Monitoring Neutron Fluxes from Proton Therapy System

A variety of proton therapy systems for generating neutrons to use in radiation oncology have been developed, using high intensity well-collimated proton therapy systems.¹ Results of a feasibility study to generate neutron beams suitable for use in radiation therapy show that the neutrons must possess energy in the range of 20-180 MeV.² Protons can be impinged on the weakly bound beryllium (⁹Be) target with particles possessing different energies and operating with different parameters to generate the desired neutron beams. On the other hand, lithium can act as another source of neurons if it is enriched to 99.8% with ⁷Li using the forward direction mechanism in a collimator.³

Collimators

The first collimator is made of Iron cylinders, which has various openings in four axial groups. It is 1 meter long. The collimator can project the beams at different solid angles. The building materials for the second and third collimators are made of sandwiched iron and iron respectively. The beam of neurons is projected through the channel of the collimator toward its destination. Neutron beams are targeted at a specific regions with an intensity of 60 the intensity of the downstream particles. ⁴

Neutron beam quality

The proton therapy system produces a spectrum of neutrons from the ⁷Li (p, n), which lie between the highly energetic neurons peak and the low energy neutrons region. The neutrons at the peak are equivalent to the unresolved ground state, which have an excited state in ⁷Be of Ex=0.43 MeV, which can reach an energy of 30-40 MeV. ⁵

Collimated neutrons

The intensity of collimated neutrons measure 5×10⁴ neurons/mm of the ⁷Li target/µA of the proton beam incident on a region, which is defined by a solid angle of 60µsr.⁶ The flux density depends on the distance of the neutrons from the irradiation source, which is always 60 times more intense nearer the source. Scientific research has demonstrated that smaller intensities of 0.2 to 10µA of the proton beam are equally important to study and can be attained. ⁷

Neutron flux measurements

The methods of measuring flux density of the neutrons include the of pressurized He-3 proportional counter. The energy of the protons can be increased by adjusting the ADC discrimination system and detection rate increases. The method yields accurate results because they agree with theoretical figures. ⁸

Conclusion

This field is worth the PhD research because of the significant contributions the results could make for controlling the flux density of neutrons used in radiation oncology.

Reference List

Linz, U, & Alonso, J. What will it take for laser driven proton accelerators to be applied to tumor therapy? Physical Review Special Topics-Accelerators and Beams, vol. 9, no. 10, 2007, p. 094801.

Nigg, D.W, Computational dosimetry and treatment planning considerations for neutron capture therapy. Journal of neuro-oncology, vol. 1, no. 62, 2003, p. 75-86.

Pérez-Andújar, A, Newhauser, W.D & DeLuca Jr, P.M, Neutron production from beam-modifying devices in a modern double scattering proton therapy beam delivery system. Physics in medicine and biology, vol. 4, no. 54, 2009, p. 993.

Satija, R, Jacobson, D. LArif, M. Werner, & S.A, In situ neutron imaging technique for evaluation of water management systems in operating PEM fuel cells. Journal of Power Sources, vol. 2, no. 129, 2004, p. 238-245.

Footnotes

1. A, Pérez-Andújar, W.D, Newhauser & P.M DeLuca Jr, Neutron production from beam-modifying devices in a modern double scattering proton therapy beam delivery system. Physics in medicine and biology, vol. 4, no. 54, 2009, p. 993.
2. U, Linz & J. Alonso, What will it take for laser driven proton accelerators to be applied to tumor therapy? Physical Review Special Topics-Accelerators and Beams, vol. 9, no. 10, 2007, p. 94.
3. ibid. p. 127.
4. J, Chang, C. H, Obcemea, J, Sillanpaa, J, Mechalakos & C, Burman, Use of EPID for leaf position accuracy QA of dynamic multi-leaf collimator (DMLC) treatment. Medical physics, vol. 7, no. 31, p. 2091-2096.
5. ibid. 3000.
6. D.W. Nigg, Computational dosimetry and treatment planning considerations for neutron capture therapy. Journal of neuro-oncology, vol. 1, no. 62, 2003, p. 75-86.
7. ibid. p. 100.
8. ibid p. 120.