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Abstract
PurposeStereotactic radiosurgery (SRS) can be delivered with a standard linear accelerator (linac). At institutions having more than one linac, beam matching is common practice. In the literature, there are indications that machine central axis (CAX) matching for broad fields does not guarantee matching of small fields with side ≤2 cm. There is no indication on how matching for broad fields on axis translates to matching small fields off axis. These are of interest to multitarget single‐isocenter (MTSI) SRS planning and the present work addresses that gap in the literature.MethodsWe used 6 MV flattening filter free (FFF) beams from four Elekta VersaHD® linacs equipped with an Agility™ multileaf collimator (MLC). The linacs were strictly matched for broad fields on CAX. We compared output factors (OPFs) and effective field size, measured concurrently using a novel 2D solid‐state dosimeter “Duo” with a spatial resolution of 0.2 mm, in square and rectangular static fields with sides from 0.5 to 2 cm, either on axis or away from it by 5 to 15 cm.ResultsAmong the four linacs, OPF for fields ≥1 × 1 cm2 ranged 1.3% on CAX, whereas off axis a maximum range of 1.9% was observed at 15 cm. A larger variability in OPF was noted for the 0.5 × 0.5 cm2 field, with a range of 5.9% on CAX, which improved to a maximum of 2.3% moving off axis. Two linacs showed greater consistency with a range of 1.4% on CAX and 2.2% at 15 cm off axis. Between linacs, the effective field size varied by <0.04 cm in most cases, both on and off axis. Tighter matching was observed for linacs with a similar focal spot position.ConclusionsVerification of small‐field consistency for matched linacs used for SRS is an important task for dosimetric validation. A significant benefit of concurrent measurement of field size and OPF allowed for a comprehensive assessment using a novel diode array. Our study showed the four linacs, strictly matched for broad fields on CAX, were still matched down to a field size of 1 x 1 cm2 on and off axis.
Highlights
In stereotactic radiosurgery (SRS), small highly modulated beams are used to deliver ablative doses to brain metastases in single or hypo-fractionated regimens.[1]
output factors (OPFs) were reported as a % of that in the nominal square field of side 10 cm on central axis (CAX)
OPF are pictured in Figures 2–5 as a function of Seq, eff
Summary
In stereotactic radiosurgery (SRS), small highly modulated beams are used to deliver ablative doses to brain metastases in single or hypo-fractionated regimens.[1] Stereotactic radiosurgery is an established treatment for patients with one to four metastases.[2,3,4] Prospective and retrospective studies have reported on its feasibility and clinical benefit for patients with ≥5 metastases.[5,6,7]. Stereotactic radiosurgery can be delivered with a standard C-arm linear accelerator (linac) equipped with a high-definition multileaf collimator (HD-MLC), a 6 degree-of-freedom (DoF) robotic treatment couch,[8] and a system for image guidance. For patients with more than one metastasis, treatment cost and time can be minimized with multitarget single-isocenter (MTSI) planning,[9,10,11] where the isocenter is placed between targets. Targets can range from small (0.7 cc) to larger volumes (30 cc),[12] and can be located up to 10 cm from the isocenter.[13]
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