Dosimetric characterization of Elekta stereotactic cones

Abstract

PurposeDosimetry of small fields defined by stereotactic cones remains a challenging task. In this work, we report the results of commissioning measurements for the new Elekta stereotactic conical collimator system attached to the Elekta VersaHD linac and present the comparison between the measured and Monte Carlo (MC) calculated data for the 6 MV FFF beam. In addition, relative output factor (ROF) dependence on the stereotactic cone aperture variation was studied and penumbra comparison for small MLC‐based and cone‐based fields was performed.MethodsCones with nominal diameters of 15 mm, 12.5 mm, 10 mm, 7.5 mm, and 5 mm were employed in our study. Percentage depth dose (PDD), off‐axis ratios (OAR), and ROF were measured using a stereotactic field diode (SFD). BEAMnrc code was used for MC simulations.ResultsMC calculated and measured PDDs for all cones agreed within 1%/0.5 mm, and OAR profiles agreed within 1%/0.5 mm. ROF obtained from the measurements and MC calculations agreed within 2% for all cone sizes. Small‐field correction factors for the SFD detector Kfield,3 × 3(SFD) were derived using MC calculations as a baseline and were found to be 0.982, 0.992, 0.997, 1.015, and 1.017 for the 5, 7.5, 10, 12.5, and 15‐mm cones respectively. The difference in ROF was about 10%, 6%, 3.5%, 3%, 2.5%, and 2% for ±0.3 mm variations in 5, 7.5, 10, 12.5, and 15‐mm cone aperture respectively. In case of single static field, cone‐based collimation produced a sharper penumbra compared to the MLC‐based.ConclusionsAccurate MC simulation can be an effective tool for verification of dosimetric measurements of small fields. Due to the very high sensitivity of output factors on the cone diameter, manufacture‐related variations in cone size may lead to considerable variations in dosimetric characteristics of stereotactic cones.

Evaluation of the IAEA‐TRS 483 protocol for the dosimetry of small fields (square and stereotactic cones) using multiple detectors

“…The average of six measurements for each detector showed good agreement overall, with the 5 mm cone yielding the largest variation (1.4%), with an RSD of 0.7% (Table ). The remaining cone sizes differed by ≤0.4% (RSD ≤ 0.3%), and agrees with the fact that smaller sized cones are known to yield higher variations in their output factor response and dosimetric characteristics . Interestingly, the results for the 5 mm cones showed a difference in output factors which may be related to the detectors production date.…”

“…The remaining cone sizes differed by ≤0.4% (RSD ≤ 0.3%), and agrees with the fact that smaller sized cones are known to yield higher variations in their output factor response and dosimetric characteristics. 11,21 Interestingly, the results for the 5 mm cones showed a difference in output factors which may be related to the detectors production date. Two microDiamond detectors with 123000 serial numbers produced slightly lower values of 0.683 and 0.680, while the 122000 serial numbered detectors yielded values of 0.690 and 0.691.…”

SciFi detector and associated method for real‐time determination of profile and output factor for small fields in stereotactic radiotherapy

“…The use of cones is typically preferable for targets smaller than the leaf width since they provide higher mechanical stability and sharper dose falloff compared to micro-MLC. 4 Due to the profound clinical consequences of incorrect beam data, there has been a strong demand for systematic and independent assessment of small radiation fields during commissioning and periodic quality assurance (QA) procedures. 5 However, such procedures are time-consuming tasks to implement and therefore remain particularly challenging.…”

“…3 To address these problems, there has been a very dynamic research activity over the last decade, in particular concerning the determination of the output factor (OF) which is a critical dosimetric parameter for the characterization of small fields used in radiotherapy. 4,[6][7][8][9] Correction factors have been proposed for small-field dosimetry based on solid-state detectors such as diodes and microdiamonds. 8,10,11 However, in the recent joint IAEA -AAPM publication of the International Code of Practice TRS-483, the use of detectors that exhibit output correction factors close to unity is recommended for the field output factor determination.…”

“Characterization of ELEKTA SRS cone collimator using high spatial resolution monolithic silicon detector array”

“…However, this method is complicated if done manually and is not practical in clinical setting as large number of profiles require long postprocessing time . Hence, different scientific papers have focused on finding proper correction factors to minimize the over‐response of the different diodes by studying their response for different beam qualities, field sizes, and types of LINACs in comparison with EBT3 and Monte Carlo simulation …”