PhoNeSPhoNeS

Photo Neutron SourcePhoto Neutron Sourcea novel a novel approachapproach to to BNCTBNCT with conventional radiotherapy acceleratorswith conventional radiotherapy accelerators

R.Bevilacqua b (riccardo.bevilacqua@ts.infn.it) – E.Vallazza a – F.Calligaris b – D.Fontanarosa a – G.Giannini b* – F.Liello b – F.Longo b

R.Longo b – G.Scian a – M.Severgnini b – P.Totaro a – K.Vittor a – R.Vidimari c – G.Bartesaghi d – V.Conti d – V.Mascagna d – C.Perboni d

M.Prest d – S.Agosteo e – G.Gambarini e – S.Gay e – M.A.Valente e – A.Monti f – A.Ostinelli f – A. Mozzanica g – L.Azario h – A. Fidanzio h

A. Piermattei h O.Borla i – E.Durisi i – F.Fasolo i – U.Nastasi i – E.Perosino i – A.Zanini i – L.Tomasino m

(*contact person: giannini@ts.infn.it, a INFN of Trieste, b University and INFN of Trieste, c A.O.U. “Ospedali Riuniti” of Trieste, d University Insubria

and INFN of Milan, e University and INFN of Milan, f “Sant’Anna” Hospital of Como, g INFN of Pavia, h INFN of Rome 3, i INFN of Turin, mA.P.A.T.)

PhoNeS (Photo Neutron Source) is an INFN project devoted to the optimization of the neutron production and moderation in radiotherapy linear accelerators. LinAc’s producing high energy (15 to 25 MeV) photon beams are becoming widespread. At this energy neutron photoproduction is unavoidable and the neutron dose must be controlled and reduced during normal radiotherapy. On the other hand, some kinds of tumors can be treated only with neutrons: for example extended tumors (lung, liver, pancreas, skin) or tumors located in particular positions (brain). This technique is known as BNCT (Boron Neutron Capture Therapy): the cells are given a drug containing 10B which undergoes fission after neutron capture, inducing heavy damages to the DNA of the cell itself. This poster will describe the moderator developed by PhoNeS and the results in terms of neutron flux and spectrum and photon contamination of the measurements performed on several radiotherapy accelerators.

ITALIAN “NATIONAL ITALIAN “NATIONAL

INSTITUTE FOR INSTITUTE FOR

NUCLEAR PHYSICS”NUCLEAR PHYSICS”

PHYSIC’S DEPT.PHYSIC’S DEPT.

TRIESTE UNIVERSITYTRIESTE UNIVERSITY

ITALYITALY

BNCT (BoronBNCT (Boron Neutron Capture Neutron Capture Therapy)Therapy)Oncological Radiotherapy is one of the most efficient weapons for tumors treatment. Energy released by

radiations causes the formation of highly reactive free radicals capable of breaking the DNA double helix, thus

interfering with cellular replication. Biological effects of radiations depend not only on the absorbed dose, but

also on the energy deposit density along the track left by the particle in the tissue, i.e. the LET (Linear Energy

Transfer). Radiations having higher LET (protons, neutrons, α particles, ions) have more chance to bring

irreversible modifications within the cell.

BNCT is a radiotherapy technique based on the capture of thermal neutrons by the isotope of the boron 10B

followed by the emission of an α particle and a nucleus of 7Li.

The fission products obtained following the thermal neutron captures on 10B are particles having high LET

value, allowing to reduce the dose delivered to the patient, obtaining the same biological effect. Moreover α

particle and 7Li ranges inside biological tissue are short (about 8 µm and 5 µm respectively) compared to

typical cell dimensions; delivering boron inside tumoral cells causes the 10B(n,α)7Li reaction to release the

most energy inside the tumoral cells themselves.

Material and methodsMaterial and methodsThe PhoNeS project is developing new ways to apply the BNCT technique exploiting, as neutron generators,

mega voltage linear accelerators already available in Hospital Radiotherapy Units, applied to a feasible

passive photo-neutron-converter structure. Materials and geometries of this structure must be carefully

examined. PhoNeS’ goal is to optimize neutron production and thermalization, while shielding from X-rays.

In this way the patient, shielded from normal therapy radiation, is irradiated with thermal and epithermal

neutrons that interact with previously injected 10B

Neutrons productionNeutrons productionNeutrons are produced via giant dipole resonance; gamma radiation from conventional radiotherapic linear

accelerators impinges on shields of high Z materials (such as lead) of the passive photoconverter, producing

an isotropic flux of fast neutrons. We build the photoconverter using lead bricks of different shapes and

dimensions. The surface of the fotoconverter orthogonal to the incident photonbeam has dimension 30 x 30

cm2, the depth is 15 cm (3 walls, 5 cm depth each). Cutaway shape of some bricks allows to better shield from

unwanted gamma radiation. Energy distribution of photoproduced neutrons is peaked around 2 MeV.

Neutrons moderationNeutrons moderationNeutrons are then slowed down to epithermal (0.4 eV < En < 0.01MeV) and thermal (En < 0.4 eV) energies,

proper for 10B capture, by elastic scattering on light nuclei, introducing feasible moderating structures of

polyethylene, deuterium oxide and graphite. Slowing down length and capture length for moderating materials

were analyzed both teoretically and using simulation toolkits; then they were studied in preliminary

measurements. Slowing down lengths for fast neutrons are dominated by high energy cross section

giant dipole resonance lead cross section for (γ,n) lead bricks 30 x 30 x 15 cm3 lead wall

lead wall plus 30 x 30 x 3 cm3 polyethylene filling heavy water moderator cavity in plexiglas cavity in carbon fibers

density

polyethylene plexiglass deuterium oxide graphite

SimulationsSimulationsSimulations with the MCNP4-GN code and with the GEANT4 toolkit have been performed to evaluate both the best

configuration of the photoconverter and of the moderator structure. We proved that it is possible, carefully studing

geometry and materials, to obtain neutrons of suitable energies for BNCT from primary electrons with energies between

18 and 15 MeV; it is also possible to significatively reduce the dose released by fast neutrons and by gamma radiation in

tissue equivalent materials to acceptable values.

