Dosimetric intercomparison for stereotactic radiotherapy of multiple brain metastases.

Background and purpose: Stereotactic radiotherapy (SRT) delivers high doses with submillimeter precision to small targets, requiring accuracy in both dose calculation and delivery. This study aimed to perform a dosimetric intercomparison of SRT accuracy across multiple radiotherapy departments. Materials and methods: The study included 30 SRT plans submitted by 24 hospitals. A head phantom, equipped with Gafchromic EBT3 film and ion chamber inserts, was used for dose measurements. Invited institutions generated SRT treatment plans prescribing 30 Gy in 5 fractions to brain targets, following their clinical protocols. Point dose and planar dose measurements were carried out at each institution, then the measured doses were compared against those calculated by their respective treatment planning systems. Agreement between planned and measured dose was assessed, with gamma passing rates calculated using 5%/1 mm and 3%/1 mm criteria. Results: An average difference of 0.2% and a range of - 4.1% to 4.3% between the ion chamber measured dose at the center of one of the targets and the calculated dose was found. For the 5%/1 mm global gamma criterion, all films achieved passing rates above 97%. All treatment devices showed sub-millimeter spatial dose delivery accuracy, with deviations under 1 mm between measured and planned dose. Conclusions: The results indicated clinically acceptable agreement between the calculated and delivered dose across all treatment delivery systems. This intercomparison provided an opportunity for radiotherapy departments to assess the quality of their SRT treatment planning and delivery and benchmark their performance against other institutions.