Evaluating cone cut in rectangular collimation in intraoral radiography: application of ALADA and radiation stewardship

In dentistry, radiography is used to identify oral pathosis and anomalies. Dental professionals are obligated to reduce patient exposure to ionizing radiation by adhering to the optimization principle (also known as ALARA: as low as reasonably achievable) [1]. This means that the patient’s radiation exposure should be kept to an acceptable minimum taking into account the diagnostic value of the radiograph.

Rectangular collimation results in at least 40% less radiation exposure to patients according to a systematic review that assessed all sensor modalities including F-speed film, PSP plates, and digital sensors [2]. However, the perceived disadvantage of rectangular collimation is the potential increase in errors like cone cutting. Cone cut occurs at the edge of the film and is often due to misalignment of the collimator and/or the film. When the film is incompletely covered by the radiation beam, a white surface on the radiograph appears. The occurrence of small cone cuttings does not necessarily affect the diagnostic quality. However, in some cases, if the size of the unexposed area (white area) is large, it can make the diagnostic area of the radiograph unreadable. In this situation, the radiograph should be retaken but the consequence is exposing the patient to twice as much ionizing radiation.

In 1983, Horton et al. [3] examined a total of 3801 radiographs. Half were taken using film with a rectangular collimator and the other half were taken with a round collimator. A total of 156 radiographs showing unexposed areas due to cone cut were detected and 86% appeared with rectangular collimation. This finding was confirmed in a study conducted by Parrot and Ng[4], which showed that there was an increase in the incidence of cone cut errors on film radiographs from 3.3 up to 20.9% when rectangular collimation was used instead of round collimation. Consequently, the patient still received less radiation with rectangular collimation when compared to round collimation, because of the reduced size of the exposure. Overall, the theme in the literature appears that patients receive a reduction in radiation dose when a rectangular collimator is used [5].

Students at the Academic Center for Dentistry Amsterdam (ACTA—dental school of the University of Amsterdam and VU Amsterdam) take radiographs with PSP plates using rectangular collimation. The aim of this study was to explore the frequency of cone cut in radiographs taken with PSP plates using a rectangular collimator, where the cone cut occurred most frequently, and whether the cone cut impacted the diagnostic quality of the radiograph and explore.

This study was designed as a retrospective analysis. For preparation, the Strengthening the Reporting of Observational Studies in Epidemiology guidelines for reporting observational studies (STROBE) [6] and the Reporting of studies Conducted using Observational Routinely collected Data (RECORD) [7] guidelines were followed. The Institutional Review Board of the Academic Centre for Dentistry Amsterdam approved this retrospective analysis (2021–91,286). This retrospective study included all radiographs taken by staff and students at ACTA from January to December 2015. In 2015, there were a total of 15 X-ray machines of 4 different brands, all equipped with rectangular collimators. The intraoral receptors used for every radiograph that was taken were photostimulable phosphor (PSP) plates.

Rectangular collimation was used for all radiographs taken in 2015. The radiographs were examined by two dental students (JK, ES) trained by their supervisor (GS) and confirmed by author DCP.

The radiographs were divided into 5 groups: bitewings (BW), front teeth (FT), superior premolar and molar (SPM), inferior premolar and molar (IPM), and no patient (NP). The NP group contained nonclinical images such as extracted teeth, referral patient images from external practices, or images too vague to classify. After the images were arranged as a full mouth series, the four groups were classified further into no cone cutting (score 0), cone cutting that does not affect diagnostic value (score 1), and cone cutting that does affect diagnostic value (score 2) (Fig. 1 and Fig. 2).

The influence on the diagnostic value of the BW was based on the following criteria:

If a radiograph met one of the items above, it was classified as unusable and assigned a score of 2. Every other case of cone cutting was usable and assigned a score of 1. The classification was only based on cone cut errors. Note, when a radiograph with a cone cut was diagnostically unusable due to factors other than cone cut, such as overexposure, it was classified as usable.

The influence on the diagnostic value of FT, SPM, and IPM was based on the following criteria:

If a radiograph met one of the criteria above, it was classified as unusable and assigned a score of 2.

