Radiation exposure of personnel during IORT: radiation protection aspects.

Transcription

1 Radiation exposure of personnel during IORT: radiation protection aspects. L. Strigari 1, V. Bruzzaniti 1, V. Landoni 1, A. Soriani 1, S.Teodoli 1, M. Benassi 1 1 Lab. Fisica Medica e Sistemi Esperti, Istituto Regina Elena, Roma, Italy strigari@ifo.it Abstract Intraoperative radiation therapy (IORT) is a technique which allows irradiating the patient directly after the surgical operation using a linear accelerator that can be situated in the operating room. The need to characterize the neutron spectra for this particular situation arises from the fact that, when neutron spectra is not fully known, it becomes necessary to be more cautious introducing a weight factor w R of 20 (maximum value); this leads to overesteem the equivalent dose due to neutrons and it indicates to introduce additional (mobile) shields for photon and neutrons radiation not easily achievable in an operating room. 1. Introduction Intraoperative radiotherapy (IORT) is a multidisciplinary procedure which combines two conventional methods of cancer treatment surgery and radiation therapy. The purpose is to deliver a large single dose to the surgically exposed tumor bed while minimizing dose to normal tissues. In the last years there has been an increasing interest on IORT technique, also because of the development of dedicated accelerators. In the world IORT is generally an adjuvant therapy, i.e., that it is given as a boost after conventional fractionated radiotherapy. A task group (No. 48) of the AAPM has developed guidelines for IORT [1]. Also an International scientific Society has been born (ISIORT) with the principal aim of gathering specifical experiences and methods. In the framework of a national project on quality assurance in Radiotherapy, the Italian National Institute of Health established a multidisciplinary working group, with clinical practice experience, in order to develop guidelines on quality assurance for IORT technique [2]. In this context, new requirements have come out, both regarding specifical clinical application both regarding a greater and simpler accelerator usability. 2. Material and Methods A new accelerator to be used in the operating room has been developed: the Light Intraoperative Accelerator Liac TM (Figure 1). Published data show that neutron contribution to adsorbed dose at 1m for medical accelerators, with energy over 10MeV, can be as much as about 0.05% of that due to the useful photon beam. When neutron spectra is not fully known it becomes necessary, to be more cautious, introducing a weight factor w R of 20 (maximum value). Using this weight factor neutron contribution could be as much as 10 times the usual photon leakage specification. This theoretical observation is taken into account when projecting a bunker for medical accelerators and it is confirmed from experimental measurements of exposure. Present address: Lab. Fisica Medica e Sistemi Esperti, Istituto Regina Elena, via E. Chianesi, Roma, Italy

2 FIG. 1. Liac TM : Light Intraoperative Accelerator This approximation applied to Liac TM leads to overesteem the equivalent dose due to neutrons and it indicates to introduce additional (mobile) shields for photon and neutrons radiation not easily achievable in an operating room; for this reason, although maximum electron beam energy of Liac TM could be 12MeV, it has been realized in a reduced power version with electron maximum energy equal to 10MeV. 2.1 Liac: Technical characteristic Liac TM is an intraoperative radiotherapy system, consisting of mobile radiant unit and a operator control rack, connected by a 10meters cable, which during the irradiation supplies the radiant unit with electrical power and transmits the treatment parameters. During the IORT session the Liac TM is not connected to electrical local system but is fed by the UPS (Uninterruptable Power Supply) hosted in the control rack. An innovative robotical system allows the Liac TM to be extremely moveable and strongly semplificates the hard-docking procedure. Liac TM radiant unit weight is less than 400Kg, so that there are not installation problems in any surgical suite; a battery system lets this unit move independently in the operative block. The standing wave S-band linear accelerating structure, specifically designed for Liac TM, is 850mm long and consists of 17 autofocusing cavities; it is supplied with a 3.1MW Magnetron, with 2.5µs pulse length and produces an electron beam of 12MeV maximum energy (Figure 2). The pulse repetition can vary from 1 up to 20p.p.s. The output beam has a 3mm diameter and is collimated by a sterilizable cylindrical perspex applicator, which is 60cm long and can have different diameters and terminal bevelled angles. The dose homogeneity on the surface to be treated is, generally, guaranteed by a 100µm brass foil scattering filter inserted after the titanium window. This technique allows the optimization of the accelerator performances keeping the level of stray radiation below the required limits. A new type of dosimetric system has been implemented to monitor the beam. It is based on a properly designed resonant cavity. The signal, proportional to the absorbed dose, is picked up from the cavity,