The simulation with Geant4 toolkit of the anthropomorphic phantom Jimmy, realized by the INFN of Turin for neutron

dosimetry, was done in the preliminary studies of the PhoNeS project.

Another important step has been the simulation of the head of the electron accelerator; since the engineering design is

covered by the industrial secret, the head of the e-LinAc was reconstructed backward in the simulation toolkit, from

measuraments results.

head of the LinAc

40 cm

40 cm

5 cm

5 cm

10 cm

20 cm

conv0

conv 3

conv 5

conv 1

conv 2

simulation runanthropomorphic phantom Jimmy and simulationsimulation of a configuration

MCNP-GN simulation results for different configurations

MeasurementsMeasurementsFive sets of measurements were performed with the PhoNeS prototype, since August 2005, at radiotherapy departments

in different hospitals, in Italy and in Austria, applying the photoconverter to linear accelerators with accelerating

potentials of 18 MeV and 25 MeV. Bubble detectors for thermal and for fast neutron dosimetry were used to evaluate

dose released and neutron fluence in the 20 x 20 x 10 cm3 cavity of the prototype. Bubble dosimeters were introduced in

suitable tissue equivalent phantoms, realized at the INFN Trieste machine shop. In addition TLD (thermoluminescent

detectors) couples were used to measure dose released by neutron in thermal energy range. At the St.Johanns – Spital

Hospital in Salzburg (Austria) and at the Policlinico Gemelli in Rome, X-ray contamination in the neutron field were

measured with radiographic film, actually used in radiotherapic gamma dose evaluation. At the Mauriziano Hospital in

Turin and at the Sant’Anna Hospital in Como, a BDS spectrometer were also used to measure energy spectrum of fast

neutrons. At the Policlinico Gemelli in Rome, in addition to usual measurements, were used CR-39 track detectors with

boron radiators and cadmium shields to measure the parameter of cadmium ratio, that was 11.3 ± 2.5

A new polyethylene steps phantom has been developed

to focus the analysis on the epithermal component of the

neutrons energy spectrum, which is not otherwise directly

measurable with bubble dosimeters.

in hospital e-LinAc PhoNeS prototype applied to different e-LinAcse-LinAc headin hospital e-LinAc

bubble dosimeter positioning bubble dosimeters in polyethylene phantom and then in the cavity TLD dosimeter CR-39 dosimeters

measuring x-ray contamination of neutron field evaluation of primary and secondary gamma radiation, with thin lead shields

polyethylene steps phantom for ephitermal neutron spectrum analysis

10B(n, α)7Li cross section 10B(n,α)7Li reaction7Li and α particle ranges in biological tissue(M.Charlier et al., Photobiol. et radiobiol. des acides nucleiques)

10B(n,α)7Lireaction

Discussion and resultsDiscussion and resultsMeasurements showed a very good agreement with the results from the detailed simulations with the Montecarlo codes,

both of the gantry and of the photo-converter-moderator system. Measured fluences showed an almost constant

increase in the different sets, reaching and exceeding the neutron flux threshold that was established as meaningful for

the prototype (107 n cm-2 s-1). Using the maximum dose rate available with the Turin LinAc (600 MU m-1) the measured

therapeutical flux was 1.8 x 107 n cm-2 s-1. It has been proved that there’s room to many further improvements, changing

materials, gantry setup or beam composition (using for example direct electrons instead of photons). Should the

expectations be fullfilled, therapeutical flux (108 n cm-2 s-1), defined from the successful experiment TAOrMInA, will be

easily reached as well. This makes this experiment the first actual promising attempt to bring BNCT into the hospital

environment, as strongly advised by the main worldwide supporters of this therapy.

LinAcs used, their nominal energy and max dose rate: Como: Varian Clinac 1800, 18 MeV, 400 MU m-1 – Torino: Varian Clinac 2100, 18 MeV, 600 MU m-1

– Salisburgo (Salzburg): Philips SL25, 25 MeV, 400 MU m-1 – Roma: General Electric Saturne 43, 18 MeV, 400 MU m-1

Configuration (1) with: photoconverter: 30 x 30 x 15 cm3 lead; moderator: 30 x 30 x 3 cm3 polyethylene, plexiglass box (walls thickness 0.85 cm) filled with 7.5 kg of D2O. Alternative configurations (tested only in Como): (W): photoconverter 20 x 20 x 1 cm3 + 10 x 10 x 1cm3 tungsten; moderator: 30 x 30 x 2 cm3

plexiglass; (PE+FC) same that (1), but with carbon fiber boxes(walls thickness 0.3 cm) instead of plexiglass box, filled with 9.0 kg of D2O; (FC) same that (PE+FC), but 30 x 30 x 5 cm3 carbon fiber box filled with D2O instead of polyethylene moderator.

ComoW

ComoPE+FC

ComoFC

0.71±0.09 0.22±0.03 0.93±0.09

0.66±0.08 0.17±0.02 0.83±0.08

0.72±0.09 0.13±0.02 0.85±0.09

0.69±0.08 0.25±0.04 0.94±0.10 367±62 1.03±0.12 36±7

171±24 0.53±0.07 32±6

256±32 0.99±0.12 26±4

200±26 1.08±0.13 19±3

ComoW

ComoPE+FC

ComoFC

(1)(1)

Experimental neutron energy spectrum

(green) and MCNP-GN simulated spectrum