During the classification of anterior teeth into score 1 and score 2, only canine to canine was included. On an anterior radiograph, the above-mentioned criteria did not apply to premolars and molars (Fig. 5a). For the classification of IPM and SPM into score 1 and score 2, only premolars and molars were assessed, and on an IPM or SPM radiograph, the above-mentioned criteria did not apply for canines or incisors (Fig. 5b). When a radiograph was made for fistula tracing purposes, the end of the gutta percha point had to be visible. When it was missing due to a cone cut error, the radiograph was assigned a score of 2 (Fig. 5c). When a radiograph was made for fistula tracing purposes, the focus was given to the tooth/teeth where the gutta percha point ended. The entire tooth with the bone level and the periapical area had to be visible. When one of these was missing the radiograph was assigned a score of 2 (Fig. 5d). When an endodontic treatment was started, the entire tooth with the bone level and the periapical area had to be visible. If one of these was missing, the radiograph was assigned a score of 2 (Fig. 5e). If a radiograph was taken for length determination of the root canals, all silicon stops had to be visible. If one or more were missing, the radiograph was assigned a score of 2 (Fig. 5f). If the radiograph included an implant, the entire implant with the bone level and the periapical area had to be visible. If one of these was missing, the radiograph was assigned a score of 2 (Fig. 5g).

The location of the cone cut error was also assessed and divided into four groups: cranial (labelled as above (A)), caudal (labelled as below (B)), distal (D), and mesial (M) (Fig. 6). When the cone cut did not occur in a straight line parallel to the edge of the radiograph, it was assessed by the following criteria (Fig. 7):

The results were tabled in Microsoft Excel 2010. First, the number of radiographs in each group was tabulated. Thereafter, the percent total was calculated for score 0, score 1, and score 2, and for each group (BW, FT, SPM, and IPM). The location of the cone cut error was expressed as a percent total of the radiographs with cone cutting. This calculation was also done for the different types of radiographs.

A total of 53,684 intraoral radiographs were included in this study from the ACTA database. From this total, 8999 were radiographs in the NP group (nonclinical images like extracted teeth, referral images from external practices, or images that were too vague to classify). The remaining 44,685 radiographs were divided into BW, FT, SPM, and IPM groups and categorized with and without cone cut and cone cut but diagnostically usable versus cone cut and not diagnostically usable (Table 1). Of the 44,685 radiographs, 79% did not have any cone cut, 18% had cone cut but were diagnostically usable, and 3% of radiographs were not diagnostically usable due to cone cut.

Comparing anterior periapical radiographs (group FT) with lateral periapical radiographs (groups SPM and IPM), there was less cone cutting in the anterior periapical group versus the lateral periapical groups. Periapical radiographs of the lateral area showed the most cone cut errors (26%). In the BW radiographs, cone cut was noted 16% of the time, which was the least frequent compared to all other radiographs. Even though the BW group had the lowest frequency of cone cut, the percent of radiographs that were diagnostically unusable due to cone cut was similar, ranging from 2.7% for BW, 2.8% for FT, 4.3% SPM, and 4.0% for IPM. Furthermore, radiographs with a cone cut from the FT group were less likely to be assigned a score of 2 than radiographs from the BW group (as demonstrated by the ratios in Table 1).

Out of the 9337 radiographs that had cone cut errors, 22% were classified as above, 31% below, 21% distal, and 27% mesial (Table 2). Specific to BWs, cone cut errors appeared least on the above side (16%) and almost two times as much on the mesial side (34%). In the FT group, the frequency of cone cut on the below aspect was 42%, which was almost four times as much as the cone cut on the mesial side (12%). In the SPM group, the cone cut on the above side (9%) was quite low compared to the cone cut that occurred on the below side (39%) and the mesial side (35%). Moreover, it is notable that the percentage of mesial cone cut was high in the BW and SPM groups and low in the FT group. In the IPM group, the cone cut that occurred on the above side (37%) was almost twice as much as the cone cut that occurred on the other sides (21%, 20%, and 21% respectively).

This study assessed the number of cone cut errors of all radiographs taken in 2015 at ACTA and explored whether the cone cut influenced the diagnostic value of the radiographs and where at the radiograph the cone cut was most often found.

A study conducted by Shannon et al. [5] reported that the use of round collimation exposes the patient’s face to an area of 5.94 square inches of radiation. The area of radiation exposure to the patient’s face from one radiograph and one retake from a rectangular collimation is 4.82 square inches. [5] Thus, the patient is still spared 1.12 square of radiation if one retake is required using the rectangular collimator. Furthermore, the change to rectangular collimation leads to a decrease between 40 and 80% of the effective dose to the patient compared to round collimation, because of the smaller field size [2, 8]. It is worth noting that dose reduction can also be attributed to the type of sensor being used (PSP vs digital sensor) as this changes the effective dose received by the patient [8].