3 acquired and real time displayed on the control rack, with a good signal-noise ratio. This dosimeter is not affected by saturation phenomena and is temperature, pressure and humidity indipendent. FIG. 2. Autofocusing cavities The reduced power version Liac dosimetric characterization has also been performed. In particular, the radiation distribution along the beam central axis for the flat applicators and on the clinical axis for the beveled ones, the isodoses and the profiles at the depth maximum dose have been measured by means of a solid state dosimeter in a Scanditronix - Wellhofer water phantom. Similarly, the beam output linearity versus accumulated dose, short and long-term reproducibility have been evaluated with a plane-parallel ionization chamber PPC-05 (Scanditronix - Wellhofer) in a PMMA phantom. Absolute dose measurements have been performed using MOSFET dosimeters. These dosimeters have been calibrated in reference conditions at the depth of maximum using an electron beam produced by conventional accelerators (Clinac 2100 C/D). In our hospital we have a mobile linear accelerator producing electrons with energies of 4, 6, 8 and 10MeV (Figure 3), with dose rate between 5 and 20Gy/min and pulse frequency between 5 and 20 Hz. FIG. 3. PDD in water

4 Due to the need to minimize the length of the perspex applicator a scattering foil, made of brass ( µm thickness), has been introduced in the beam to produce a homogenous profile (Figure 4). FIG. 4. Profiles This technique allows the optimization of the accelerator performances keeping the level of stray radiation below the required limits. 2.1 Contamination of electron beam The National Council on Radioprotection Protection and Measurements (NCRP) has made recommendations for the protection of personnel who can be exposed to radiation [3,4]. For application of these recommendations, it is necessary to examine the radiation environment to evaluate the factors determining the likelihood of exposure and to estimate the hazard. The purpose of this work is to evaluate neutron contamination from Liac TM taking into account the differences between the structure of this accelerator respect to a conventional accelerator for radiotherapy and the differences between an operating room and a dedicated medical accelerator room (bunker). To asses radiation exposure it is necessary to take into account the contribution to scattered radiation from electrons, photons and neutrons. In particular, bremsstrahlung photons are of dosimetric importance for a contamination of the tail of dose distribution and for the choice of the true mean stopping power ratio in a contaminated or mixed beam. A photon absorbed dose contamination of 5% increases the mean stopping power in a high-energy electron beam by almost 1% in low atomic number materials, in first approximation independently of electron energy at the phantom surface [5]. The minimum energy required to remove one neutron from a nucleus, absorbing energy from a high energy electron or photon, lies between 6 and 16MeV, for most stable nuclei heavier than Carbon. Above the threshold energy, neutron production can be given by cross section of nuclear reaction. It is well known that the contamination from neutrons in a beam produced by a linear accelerator for radiotherapy (E> 10MeV) is small but not negligible [4]. Potential sources of neutron contamination are any materials on which the electron or photon beam is incident. The reaction yield from Bremsstrahlung induced in target is given by:

5 Y (, xn : E ) b e = σ q γ : X r ( Ee ) A 2 2 (, xn E mc ) x N x γ neutrons / electron (1) where: o x is the thickness of target, o X r ( E e ) is radiation length, o N is Avogadro Number, o A is the number of nucleons in a nuclide, 2 σ γ, xn : E mc is the cross section for equivalent quantum. o ( ) q e The reaction yield from direct electrodisintegration process is given by: N x Y e ( e, xn : Ee ) = σ ( e, xn : Ee ) neutrons / electron (2) A 2 where: o σ ( e, xn : Ee ) is the electrodisintegration cross section. The yield ratio from the Bremsstrahlung induced in target and from direct electrodisintegration process depends on the target thickness and on the ratio of the radiation to the total stopping power for the target. This yield ratio increases linearly with target thickness and ranges from 5.5ε for Carbon (C) and 10.2ε for Aluminium (Al) at 10MeV to 8.1ε for C and 14.4ε for Al at 15MeV, where ε is the percent of energy loss in the target. Maximum yield ratio is 57.1ε for Pb at 50MeV According to the curves relative to neutron yield from a semi-infinite slabs of 10 elements ranging from carbon to uranium [3], assuming that entire electromagnetic spectrum generated from the electrons is absorbed in the slabs and that no neutron are reabsorbed in the target, we have placed a lead block in the beam axis (figure 3) to amplify the contribution of neutron contamination deriving from the interaction of electrons and photons with the structures of the accelerator. e FIG. 3. Experimental set-up; To evaluate the exposure in mixed field of neutrons and γ rays we have used the following general equation: Q n, γ = A Dγ + B D n (3) where:

