Radiation therapy: Latest findings and technological breakthroughs

This article was first published in the ADA News Bulletin, March 2023.

"Radiation therapy: Latest findings and technological breakthroughs" is a transciption of a Dental Files podcast "Changing Tech, Changing Guidelines" which you can listen at the ADA's CPD Portal.

New guidelines on radiation safety mean we must keep up to date on the ways we keep patients safe during imaging procedures that involve ionising radiation. Like any tool that is potentially harmful, there is a need for consistent guidelines and operating protocols to make it safe for everyone in the practice environment. A recent episode of the Dental Files podcast (Ep. 43, 2022) saw oral health radiology expert Prof. William Scarfe discussing with the ADA’s Dr Peter Norton the latest findings and technological breakthroughs in this essential area of practice; due to great member interest in this subject matter, here is the podcast in written form.

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Dr Peter Norton 

Bill, welcome back to Australia. It’s great to have you here. I’d like to talk about radiation safety, particularly what the current guidance is for different age groups, because I believe the guidance differs across the lifespan.
 
Prof. William Scarfe

Yes, it does. It’s come about because in the early 2000s, there were data presented in the medical literature that indicated that the sensitivity of paediatric patients to radiation, especially those children under the age of 12, is higher. This is because children’s absorbed doses are greater and their cumulative dose is higher because their life expectancy is longer than adults. This new data has trickled into dental radiography and raised a number of concerns and questions such as: “Shouldn’t we be adjusting exposure for paediatric patients?” and “Shouldn’t we be selecting imaging procedures based on age and appropriateness?”. So your comment is very topical and pertinent.

Interestingly, when we think about radiation safety, most think that we are referring to patient radiation. But you’ve got to remember that if we minimise the dose to the patient, we’re also minimising the dose to ourselves and also to those around us who work in the workplace.

There’s actually no regulation at all regarding patient dose limits, at least in the United States; it is really on the judgement of the individual professional. Our role as oral maxillofacial radiologists, in my professional sense, but also as a general dentist, is really to be a patient advocate and to be knowledgeable about what is the current thinking on using the technologies we have available.

There are three fundamental principles involved with any dental radiographicexposure: justification or appropriateness, optimisation and protection – these form the basic elements in radiation safety.

There are numerous guidelines available in the United States, and we refer to those as being selection criteria; I think the Faculty of General Dental Practice in the United Kingdom also referred to them as a selection criterion. The second principle is optimisation – if a decision has been made to expose the patient, then you need to minimise that exposure. And the third principle relates to the protection aspect: what are specific protection measures, such as lead aprons, thyroid collars, radiation personal monitoring, what procedures should be in place to minimise exposure such as maintaining at least six metres from the patient during exposure.

As regards justification, I think it’s important to realise there are a number of documents available. The current guidance document was developed by the US Food and Drug Administration in collaboration with the American Academy of Oral and Maxillofacial Radiology.

This document provides guidance based on patient age and presentation. Patients are considered either as being in the primary dentition, in the transitional mixed dentition or as adult. Adults are subdivided as dentate or edentulous. Patient presentation is then determined according to type of encounter (new patient or recall) or clinical presentation (high or low dental caries risk or alveolar bone loss).

So, this matrix gives an understanding of what is an appropriate survey or radiographic series as, for example, a new patient who has the primary dentition. This is what’s taught in every dental school in the United States. It’s approved by the ADA and also the Food and Drug
Administration – which would be similar to your ARPANSA – the Australian Radiation Protection and Nuclear Safety Agency, that provides guidance for Australian dental practitioners in its publication entitled Code of Practice and Safety Guide for Radiation Protection in Dentistry (2005).

I think it’s important for listeners to understand that, within the last 10 or 12 years, there have been two initiatives directed specifically at children because of their increased sensitivity. The first initiative in the United States has a paediatric focus and was called the Image Gently in Dentistry campaign; it has a six-step plan that included collimation of the beam; use of thyroid collars; the use of cone-beam CT if appropriate; and also child-sizing exposure parameters for cone-beam CT. It was fairly generic and open ended.

Another group called DIMITRA (dentomaxillofacial paediatric imaging: an investigation towards low-dose radiation induced risks) is a European multicentre and multidisciplinary project focused on optimising cone-beam CT exposures for children and adolescents.

