Participants and participating centers

The survey was circulated among all 211 registered medical members of the DEGRO-WG RS and SRT in Germany, Austria, and Switzerland. Of the registered radiation oncologists, 35.5% (75/211) completed the online survey, 46.7% (35/75) of whom were working in university hospitals and 26.7% (20/75) each in non-university hospitals and outpatient radiotherapy facilities. Most responders were board-certified radiation oncologists (68/71), almost three quarters (74.7%; 53/71) of them working in a leading position (senior consultant physicians: 35/71; 49.3%; departmental heads: 18/71; 25.4%) at their respective institution (Supplementary Fig. 1). Centers treating more than 100 patients with BoM with radiotherapy annually represented the largest group, with 58.7% of participants (44/75), followed by centers with 51–100 patients per year (33.3%; 25/75). The majority of radiation oncologists (56%; 42/75) reported treating about 10–30% of these patients with stereotactic radiotherapy (Supplementary Fig. 2). Comparing university medical centers and non-university centers, there were significant differences in the number of patients treated for BoM annually: non-university centers predominantly reported treating between 51 and 100 patients per year, while university centers treated more than 100 patients with BoM annually (p < 0.05). There were no significant differences in the percentage of patients with BoM treated with SBRT, but the rate of bone SBRT tended to be slightly higher in university centers. As an example, 68.6% (24/35) reported treating 10–30% of these patients with SBRT vs. 45% (18/40) in non-university centers (Supplementary Fig. 3).

Treatment indications for palliative and stereotactic radiotherapy of bone metastases

For palliative irradiation of BoM, the majority of responding physicians (84.3%; 54/64) recommended moderately hypofractionated treatment concepts (single doses of 2.5 to 3.5 Gy); 60.9% of ROs (39/64) reported using stereotactic ablative concepts for palliation of BoM at least occasionally. However, 50.8% (31/61), 44.3% (27/61), and 27.9% (17/61) of answering physicians regarded postoperative bone radiation, bone instability, and new or progressing neurological deficits, respectively, as contraindications to bone SBRT (Fig. 1). Of the included ROs, 57.4% (35/61) performed stereotactic ablative treatment of BoM most frequently in the context of OMD. In this context, 41.3% of participants (26/63) defined osseous OMD as ≤ 5 BoM; 30.2% of respondents (19/63) answered that they deviate in their definition of oligometastases depending on clinical, anatomic, and histologic factors.

Fig. 1

Treatment indications for stereotactic body radiotherapy (SBRT) of bone metastases (a) and localizations amenable to bone SBRT in participating centers (b)

Almost two thirds of ROs (60.7%; 37/61) stated that they decide for or against bone SBRT regardless of histology. However, small cell lung cancer was the most frequently mentioned histology for which no bone SBRT was recommended (31.4%; 16/51), followed by disseminated hematological malignancies (27.5%; 14/51) and multiple myeloma (21.6%; 11/51).

The pelvic bones were reported as the most common site of bone SBRT (frequently: 51.6%; 32/62; occasionally: 37.1%; 23/62), followed by the thoracic/lumbar spine (frequently: 46.7%; 29/62; occasionally: 43.5%; 27/62) and the ribs, scapula, or sternal bone (frequently: 40%; 24/60; occasionally: 45%; 27/60). SBRT for BoM of the cervical spine was reported to be applied occasionally by 34.4% (21/61) and frequently by 26.2% (16/61), while bone SBRT for the long bones of the extremities was used occasionally by 32.3% (20/62) and frequently by 16.1% of participants (10/62; Fig. 1).

Bone instability with the need for surgical stabilization was considered as the most common absolute contraindication to SBRT of BoM (86.2%; 50/58), followed by relevant spinal cord compression (44.8%; 26/58) and limited life expectancy of less than 3 months (39.7%; 23/58; Fig. 2).

Fig. 2

Absolute contraindications to bone stereotactic body radiotherapy as mentioned by the survey participants

Patterns of care for combining bone SBRT with systemic therapies

Only 4.9% (3/62) of participating ROs declined any treatment combination of bone SBRT with systemic therapies. However, 30.6% (19/62) of ROs reported concomitant use of systemic therapies, and 32.3% (20/62) felt safe combining bone SBRT with sequential systemic treatments. Of all respondents, 32.3% (20/62) had no concerns about combining SBRT with both sequential and concomitant systemic therapy.

Concerning specific systemic therapies, many of the participating physicians declined concomitant use of either targeted therapies (27/42; 64%), chemotherapy (22/42; 52.4%), or immunotherapy (19/42; 45.2%) with bone SBRT. Especially the combination of BRAF/MEK inhibitors with bone SBRT was negated most frequently (6/12; 50%). Interestingly, 30.9% (13/42) of ROs expressing concerns about systemic therapies reported stopping hormone-modifying therapy such as androgen deprivation therapy during bone SBRT (Fig. 3).

