Interventional radiotherapy (IRT, brachytherapy) is a radiotherapy technique, in which radioactive sources are placed directly into or near the tumor site. IRT procedures are widely used in modern oncology, because they allow for treating different anatomical locations while delivering high doses and sparing the surrounding organs at risk [1]. Moreover, IRT is a procedure that requires several phases, which could be briefly classified as the clinical indication, implant, treatment planning, and delivery. Historically, several classification systems have been used for IRT, and they were based on the type of clinical purpose, type of implant and timing of the implant, dose-rate, and type of loading for treatment delivery.
According to IRT clinical indication, it is possible to identify radical IRT when: 1. The aim of treatment is to cure, and IRT is delivered as the only treatment modality or as a boost to external beam radiotherapy [2]; 2. Post-operative purpose when IRT is employed after a surgical intervention to improve local control [3]; and 3. Palliative intent when the only reasonable clinical result expected is a reduction of clinical symptoms [4]. With regard to the type of implant, different types of IRT approaches can be distinguished, including contact [5], intra-cavitary [6], intra-luminal [7], and interstitial [8] techniques. Another way to classify the types of implants is to divide them into permanent or temporary implants [9].
In terms of dose-rate, low-dose-rate (LDR) [10], pul-sed-dose-rate (PDR) [11], or high-dose-rate (HDR) [12] can be classified. Specifically, LDR is defined as a dose of 0.4-2.0 Gray (Gy)/h, and HDR is identified as a dose of > 12 Gy/h [13], with typical irradiation distance.
It is also important to emphasize that modern IRT mostly relies on remote afterloading units, which allow for significant decrease of radiation exposure on interventional radiation oncologists and radiation therapists [14, 15]. Another aspect to consider when classifying different IRT techniques is the overall number of catheters used to deliver the dose. In this regard, there are mono-catheter implants (for example, in adjuvant vaginal applicators where only the central channel is active) [7] or multi-catheter implants [8]. Finally, the role of anesthesiologist during the procedure is of paramount importance, since various procedures require local anesthesia (including spinal anesthesia), analgosedation, or general anesthesia [16].
In Table 1, the different classification systems are summarized according to phases of IRT process they are based on.
Table 1
Different classification systems used in interventional radiotherapy (brachytherapy)
It is worth to underline that IRT has been recognized as a relevant part of modern oncology [17, 18], and is currently the subject of a huge interest, being used after a slow but inevitable decline in previous years [19]. The main reasons for such renewed interest in IRT are recent technological innovations, which have dramatically changed the clinical practice [20].
The first revolution occurred with the introduction of 3D image guidance for simulation and treatment planning [21]. The second point of paramount relevance is the launching of intensity-modulated external beam radiotherapy with dedicated 3D treatment planning systems in clinical practice, allowing for dose escalation and organ-sparing during EBRT [22]. These two technological advancements enabled discussions about image-guided and intensity-modulated IRT, respectively. To highlight the differences between the old technique and the new approach, some authors used a different naming, replacing “brachytherapy” with “interventional radiotherapy”.
All the afore-mentioned classification systems are useful and currently implemented in the clinical practice. However, none of them consider the varying degree of complexity that characterizes interventional procedures.
Specifically, IRT tools and strategies are continuously changing and updating. On the one hand, the role of image guidance is gaining more and more relevance [23], and on the other hand, the close and collaborative role of surgeons during some IRT procedures allows for safer and higher quality implants [24].
There are several aspects of interventional procedures regarding the complexity of implant phase, including the type of implant, imaging used during the procedure, and the role of multi-disciplinary team in operating room, as summarized in Table 1. Therefore, the implant complexity results from the combination of three different aspects:
Type: As mentioned before, the type of implant may be contact, intra-cavitary, intra-luminal, or interstitial. The clinical and technical difficulties in the management of different types of implants are directly correlated with the expertise of interventional radiation oncologist. In addition, implants could be either based on single-catheter or multiple catheters.
Equipment: The imaging used during implantation can be visual only (or requiring specific expertise, such as dermoscopy), and can rely on radiography, ultrasound, or CT, which are manageable by the radiation oncologist. Moreover, in selected cases, the imagining can require endoscopy (performed directly by IRT physician in selected centers only) or MRI.
Team: This point is related to the need of a multi-disciplinary team in the operating room that, in addition to the interventional radiation oncologist, can comprise an anesthesiologist or, in certain circumstances, additional specialists, such as the surgeon to manage possible acute intra-operative complications, or medical physicist to support the implant procedure.
Based on the different procedural aspects, a system can be proposed to classify the complexity of procedures according to the overall score of the above-mentioned aspects of interventional procedures (Table 2), including the type of implant, primary imaging used during implantation, number of catheters, participation of an anesthesiologist, and multi-disciplinary team involvement.
Table 2
Different scores assigned according to the COMIRI system (scores are given in brackets)
In Table 3, the scores of various typical procedures in commonly treated sites with four different degrees of complexity according to the scores with the following intervals, are presented: low (≤ 5), moderate (between 6 and 8), high (between 9 and 10), and very high (≥ 11). However, some clinics might use different approaches with diverse overall scores obtained for the same procedures.
Table 3
Complexity levels of implants in different anatomical sites
It is absolutely fundamental to emphasize that the current proposal aimed at adequately framing a specific phase of IRT procedure, which is the implant. However, no association should be made between the complexity of implant and the complexity of subsequent phases. In fact, even contact skin radiotherapy that has a relatively low complexity implant is characterized by the same level of complexity regarding subsequent phases, relies on an accurate patient diagnosis, and both on imaging [25] and intensity modulation [26], in order to prevent toxicities [27].
There is a disparity in terms of access to IRT across different countries, and several domains should be empowered to foster the spread of this effective technique, including the clinical practice, education, research, and communication [28]. It is desirable that the correct understanding of the implant complexity can lead to changes in reimbursement systems, which may improve sustainability of IRT [29].
In addition, there are several dose calculation methods used in IRT. TG-43 relies on standardized data assuming a homogeneous medium, such as water, and is clinically validated. In contrast, TG-186 uses advanced algorithms to account for tissue heterogeneities and applicator characteristics, offering more accurate and personalized dose distributions. While TG-186 is not yet fully clinically validated, it should be further implemented to enhance dosimetric accuracy in brachytherapy [30]. In particular, the addition of the COMIRI classification system should adequately indicate the need for suitable resources according to the level of complexity for different IRT procedures in terms of personnel expertise, equipment availability, and multi-disciplinary teamwork.