Originally, we are all dependent upon the ALARA idea, which is ‘as low as reasonably achievable’, and that came from the 1950s. This concept was modified to ALADA: ‘as low as diagnostically acceptable’.

In 2017, the DIMITRA group proposed a concept that involves adjusting exposure to children, particularly related to cone-beam CT, and they call this ALADAIP: ‘as low as diagnostically achievable, being indication oriented and patient specific’. It essentially boils down to, when the clinician has made the choice to irradiate a patient, you’ve got to ask first the question why, and secondarily, what are you looking for? Then you need to adjust the exposure settings accordingly. There is a lot of guidance out there but, unfortunately, many of the concepts, practices and procedures have not been reflected in legislation. But that’s the same whether it’s here in Australia or in the United States.
 
Dr Peter Norton

I’d like to go back to a couple of topics you raised. The first one is collimators; I’m going to come clean and say I’m a bit nervous about the use of collimators because, to me, it’s hard enough to get a good image on a small child who may or may not sit still, let alone reducing my field size. What are your thoughts on that?
 
Prof. William Scarfe

Why would you use rectangular collimation from a dose point of view? Well, it decreases dose by at least 40% and up to 75%. So if you’re not going to use round collimation because of the reasons you have stated, you’ve got to incorporate something else that assists you in reducing dose – that would be lead aprons or digital receptors, etc. Storage phosphors are very tolerant to wide variations in exposure, whereas direct digital receptors are intolerant to overexposure. They become saturated or overexposed at relatively low exposures. So I think that the use of digital, also called wired or corded technology, is a little bit different than storage phosphors. Storage phosphors can be a straight replacement for film and it is not necessary to adjust x-ray equipment exposure settings, Direct digital receptors, on the other hand, are more sensitive to radiation and therefore much lower exposure settings can be used which is important if round collimation is used.

We have found, in our teaching institution, it’s hard enough to get appropriate region-of-interest coverage with rectangular collimation – however, speaking with other colleagues, once you make the commitment to rectangular collimation, you get used to it. There are certain areas where you can’t use it; however, it is important that practitioners are aware of the substantial dose reductions to patients using this option.
 
Dr Peter Norton

Does that mean that we should be using rectangular collimation or aprons and thyroid collars on all age groups or just on children?
 
Prof. William Scarfe

It’s an interesting question, because the thinking now is not about the overall detriment to the patient – which has been reported for CBCT radiography as well as for intraoral and panoramic – but it’s actually about where the dose is being delivered. In all dental radiographic procedures, a dose is being delivered not only to the salivary gland, which is a relatively sensitive organ, but it’s also being delivered to the thyroid gland, as well as the bone marrow in the chest. Really, we have to think about protecting those specific organs.

We can’t get away from irradiating bone marrow, because it’s part of the mandible – and part of the cervical spine – but we can try to limit the exposure. My feeling is that, if you’re not using rectangular collimation, you should be trying to reduce the dose to the patient by some other means, and that includes the use of the lead apron.
 
Dr Peter Norton

Speaking of lead aprons, I’ve heard some practitioners (I don’t know how well informed they were) talk about the possibility of trapping radiation underneath and causing it to bounce around the patient for longer. Is that a real thing?
 
Prof. William Scarfe

There has only been one particular article that did indicate this component of “radiation entrapment”. The idea behind it is that when we radiate a face, some of that radiation, if it’s directed inferiorly, for example, when taking periapicals of the maxillary central incisors, you’re going to direct that through the mouth, through the throat. That’ll be direct radiation. If you think about it, there’s no such thing, as it doesn’t bounce off lead; it is absorbed by lead. Lead is the best absorber of radiation. The absorption of this radiation by lead counteracts that argument that it’s bouncing around – ricocheting, as it were. There is actually some reflection back – there is some truth to that – but it’s not in the levels that would be appreciable. So this concept that the lead apron is bad for you is not a concept that’s widely held.
 
Dr Peter Norton

Are thyroid collars and lead aprons at all useful for extraoral imaging?
 