Fig. 3

Patterns of care for combining bone stereotactic body radiotherapy (SBRT) with systemic therapies. a Continuation of treatment with systemic therapies in combination with bone SBRT; b types of systemic therapy for which no concomitant use with SBRT is recommended; c substance classes for which no concomitant use with SBRT is recommended

Adding bone-modifying agents to bone SBRT was recommended by 56.5% of ROs (35/62) for most treated patients (i.e., > 50% of patients) and by 35.5% (22/62) for at least some patients (25–50% of patients).

Treatment planning and application of bone SBRT

Reproducible patient positioning for bone SBRT was seen as a key issue by treating ROs, and the majority of participants reported regular use of dedicated positioning equipment such as rigid masks and/or vacuum cushions (79.6%; 39/49). Additional motion-management systems to control patient surface areas or respiratory motion are used by 55.1% of treating ROs (27/49).

For treatment planning, the vast majority of physicians recommended additional diagnostic imaging such as Magnetic resonance tomography (MRI) (91.9%; 45/49), diagnostic Computed tomography (CT) (71.4%; 35/49), and to a lesser extent, Positron emission tomography (PET/CT) imaging (44.9%; 22/49). For bone SBRT planning, a CT slice thickness > 1 but below 2 mm was recommended by 54.5% of participants (24/44), and 34.1% of ROs (15/44) recommended a slice thickness of ≤ 1 mm.

For the application of bone SBRT, the majority of ROs use conventional linear accelerators (46.3%; 19/41) most often, followed by conventional linear accelerators with X‑ray verification for SRT (37.2%; 16/43) and dedicated linear accelerators with external X‑ray verification for SRT (28.2%; 11/39). Only a minority of treating ROs use specialized systems such as MR-linear accelerators (8.6%; 3/35) and robotic radiosurgery (24.3%; 9/37) at least rarely.

For treatment verification, CT (cone-beam or fan-beam) imaging before treatment was reported as the commonly used modality (84.8%; 39/46), and additional CT after couch repositioning is also preferred by 42.5% of participating ROs (17/40).

Surface guidance/surveillance was used regularly by 48.6% of participating ROs (18/37; Fig. 4).

Fig. 4

Patterns for treatment planning and application of bone stereotactic body radiotherapy (SBRT). a Imaging modalities required for treatment planning; b slice thickness for planning CT scans (in mm); c linear accelerator specifications used for bone SBRT: d treatment verification imaging used for bone SBRT (2D imaging including stereoscopic imaging)

Dose concepts and contouring for bone SBRT

Contouring and dose prescription concepts were separately recorded for spinal and non-spinal BoM. Most participating ROs stated the frequent use of compartment-based anatomic target volume concepts based on existing guidelines for spinal SBRT (59.5%; 25/42), and the frequent use of simultaneous integrated boost/dose-painting techniques was reported by 57.4% (27/47). Isotropic expansions of the gross tumor volume without a clinical target volume (25.5%; 12/47) or with a clinical target volume (15.6%; 7/45) or whole vertebral body treatment concepts (18.1%; 8/44) were less frequently used.

For non-spinal metastases, simultaneous integrated boost/dose-painting concepts were regularly prescribed by 44.7% of ROs (21/47), and isotropic GTV–PTV expansion was most commonly applied (51.2%; 23/45). In this context, the majority of ROs recommended a safety margin from the GTV to the PTV of 3–5 mm (63.1%; 12/19).

Dose and fractionation concepts varied considerably for both spinal and non-spinal SBRT. The use of single-fraction bone SBRT was rarely reported by participating ROs, with only 16.7% of ROs (7/42) using single-fraction treatments at least occasionally for non-spinal metastases and 10.6% of ROs (5/47) using single-fraction treatments at least occasionally for spinal metastases; 61.9% (26/42) and 66.0% (31/47) of participants ruled out the use of single-fraction bone SBRT for non-spinal and spinal metastases, respectively. Hypofractionated bone SBRT applied daily was the most frequently used method for both non-spinal (47.5%; 19/40) and spinal metastases (61.9%; 26/42; Fig. 5). Of participating ROs, 25.6% (10/39) and 40.5% (17/42) reported employing dose-painting concepts with integrated boost doses frequently for bone SBRT of non-spinal and spinal metastases, respectively.

Fig. 5

Dose concepts and contouring patterns for bone SBRT. Target volume concepts for SBRT of a spinal metastases and b non-spinal metastases; c dose concepts for SBRT of c spinal metastases and d non-spinal metastases

The question about specific dose concepts for both spinal and non-spinal BoM was answered by too few participants for clear conclusions to be drawn.