Clinical Radiation Generators
Presently there are two major types of xray generators. Previously the xray generators included a series of what were affectionately called "bombs". There were several types but they all used a radioactive element to produce xrays. The radium "bomb" produced a megavolt beam. The cesium "bomb" used refuse material left over after the uranium fission reaction in reactors, and the cobalt "bomb" used cobalt that had captured neutrons inside a nuclear reactor. Specifically, cobalt has a high Barnes cross-section, which is to say that it will absorb lots of neutrons from inside the reactor. This property is used in the construction of control rods that are used to slow the nuclear reaction preventing it from getting too 'hot' and causing a 'meltdown'. There are no "bombs" in use in Australia. The last one to my knowledge was the cobalt machine at RNSH in Sydney which was operation up to the start of 1996. The cobalt machine is still in wide use in the undeveloped world.
| DIVERSION! Incidentally, the production of cobalt machines was undertaken by Johns in Saskatoon in Canada. You can find the first and second cobalt machines at the Saskatoon Cancer Centre. The first machine is suspended in the foyer. You have to look up when you walk through the front door. Also you will have passed the second machine beside the concrete path on the way in. |
Low energy xray generators (15kV - 300kV)
- Kilovoltage Units
- Grenz-ray therapy (15-20kV)
- These machines produce very low energy xrays that deposit a large portion of their energy in the upper few millimetres of the surface. There are few clinical applications for such a beam, but the treatment of solar keratosis fits the bill nicely. A single dose of 20Gy using 15kV will eradicate solar keratosis with a high probability of success and leave a faintly hypopigmented area. It is possible to use a Grenz ray machine and remain in the room while wearing a lead apron and thyroid shield (as used in diagnostic radiology).
- Contact therapy (40-50kV)
- Some of the multienergy superficial machines have energies in this area. It is appropriate for things a little thicker than keratoses, but which you would still describe as 'flat' and without the expectation of deep invasion.
- Superficial therapy (50-150kV)
- Many departments have a superficial or SXR machine. The machines usually produce a range of energies such as 50kV, 100kV, and 150kV, although you may also see numbers like 75kV and 125kV. Before you use any SXR beam, you should look at the depth dose curves for the desired field size and then specify energy and SSD.
- The SXR beam is somewhat inappropriately named because it isn't really that 'superficial'. Many things are named in comparison, and "superficial" was used to differentiate from "deep" - which isn't that deep either! We humans are really good at labeling the latest and greatest with superlative and absolute terms that time shows to be unfortunate! "Superficial" that can be a little deep, "Deep" that is a little superficial, "Conventional" that is decidedly old hat …. good one guys! Keep it up!
- Orthovoltage or deep therapy a.k.a. DXR (150-500kV)
- These machines represent the epitome of the xray generator based on potential difference and transformers. It is very difficult to build a DXR machine of more than 500kV because the current will just arc though the insulation at these potentials. The machines are devised to use filters can achieve HVLs of between 1 and 4 mm of Cu. Unfortunately, …. or fortunately depending on your view …. I was trained in a setting where beams were designated by kV rather than HVL, so I admit to not really understanding HVLs in a practical sense.
- The machine can be used at a number of different SSDs. Longer SSDs have a more penetrant beam for same field size and energy. The beams are collimated by cones out of the window, and with 2-3mm Pb cut-outs on the skin surface.
- Sometimes you will come across another doctor who suggests that we use DXR for a prostate cancer or breast cancer. This really shows the age of the doctor!
- Grenz-ray therapy (15-20kV)
Across all these beams, the depth dose depends on the beam energy (that is its HVL) and the filtration used, as well as the skin source distance (SSD) and field size. In general, all of these beams above have a depth distribution which delivers $D_{max}$ at surface. The position of the other isodoses will vary, but for the orthovoltage range the approximate depths are 90% at 1 cm, 50% at 5 cm and 10 % at 10 cm. This shallow dose distribution is very good for treating superficial tumours (from the skin to the depth of a rib. However there is no skin sparing, only superficial depth dose distribution, and the photons with an energy below 100kV will deliver a higher dose to bone. This has the effect of creating a dose shadow deep to bone. Becasue the beams are of low energy there is also increased scattering (yes, increased is a comparative terms and we need to state "increased compared to …", and it means "increased compared to a MV beam"!)
Megavoltage Therapy
Across the globe, there are two main ways that megavoltage therapy is deployed. The linear accelerator is most common in developed countries, and it would be a surprise to find anything but linear accelerators in North America, Australia, NZ or the UK. In under-developed countries, there are radionuclide based alternatives that are easier to operate and which have a much better up time profile, as well as being cheaper!