissues receive a lower overall dose.
Recent treatment modalities - Tomotherapy
Photoneutrons: NCI-WG 2001 recommendations Total-body dose for IMRT patients is higher, generally increasing with the number of monitor units used for treatment. The potential for complications related to this increased dose should be recognized and considered. Increased neutron production for high-energy machines used for IMRT should be considered. Increased workload values for IMRT (may be 2 5 times larger than in conventional therapy) should be considered for the leakage/transmission part of the treatment room shielding. Because IMRT is inherently less efficient (per MU) than conventional RT, vendors should consider the use of more internal shielding in the design of future IMRT machines.
Neutron dosimetry Conversion coefficients Neutrons particle fluence to ambient or personal dose equivalent influence of phantom (multiple scattering) and quality factor
Neutron radiation protection dosimetry Utilization of physical phenomena resembling dose (equivalent) deposition in tissue. Measurements of LET spectra and convolution over Q(L). Design of systems mimicking the fluence to dose equivalent conversion coefficient
Neutron detection by thermoluminescence Neutrons are particles without charge, difficult to detect TL induced by secondary particles from nuclear reactions: (n, ) (n,p) (n,d) etc TLD for neutrons have high concentration of isotopes with high cross section to neutrons in LiF ~ 7,4% of Li is 6 Li n Radiation LiF 6 Li(n, ) 3 H
Some neutron capture cross sections 3 He(n,p) 3 H 10 B(n, ) 7 Li 6 Li(n, ) 3 H
Albedo-based neutron dosimeters
TL albedo neutron dosimeter response
Plastic Nuclear Track Detectors Particles of ionizing radiation cause molecular size damage in solid material The damage can be enlarged to microscopic range by chemical etching
How CR-39 PNTDs Work
How CR-39 PNTDs Work
How CR-39 PNTDs Work
Atomic force microscopy of latent tracks Latent tracks 70-100 nm and must be enlarged to 5-30 µm to be visible with optical microscopy Tracks are enlarged by chemical or electrochemical etching
Atomic force microscopy of latent tracks
How CR-39 PNTDs Work
Result of Etching
Typical microscopic view-field with developed tracks Overlapping Tracks Identified Tracks Track density is proportional to the exposure value [dose] With calibration the [equivalent] dose can be deducted
Neutron response mechanism In practice, converter layers are utilized: Neutrons are non ionizing radiation, but CR-39 (C 12 H 18 O 7 ) is sensitivity to fast neutron recois Neutrons interact with H producing recoil protons Recoil protons create latent tracks in the CR-39 material - Converters modify (improve) the energy dependence of the reponse - Converters also protect the CR-39 chip against alpha particles from environmental radon
Neutron energy dependent response Good energy dependence of response But, relatively low sensitivity
Sensitization by CO 2 pretreatment
Superheated emulsions Fluorocarbon droplets kept in a steady superheated state by emulsification in compliant gels. Bubble nucleation triggered by neutrons above selectable threshold energies. Can be totally insensitive to photons.
Some current technologies
Neutron energy dependence of emulsions Composition closely tissue equivalent Insensitive to photons Response resembling kerma equivalent coefficient The emulsions can measure dose equivalent in phantom without disrupting charged particle equilibrium
Isocenter neutron doses
Neutron doses from various x-ray beams 18 MV x-rays 15 MV x-rays 10 MV x-rays
In vivo photoneutron measurements
SDD fluence response vs quality-factor weighted kerma coefficient and photoneutron spectrum
Bubble counting by scattered light Photo diodes LEDs Instant read out Rate insentive Position sensitive
Study of a prostate treatment
Characteristics of various phantoms for out-of-field measurements
Clinical simulations of prostate radiotherapy BOMAB = BOttle MAnnikin ABsorber phantom, industry standard (ANSI 1995) for calibrating whole body counting systems
Irradiation channels ( pipes )
BOMAB CT scan and simulated organs Bladder Rectum Transverse plane section view Prostate PTV Boost volume Sagittal Sagittal plane section view plane section view 5 cm 5 cm 5 cm 5 cm
Organs of interest: bladder, prostate, rectum
Organs of interest: bladder, prostate, rectum
Organs of interest: bladder, prostate, rectum
Treatment modalities in Pisa and Krakow Pisa Varian Clinac 2100 C MU/ 2 Gy Modalities Krakow Varian Clinac 2300 CD 6 MV single 10x10 cm 2 field (ref) 251 6 MV single 10x10 cm 2 field (ref) 15 MV single 10x10 cm 2 Used for reference field (ref) 218 18 MV single 10x10 cm 2 field (ref) 6 MV 4-field MLC 15 MV 5-field MLC 6 MV IMRT 266 432 18 MV 4-field MLC 6 MV IMRT 18 MV IMRT 6 MV VMAT (RapidArc) 481 6 MV Tomotherapy MU/ 2 Gy 240 199 277 218 466 350
Treatment modalities 5 fields with MCL
Treatment modalities IMRT
Treatment modalities - VMAT
Dose profiles from different treatment plans
LEG S Dosimeter in-phantom placement BTI sensitive detector length: 7.0 cm SDD sensitive detector length: 2.5 cm Measurement points center of sensitive length D = 6.8 D = 12 D = 16.8 D = 31.8 D = 46.8 D = 56.8 D = 12 D = 27 D = 42 D distance from outer flange legs side. Dimensions in cm
μsv/gy IMRT neutron dose as a function of nominal energy 1,E+03 1,E+02 1,E+01 18 MV - Krakow 6 MV - Pisa 1,E+00 0 5 10 15 20 Penetration distance along the sagittal plane (cm) 5 cm is bladder, 10 cm is prostate and 15 cm is rectum
μsv/gy MLC multiple field treatments 1,E+04 PTV Krakow 18 MV SDD PTV Pisa 15 MV SDD 1,E+03 PTV Krakow 18 MV PADC PTV Pisa 15 MV PADC 1,E+02 1,E+01 1,E+00-10 0 10 20 30 40 50 Off-axis distance (cm)
μsv/gy Reference phantom literature comparison - SDD 6,E+03 5,E+03 4,E+03 3,E+03 This work (SDD, 20 MV) This work (BDPND, 20 MV) d'errico 2000 (SDD, 18 MV) d'errico 2000 (SDD, 15 MV) Awotwi-Pratt 2007 (SDD, 15 MV) Lin 2007 (BDPND, 15 MV) This work (SDD, 12 MV) 2,E+03 1,E+03 0,E+00 0 50 100 150 200 250 Off-axis distance (mm)
TrueBeam 10 cm x 10 cm field results
Conclusions and Prospects The Pisa BOMAB proved a viable approach The selected neutron dosimetry methods appear reliable and reproducible But, measurement times are long and additional campaigns are needed for a systematic analysis Tomotherapy appears to deliver the smallest neutron doses A small neutron contamination is present already at 6 MV and warrants further investigation