6 o Q n,γ is the total response due to combined effects of the neutrons and γ rays; o A is the response per unit of dose adsorbed in tissue for γ rays; o B is the response per unit of dose adsorbed in tissue for neutrons; o is γ rays dose adsorbed in tissue; D γ o and D n is neutrons dose adsorbed in tissue. In the mixed field measurements by means of two dosimeter having different value of A/B can be used to obtain the dose adsorbed D and D, solving simultaneously the equations derived by equation 3. γ n Measurements of dose equivalent for neutrons has derived by LB6411 neutron probe, with optimised energy response, in accordance with the new dose equivalent conversion factors defined by ICRP 60 [6], with a significantly improved response and a better detection limit as compared to conventional rem counter [7]. Sensitivity of overall system is 3.15 counts/nsv relative to 3 MeV. -3 The response for photon radiation is ( 0.69 ± 0.05) 10 counts/nsv, for a large energy range. Inovision Ion Chamber Survey Meter Model 451B has been used for stray radiation measurements. This is an air ionisation chamber instrument calibrated in exposure rate units for gamma and x-ray in the energy range of 20 kev to 2 MeV. β radiation component has been shielded by mean of 2 mm lead foil. 3. Results In order to evaluate beam contamination and calculate the photon-shielding requirement, we have measured Bremsstrahlung yield. We have measured the equivalent dose a 1 meter around the Liac TM, for 100 pulses (figure 4), with different scatter angle. FIG. 4. Attenuation curves of scattered photon around IORT accelerator We have also measured the equivalent dose around the Liac TM, for 10Gy to patient with different distance by patient (figure 5).

7 FIG. 5. Equivalent Dose for 10Gy to patient around IORT accelerator Stray radiation at 0, at 1 meter and at 9MeV, is less than 0.5% of maximum adsorbed dose in the patient. Bremsstrahlung yield at 90 is less than 1.5% of stray radiation at 0 and at 135 less than 0.3%. At maximum effective energy neutron dose equivalent is less than 140nSv with electron dose absorbed equal to 10Gy. Assuming workload of 200 IORT treatments per year, it can be estimated that neutrons dose is less than 0.03mSv/year at 1m. 4. Conclusion While lead panels of adequate thickness easily shield electrons and photons of Liac TM, scattered neutrons dose equivalent depends on the energy spectrum and on the constructive characteristics of the accelerator. Answer of the system can be used in order to derive answer for lighter materials at greater energies, in agreement with experimental curves [3]. Neutron yield production per unit incident electron beam power, for a semi-infinite target, is significant only for materials heaver of Al, at energy greater of 15MeV. Re-evaluations of biological effects due to exposure to neutron radiation leads to consider more cautious weight factors respect to the ones reported in NCRP No.79 and as a consequence a change in the correction factor between fluence and equivalent dose as a function of neutron spectra mean energy reported in paragraph 2.3 of NCRP No.79. A more accurate characterization of the source, both photon spectra and neutron contamination, may give a better estimate of the risk for personnel thus a better choice of the methods of protection and subsequent technical solutions. A simulation with Geant-4 of Liac TM IORT is in progress for full power version dosimetric characterization and evaluation of personnel exposure, taking into account the quality of radiation.

8 References 1. Palta JR, Biggs PJ, Hazle JD, et al. Intraoperative electron beam radiation therapy: Technique, dosimetry, and dose specification: Report of task force 48 of the radiation therapy committee, American Association of Physicists in Medicine. Int J Radiat Oncol Biol Phys 1995;33: Istituto Superiore di Sanità, Italy. Guidelines for intra-operative radiation therapy. ISSN , Rapporti ISTISAN 03/1 IT. 3. National Council on Radioprotection Protection and measurements Neutron contamination from medical electron accelerators. NCRP Report No. 79, National Council on Radioprotection Protection and measurements Radiation Protection Design Guideline for MeV Particle Accelerator Facilities. NCRP Report No.51, International Commission on Radiation Units and Measurements Radiation Dosimetry: electron beams with energies between 1 and 50MeV. ICRU Report No. 35, International Commission on Radiation Protection, Recommendations of the International Commission on Radiation Protection, ICRP publication No. 60, Burgkhardt B, Optimierung einer Neutronen Äquivalentdosisleistungs-Meßsonde; Strahlenschutz: Physik und Maßtechnik FS T Verlag TUV Rheinland 1994.