Session Item

With increasing indications and availability of stereotactic body radiotherapy (SBRT) a need for harmonization is emerging to maximise consistency in delivery and reporting of dosimetric parameters.

Within a national effort to develop safe and efficient SBRT for all metastatic sites, two phase II SBRT trials targeting bone (BONY-M) and soft tissue metastases (SOFT) opened for inclusion in late 2019. Both trial protocols provided detailed information on SBRT dosimetry and delivery. We hypothesized that more detailed guidelines and involvement in clinical trials would increase harmonization in applied dosimetric parameters. To test this, we performed a national survey in 2019 and repeated it in 2021. 

The survey, consisting of 7 general questions and 18-26 questions related to specific anatomical sites, was sent to radiotherapy departments in December 2019 and in September 2021. The survey responses were compared with national guidelines (brain, lung and liver) and trial protocols (soft tissue and spine). Here we report on change in SBRT availability and dosimetric parameters.

SBRT-responsible physicists at the sites offering stereotactic radiosurgery (SRS) or SBRT at the time, filled out the survey. Figure 1 shows availability of SRS/SBRT indications and technology. Treatment planning systems differed, but all applied advanced dose calculation algorithms (type B or better).

Brain SRS heterogeneity was high: 6 different fractionations, dose prescription to either GTV (4 centres) or PTV (1 centre), highest single fraction dose of 18Gy (2 centres) or 20Gy (3 centres) and 5 different target coverage objectives. National SRS guidelines don’t provide any dosimetric details.

Multiple lung SBRT fractionations were applied, not all described in the guidelines. PTV coverage of 67%, simply specified in the guidelines, was clinically applied as PTV D99% or D98%. GTV coverage, not specified in the guidelines, was applied in three different ways.

Liver SBRT fractionations from guidelines and SOFT protocol (with one additional fractionation at one institution) were applied. Clearly prescribed GTV and PTV coverage from the guidelines was applied at all centres, with two having an additional GTV coverage aim.

The dosimetric parameters did not change in the 2-year period for brain, lung and liver SBRT.

For soft tissue SBRT, the three centres participating in the trial used target coverage per trial specification, which differed from the fourth centre, first starting with soft tissue SBRT in 2021.

In spine SBRT, target coverage consistency improved between the centres in 2021, with four of them participating in the phase II trial in 2021 and only two at the time of the first survey.

An overview of guidelines/protocol adherence and deviations in applied dosimetric parameters between the centres is shown in Figure 2.

Agreement in dosimetric parameters between SBRT centres increased with amount of dosimetric details provided in guidelines and trial protocols.