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It is not uncommon for researchers to be unaware of the contribution that irradiation filters make to improving beam quality when performing preclinical irradiation studies.
In our previous articles, we discussed how beam quality defines the penetrating capability of an X-ray irradiator’s beam and is described by two characteristics – average beam energy and beam homogeneity, where higher average energy and greater homogeneity are hallmarks of higher beam quality.
As a method for removing lower energy photons and retaining higher energy photons, beam filtration increases both beam homogeneity and the average energy of the beam, thereby improving beam quality to achieve a more penetrative, predictable, and reproducible dose.
However, there is also a flip side to filtration that researchers should be aware of and should consider when planning their studies. Understanding the quality of the beam that a system can generate is important, as is understanding the full effects of filtration on their irradiation study.
The half-value layer (HVL) is a parameter used to describe and quantify the quality of an x-ray beam. The HVL is dependent on the penetrability of the beam and the inherent and additional (selectable) filtration used in the system.
HVL is defined as the thickness of a given filter material (usually aluminum or copper), measured in mm, that is required to reduce the air kerma rate* to one half of its original value (original value =1). Though the HVL provides no information regarding the amount or the energy spectrum of photons, it is common practice to describe the quality of an x-ray beam in terms of the energy of the beam and its HVL.
* Air kerma rate is the speed at which radiation energy is transferred to a mass of air. It is measured in units like milliGray per second (mGy/s) or Gray per minute (Gy/min), and it describes a radiation (X-ray or gamma-ray) beam intensity over time.
The HVL of the X-ray source in KUBTEC Scientific’s XCELL® 180 benchtop irradiator was measured according to the protocol from The American Association of Physicists in Medicine (AAPM) TG-611. For the full experimental details and all data sets obtained, please see LINK.
In summary, X-ray beams were generated using inherent filtration. An initial dose measurement was obtained and used to calculate the HVL dose values as needed. Dose measurements were then acquired using selectable filters with progressive attenuation capability, either alone or in combination. The filters were placed in path of the beam, and beams were measured until the calculated HVL dose values were reached; datasets were recorded for each filter/filter combination. A Homogeneity Factor (HF) was also calculated and recorded for each dataset. Note that the HF yields information on the quality of the beam, where a larger HF means more beam homogeneity.
It was observed that as filtration increased, the HF also increased. In irradiation, the goal of filtering the x-ray beam is to attenuate the lower energy photons, which in turn increases the average energy of the beam. As the filters become thicker and of higher atomic number, more lower energy photons are attenuated, and the remaining photons are of a more uniform energy distribution.
As expected, it was also observed that filters that cause more beam attenuation yielded a lower dose. At the most extreme, an X-ray beam was filtered using only inherent filtration and was designated to deliver 100% of the dose. Adding a filter with the most attenuation capability (i.e., the highest density material and the thickest filter option) resulted in an output that was reduced to ~9% of the initial dose. However, as the initial beam was filtered only through inherent filtration, which is the least attenuating filter, it was comprised of a larger range of energies, not all of which will contribute to a therapeutic biological effect due to the diminished penetrative capability of the beam.
The observations stated above make two principles clear:
Decreasing beam intensity will require longer exposure times. This principle is true when using irradiation filters to improve beam quality on any X-ray irradiator system. The tradeoff of using progressive filters for higher beam quality (i.e. better penetration and homogeneity) is progressively longer exposure times. This is a tradeoff that is generally considered to be advantageous and worth making, because removing lower energy beams decreases the risks associated with delivering a superficial dose during in vivo irradiation.
When determining to what extent filtration should be used in a study, beam quality should be given strong consideration. However, there are other factors that must also be considered and may conflict with achieving the highest beam quality. These factors include: