Photon beams

K. H. Brown, Proc. Soc. Photo-Opt. Instr. Eng. 2438, 33 (1995). A good source of information on the new exposure technologies can be found in the Proceedings of the International Conference on Electron, Ion and Photon Beam Technology and Nanofabrication, pubhshed annuaUy in Issue 6 of the Journal of Vacuum Science and Technology B. 

An excellent agreement with the X-ray photoionization spectra of ethylene, butadiene and hexatriene (7) is obtained (12) (Figure 3) when including in our calculations the Gelius (36) photoionization cross sections for an Alka photon beam, by means of Eqs. (4) and (5). Such a direct comparison is impossible for octatetraene, a compound for which there is no available XPS data. 

Figure 1. Photoelectron circular dichroism angular distribution [/icp(0) - /rcp(6)] for the hv = 10.3-eV photoionization of (/ )-camphor, as imaged with the photon beam propagating along the X axis. The x,y axis scales are the physical pixel coordinates of the detector.

It is very obvious from the peaks and troughs displayed in Fig. 1 that the anticipated dissymmetry between forward and backward electron ejection directions (relative to the photon beam direction) is borne out by experiment. Moreover, one sees that the dissymmetry lies in opposite directions for ionization of the two energetically accessible orbitals observed here. 

At a phenomenological level too, there are differences since the CDAD effect disappears in directions parallel to the photon beam, whereas PECD asymmetry is maximized in these directions. Conversely, the PECD asymmetry disappears in directions perpendicular to the photon beam where the maximum CDAD asymmetry can be found. 

Within its orbit, which has some of the characteristics of a molecular orbital because it is shared with electrons on the surrounding atoms, the electron has two possible spin multiplicity states. These have different energies, and because of the spin-multiplicity rule, when an (N-V) center emits a photon, the transition is allowed from one of these and forbidden from the other. Moreover, the electron can be flipped from one state to another by using low-energy radio-frequency irradiation. Irradiation with an appropriate laser wavelength will excite the electron and as it returns to the ground state will emit fluorescent radiation. The intensity of the emitted photon beam will depend upon the spin state, which can be changed at will by radio-frequency input. These color centers are under active exploration for use as components for the realization of quantum computers. 

The rate of the formation of photoexcited electron-hole pairs, Gix), is given as a function of the intensity of photon beam h, the absorption coefiicient of photons a, and the depth of photon-penetration x as shown in Eqn. 10-12 [Butler, 1977] ... 

Figure 2 The photoabsorption (c), photoionization (o-,-), and photodissociation (cr

At present, in industrialized countries, about 70% of the cancer patients are referred to a radiation therapy department for at least part of the treatment. The majority of them are treated with conventional photon beam therapy (i.e., the reference treatment modality as defined in Table 2.1). 

Does Effectiveness of Photon Beam Therapy Reach a Plateau ... 

However, the impressive development and progress in conformal therapy with photons raise a difficult issue to what extent has photon beam therapy reached a kind of plateau as far as physical selectivity is concerned This important question is still controversial and heavily debated. If photon therapy had reached a plateau, the search for improvements would be directed to alternative radiation modalities [8,9]. 

An alternative to further improve or optimize the photon techniques is to replace the conventional photon beams with new types of radiation. Indeed, since the beginning of radiation therapy, the radiation oncologists have always been eager to search new types of beams (different from conventional x-rays/photons) in order to improve the efficacy of radiation therapy. In principle, different approaches can be adopted (Table 2). 

Table 2 New Types of Beams in Radiation Therapy (Alternative to Conventional Photon Beam Therapy)...

However, clinical evidence has demonstrated that, from a pure safety point of view, a physical selectivity at least as good as that obtained with conventional photon beams is absolutely required for neutrons as well. 

Fractionated photon beam therapy, as defined in Table 3, is generally accepted as the reference therapy modality. This is due to its broad application, the experience accumulated over several decades, and its recognized effectiveness [4,5,8]. 

External photon beam therapy, involving (multiple) beams of conformally adjusted size and shape, adequately orientated to cover the PTV(s) in a homogeneous way, with photon energy ranging from a few megavolts to about 20 MV, using fractionated irradiation with daily fractions of 2 Gy, live times a week, over 4-6-7 weeks depending on the clinical situation. 

With hadrons (i.e., neutrons, protons, and heavy ions), new radiation qualities are introduced in therapy. The distributions of the ionizations (and energy deposition events) along the particle tracks are different, and, as a result, different and increased biological effects (at equal absorbed dose) may be expected compared with the conventional photon beams. Fig. 1 illustrates the differences in dose necessary to produce a given biological effect as a function of radiation quality [15]. 

In external photon beam therapy, weighting factors, W /p., are currently estimated based on the a/(i model to compensate for differences in fractionation. 

Today, in the majority of centers still active in neutron therapy, the technical conditions are becoming progressively comparable to those in modern photon beam therapy. In addition, a few new high-energy facilities have been proposed (e.g., in China, Germany, Poland, Slovakia, and South Africa). 

Figure 11 Clinical loco-regional control in patients treated with mixed (neutron/photon) beams or photons only (RTOG randomized trial) for locally extended prostatic adenocarcinoma. (From Ref. 34.)...

Figure 14 Proton beam irradiation of a deep-seated large tumor. Single Bragg peaks of different energy are combined, in adequate proportions, to obtain a homogeneous dose distribution at the level of the SOBP. The depth-dose curve of a photon beam, shown for comparison, is inferior compared to the proton curve. However, an optimized multifield photon treatment allows to reach better irradiation conditions. (From Ref 43.)...

For comparison, the best estimates of local control rates currently obtained with conventional photon beam therapy are given in parentheses. 

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