When using rectangular collimation and a retake is necessary, the patient would not even receive the same amount of ionizing radiation as by taking the radiograph with a round collimation in the first place. Within the present study, if 96% of the radiographs, from which the diagnostic value was not affected by cone cut, were made with a round collimator, every patient would have received approximately double the amount of ionizing radiation [8]. Similar results were found in a previous study which examined cone cuts in 6763 radiographs taken by junior dental students and found the frequency of cone cut error leading to a retake to be 2.7% [9]. Furthermore, similar to our study, the cone cut was noted to be the highest in the IPM region [9]. In summary, the rectangular collimator is effective in reducing radiation exposure to a patient.

Radiographs are frequently taken in the dental office, and as a result, the head and neck are exposed to higher doses of radiation over a patient’s lifetime. This may have the potential to induce salivary gland cancer [10], thyroid cancer [11, 12], and intracranial meningioma [12, 13]. However, it is important to note that studies associating cancer with dental radiographs were done when radiation exposure was higher with older technology [14]. Regardless of new and improved technology, dental practitioners still must follow the optimization principle [1]. Based on the findings of this study, a rectangular collimator greatly reduces the amount of radiation a patient receives without interfering with the diagnostic value of the radiograph and therefore is a textbook way for dentists to adhere to the optimization principle. Furthermore, if students can be trained to successfully take quality PSP images with the rectangular collimator, as reported by Freyche Vazquez [15] in 2023, clinicians would also likely be able to be trained to take quality images while reducing radiation to the patient with collimation.

Limitations of this study include the use of four different types of X-ray machines. The brands were not subanalyzed as it was not possible to determine which radiographs came from which machine. The radiographs were also taken by dental and dental hygiene students which may influence the study results as on one hand every student underwent training with the rectangular collimator, and on the other hand, students are less experienced in taking radiographs than dentists. Thus, we are unable to determine if the partial exposures are exclusively related to the rectangular collimator or the operator experience level, and as such, if the results would be different if round collimation was used. Furthermore, the criteria for score 1 and score 2 were different for the BW group versus the FT, SPM, and IPM groups. The results showed a lower percentage of cone cut in the BW group compared to the other groups and this may have been influenced by the different criteria. This was because the periapical radiographs had more specific criteria, and if one area was missed, the radiograph was immediately assigned a score of 2. Future directions include a prospective study including why the radiographs were prescribed. This data would help inform if the diagnostic value was affected due to cone cutting as the present data is unable to determine if a retake was truly necessary. For a prospective or multiple-center study, it is worth considering the level of education and use of the same type of radiographic machine, standardizing the characteristics of each radiographic machine such as stability and number of unit arms, and comparing rectangular and round collimation for the same group of professionals. Future research exploring the cause of the cone cut errors may also be useful. For example, whether the collimator was misaligned, or if the patient moved or swallowed.

Alongside these results to support the use of a rectangular collimator is the dental care professional’s obligation to adhere to the optimization principle [1] and consequently practice radiation stewardship. Radiation stewardship is analogues to antibiotic stewardship which is the movement to measure and improve antibiotic prescriptions to reduce the negative outcomes of unnecessary or improper use (CDC) [16]. Antibiotic stewardship influences recommendations from (inter)national guidelines and therefore clinical decisions in daily practice. Radiation stewardship is the implementation of the optimization principle. In medical and dental radiography, this principle was further detailed by the introduction of the acronym ALADA in 2017 [17]. The term ALADA stands for “as low as diagnostically acceptable” and is sometimes extended to “as low as diagnostically acceptable being indication-oriented and patient-specific” (ALADAIP) [17]. The purpose of this term is to emphasize the need to optimize radiation rather than simply minimize it. Several strategies exist to minimize radiation exposure to the patient including beam collimation [18]. Thus, even though new radiation technology limits the overall radiation a patient receives, proper, individualized prescription of radiographs remains necessary [18]. By practicing according to these principles, the dental professional practices an important point of radiation stewardship.

The frequency of cone cut errors interfering with diagnostics from using a rectangular collimator in intraoral radiography is very limited, and thus, applying the ALADA principle and radiation stewardship, the dental practitioner should choose rectangular collimation instead of round collimation.