Purpose Stereotactic 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. Methods We 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. Results Among 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. Conclusions Verification 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.

Matching multiple linacs to common baseline data allows patients to be treated, and patient‐specific quality assurance (PSQA) to be completed on any linac. Stereotactic body radiotherapy (SBRT) requires higher levels of accuracy and quality assurance than routine radiotherapy. The achieved linac matching must therefore be evaluated before distributive treatment or PSQA models can be implemented safely. This investigation aimed to propose metrics for defining linacs to be matched for SBRT deliveries, assess 12 linacs against these criteria, and determine if a distributive PSQA model could be implemented by reviewing the rates of false PSQA results. Ten SBRT spine plans were delivered by 12 matched Elekta linacs and measured using one of seven SRS MapCHECK devices. For gamma criteria of (3%, 2 mm), 96.9% of equivalent location detectors, showed a range of gamma ≤ 1.0 and 99.9% showed a standard deviation of ≤ 0.5. For criteria of (3%,1 mm) and (2%,1 mm), these ranges decreased to 92.1% and 80.2% while the standard deviations decreased to 99.3% and 95.7%, respectively. The dose differences showed that 43.6%, 82.7%, and 91.4% of detectors had a dose range of ≤ 3.0%, ≤ 5.0%, and ≤ 6.0%, respectively. Standard deviations of dose differences were 1.5%, 2.5%, and 3.0% for 94.1%, 98.3%, and 99.5% of detectors, respectively. For the fleet of linacs, distributive PSQA yielded false results for 0.0%, 17.7%, and 33.0% of plans, equivalent to 1.2%, 3.5%, and 9.4% of detectors when using gamma criteria of (3%,2 mm), (3%,1 mm), or (2%,1 mm), respectively. These linacs could be considered matched for SBRT treatments and implement a distributive PSQA model when gamma analysis was completed with a criterion of (3%, 2 mm). For stricter criterion of (3%,1 mm) or (2%,1 mm), they did not meet the proposed metrics.

Purpose: An investigation was conducted of an elastomeric material, VisiJet M2 (3D systems, USA) for use as 3D bolus within high energy photon beams for radiotherapy. Personalized conformal bolus material on complex structures like the nose can be challenging. This material was evaluated for its clinical feasibility due to its pliability and comfort compared to alternatives. Method: Regular slabs of bolus were created of various thicknesses for dosimetric and non-dosimetric characterization. Verification culminated with the creation of a custom nose bolus for an end to end verification using an anthropomorphic head phantom. In vivo dosimetry using Gafchromic EBT3 (Ashland, USA) film validated delivered doses from a 6 MV conformal field and a pair of 6 MV volumetric modulated arc therapy (VMAT) beams. Results & Conclusion: Non-dosimetric and dosimetric tests were conducted to assess clinical suitability. The bolus was precisely created using stereolithographic (SLA) methods and presented a compliant and uniform water equivalent material with elastic memory. Measurement yielded a physical density of 1.10 g cm −3 and 1.06 relative to water electron density, and the bolus to skin distance was measured to be a maximum of 3 mm. A maximum measured dose difference of<2% was observed for dynamic treatment. Based on the investigation conducted, and the benefits presented for patient comfort while being uniform and water equivalent, and correctly represented within the treatment planning system (TPS), this material has the potential for clinical use for patient specific custom bolus.

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