Prof. William Scarfe

Most definitely, thyroid collars and lead aprons are useful. There have been some studies recently that indicated there was very little difference in cone-beam if you put a lead apron on or not. But they have been refuted by two more recent studies by colleagues, that in fact indicate there is appreciable difference to particular organs – one of which is the breast tissue, and the other is the thyroid gland. Those two areas contribute substantially to the overall effective dose to the patient. I think the use of these devices, apart from the psychological aspect that patients are still used to these things, do have benefit in extraoral imaging. In cephalometric and panoramic imaging, I would not suggest the use of thyroid collars because, obviously, there are some structures within the neck such as the hyoid bone that can be obscured by them. But certainly, if we’re doing a cone-beam CT of a child, particularly of the maxilla, which we quite often do for impacted canines, we’ll provide them with a thyroid collar to minimise that radiation load.
 
Dr Peter Norton

For a 2D panoramic x-ray film though, should we be using thyroid collar?
 
Prof. William Scarfe

No, not at all. The nature of panoramic imaging actually projects the primary x-ray beam over the back shoulder from below. If you’re using a thyroid collar, that image is going to be projected higher on the image, perhaps involving the lower portion of the anterior symphysis of the mandible, so I wouldn’t use thyroid collars at all on that. Essentially, we use a reverse gown technique, as it were, with lead aprons. We pull it up at the front, and we don’t pull it up at the back because, once again, the x-ray beam shoots from approximately 7 to 12 degrees below the shoulder. You don’t want it actually going through the shoulder like a polo neck.
 
Dr Peter Norton

Does a thyroid collar actually do much? It doesn’t seem to cover much real estate on the body. Does it have to be on tightly in order to work? What are the guidelines?
 
Prof. William Scarfe

Thyroid collars should cover the thyroid. It’s not a fashion statement; it’s got to cover that structure completely. What it does do is prevent scatter radiation, which occurs due to the primary beam interacting with the jaws and especially high density objects such as dental amalgam.

The thyroid collar doesn’t go on the back of the neck, it goes on the front. And the thyroid collar is there to prevent not direct exposure but scatter radiation. It also has a secondary effect of preventing scatter radiation from reaching the chest cavity, or the bone marrow in the sternum.
 
Dr Peter Norton

How do you go about easing patient or parent concerns about radiation exposure with diagnostic imaging?
 
Prof. William Scarfe

I think the most important thing is that clinicians refer to and provide patients with educational materials that are available through trusted sources. The Australian Dental Association has produced a hand-out, which describes procedures and provides very useful infographics relating radiation exposure of dental x-rays to everyday life activities and other medical procedures as well (that’s available from the arpansa.gov site).

I think in discussion with the patient, providing the patient with information on what the dose of dental radiographic procedures is, relative to other procedures, is important. That’s what we really do. We should provide patients with literature and support the literature with our comments that it is a safe procedure, relative to other forms of everyday existence and also relative to other imaging – for example, breast mammography, a lumbar spine, or items of that nature.
 
Dr Peter Norton

In the lead-up to today, I was speaking with an orthodontic colleague who was very excited that you were going to be involved in this interview, and he specifically asked me to ask you about the CBCT statements from the American Academy of Maxillofacial Radiology, particularly as it relates to CBCT and orthodontics. Now, this orthodontist was saying that his understanding of the guidelines was that,in general, CBCT should be avoided in children. And if taken, there should be small fields of view, using the lowest appropriate diagnostic radiation. Do those guidelines still apply?
 
Prof. William Scarfe

That document was developed over a period of three years, based upon the literature up until 2013. There are certainly more exposure controls in CBCT equipment that are available, particularly for children. An example of that is, if you take a maxillary and mandibular scan on a child, it will automatically reduce the size of the diameter of the field of view and therefore exposure compared with an adult. Moreover, changing the exposure and judicious selection are the most important. I work with our orthodontic department and I read all their scans. We abide by the original guidelines.

It’s not useful for Class 1 malocclusion patients generally, but if a Class 1 patient comes in with an ankylosed, submerged tooth, and you really want to know what you’re up against, then a small field of view with the appropriate resolution is going to help. Quite often, there’s no amount of intraoral imaging that’ll assist you with that third dimension, so I think those guidelines are still valid. They could be modified somewhat, incorporating some of the technologies that are available to minimise the dose.
 
Dr Peter Norton

Bill, as always, it’s been wonderful chatting, so enjoy the rest of your time in Australia. And thank you for speaking with the Dental Files.
 
Prof. William Scarfe

Thank you very much. It’s been my pleasure.