The response rate in this survey was 22%, which was found acceptable comparing to other surveys using similar methodology [6, 7].
Clinicians generally agreed that an ablative strategy for selected patients with OMD could result in cure or long-lasting disease control. Up to three metastases in a maximum of two organs were the preferred threshold the responders were willing to treat when applying an ablative strategy for patients with OMD, and only 7% were willing to treat up to five metastases. Due to a small number of responders, analysis of agreement among different organ-specific specialists were not possible. For OPD, the picture was more blurred with a tendency to accept fewer metastases for an ablative treatment strategy when the patients were diagnosed with OPD compared to OMD. However, the responders agreed that ablative treatment could be an option for patients with OPD in selected cases. Our findings align with results from the large international survey from 2017, based on > 1000 responders from several international societies for radiation oncology. They found that most physicians (69%) were willing to target two to three oligometastatic lesions with SBRT in an individual treatment course [6]. However, in three other surveys, the threshold was higher. In the OLIGO-AIRO (The Italian Association of Radiotherapy and Clinical Oncology) survey [7], 78% of responders considered up to five metastases as the most appropriate definition of OMD, and in the “Elekta International Oligometastasis Consortium” survey [5], five out of seven responders recommended SBRT for up to five metastases in patients with OMD. In the AIRO‑Lombardy survey (The Lombardy Section of the Italian Society of Oncological Radiotherapy) [8], 62% defined an oligometastatic state if the number of metastases was less or equal to five. In reference to the threshold of organs defining an OMD state, the OLIGO—AIRO survey [7] also found most of their responders (70%) willing to accept more than one involved organ when defining OMD. In contrast, the AIRO-Lombardy survey [8] found that their responders favoured limiting the threshold to one organ, but only with marginal support (54% versus 46%).
Among different cancer types, the responders in this survey opted that patients with colorectal-, breast-, lung-, kidney-, and prostate cancers were most suitable for an OMD strategy with SBRT. These findings are in concordance with the patient recruitment in recently published phase-2 studies [11–15]. The randomised phase 2 trial, SABR—COMET [14], recruited patients with a broad spectrum of different cancer types, and the four most frequent primary tumours were CRC (18%), lung cancer (18%), breast cancer (18%), and prostate cancer (16%), representing most common cancer types. However, rarer cancer types like kidney cancer may also benefit from an OMD strategy with SBRT [19, 20].
The responders agreed that metastases in the CNS, lung, bone, and liver were most suitable for an OMD strategy with SBRT. However, there were more discrepancies among the responders for other soft tissue metastases, as adrenal glands and lymph nodes. These findings differ from the OLIGO-AIRO survey results, where responders answered that lymph nodes (46%) were the site most susceptible for a local treatment. However, the following preferences matched the responses from our survey; bone (45%), lung (40%), brain (36%), and liver (7%) [7].
Most of the responders (60%) found it necessary to discuss patients with OMD at a tumour board, but only 25% described that this was the case in the daily clinical practice. In the OLIGO-AIRO survey, a similar part of the responders (66%) preferred an interdisciplinary discussion when managing patients with OMD [7].
The results across the different surveys depend not only on the responder’s perception of the definition and treatment of OMD and the available ablative techniques. To a high degree, it reflects the selected responder group and the design of the survey questions. In the survey from the Elekta International Oligometastasis Consortium [5], responders were seven highly selected SBRT experts. In the OLIGO-AIRO survey [7] and in the large international survey [6], responders were members of large national and international radiation oncology societies. Our responders were less selected and consisted of both medical- and clinical oncologists as well as trainees. This may have impacted the responses. Several responders stated they did not know enough about SBRT to choose this strategy. The postgraduate six-year-long specialist training program in Denmark educates clinical oncologists to master both medical oncology for solid tumours and radiation oncology. However, a fraction of the oncologists is educated as medical oncologists or work merely within the medical oncology field. This was considered when we designed the survey, as aspects concerning the technical use of SBRT were not included.
In Denmark, there is a broad access to radiotherapy and SBRT, although not all centres offer SBRT. This is reflected in the survey as none of the responders chose lack of human resources or lack of the necessary equipment as a reason for not referring to SBRT.
Most of our responders expected that cure or long-term survival could be achieved for selected patients with OMD. However, we still need clearer evidence of the clinical benefit of an ablative treatment strategy, and we lack tools to select these patients.