Detection of rotational errors in single-isocenter multiple-target radiosurgery: Is a routine off-axis Winston-Lutz test necessary?

PURPOSE: Recently the use of linear accelerator (linac)-based stereotactic radiosurgery (SRS) has increased, including single-isocenter multiple-target SRS. The workload of medical physicists has grown as a result and so has the necessity of maximizing the efficiency of quality assurance (QA). This study aimed to determine if measurement-based patient-specific QA with a high-spatial-resolution dosimeter is sensitive to rotational errors, potentially reducing the need for routine off-axis Winston-Lutz (WL) testing. METHODS: The impact of rotational errors along gantry, couch, and collimator axes on dose coverage of the gross tumor volume (GTV) and planning target volume (PTV) was determined with a 1-mm GTV/PTV expansion margin. Two techniques, the off-axis WL test using the StereoPHAN MultiMet-WL Cube (Sun Nuclear Corporation, Melbourne, Florida, USA) and patient-specific QA using the SRS MapCHECK (Sun Nuclear Corporation, Melbourne, Florida, USA), were assessed on their ability to detect introduced errors before target coverage was compromised. These findings were also considered in the context of routine machine QA of rotational axis calibrations. RESULTS: Rotational errors significantly impacted PTV dose coverage, especially in the couch angle. GTV dose coverage remained unaffected except for with large couch angle errors (≥1.5°). The off-axis WL test was shown to be sensitive to rotational errors with results consistently exceeding tolerance levels when or before coverage fell below departmentally accepted limits. Although patient-specific QA using the SRS MapCHECK was previously validated for SRS, this study showed inconsistency in detection of rotational errors. CONCLUSIONS: It is recommended that off-axis WL testing be conducted regularly to supplement routine monthly machine QA, as it is sensitive to errors that patient-specific QA may not detect. This frequency should be determined by individual departments, with consideration of GTV-PTV margins used, limitations on target off-axis distances, and routine mechanical QA results for particular linacs.

The use of collimator angle optimization and jaw tracking for VMAT-based single-isocenter multiple-target stereotactic radiosurgery for up to six targets in the Varian Eclipse treatment planning system.

PURPOSE: Island blocking occurs in single-isocenter multiple-target (SIMT) stereotactic radiotherapy (SRS) whenever targets share multi-leaf collimator (MLC) leaf pairs. This study investigated the effect on plan quality and delivery, of reducing island blocking through collimator angle optimization (CAO). In addition, the effect of jaw tracking in this context was also investigated. METHODS: For CAO, an algorithm was created that selects the collimator angle resulting in the lowest level of island blocking, for each beam in any given plan. Then, four volume-modulated arc therapy (VMAT) SIMT SRS plans each were generated for 10 retrospective patients: two CAO plans, with and without jaw tracking, and two plans with manually selected collimator angles, with and without jaw tracking. Plans were then assessed and compared using typical quality assurance procedures. RESULTS: There were no substantial differences between plans with and without CAO. Jaw tracking produced statistically significant reduction in low-dose level parameters; healthy brain V10% and mean dose were reduced by 9.66% and 15.58%, respectively. However, quantitative values (108 cc for V10% and 0.35 Gy for mean dose) were relatively small in relation to clinical relevance. Though there were no statistically significant changes in plan deliverability, there was a notable trend of plans with jaw tracking having lower gamma analysis pass rates. CONCLUSION: These findings suggest that CAO has limited benefit in VMAT SIMT SRS of 2-6 targets when using a low-dose penalty to the healthy brain during plan optimization in Eclipse. As clinical benefits of jaw tracking were found to be minimal and plan deliverability was potentially reduced, a cautious approach would be to exclude jaw tracking in SIMT SRS plans.

Consistency of small-field dosimetry, on and off axis, in beam-matched linacs used for stereotactic radiosurgery.

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.

eXaSkin: A novel high-density bolus for 6MV X-rays radiotherapy.

PURPOSE: To evaluate eXaSkin, a novel high-density bolus alternative to commercial tissue-equivalent Superflab, for 6MV photon-beam radiotherapy. MATERIALS AND METHODS: We delivered a 10 × 10 cm2 open field at 90° and head-and-neck clinical plan, generated with the volumetric modulated arc therapy (VMAT) technique, to an anthropomorphic phantom in three scenarios: with no bolus on the phantom's surface, with Superflab, and with eXaSkin. In each scenario, we measured dose to a central planning target volume (PTV) in the nasopharynx region with an ionization chamber, and we measured dose to the skin, at three different positions within the vicinity of a neck lymph node PTV, with MOSkin™, a semiconductor dosimeter. Measurements were compared against calculations with the treatment planning system (TPS). RESULTS: For the static field, MOSkin results underneath the eXaSkin were in agreement with calculations to within 1.22%; for VMAT, to within 5.68%. Underneath Superflab, those values were 3.36% and 11.66%. The inferior agreement can be explained by suboptimal adherence of Superflab to the phantom's surface as well as difficulties in accurately reproducing its placement between imaging and treatment session. In all scenarios, dose measured at the central target agreed to within 1% with calculations. CONCLUSIONS: eXaSkin was shown to have superior adaptation to the phantom's surface, producing minimal air gaps between the skin surface and bolus, allowing for accurate positioning and reproducibility of set-up conditions. eXaSkin with its high density material provides sufficient build-up to achieve full skin dose with less material thickness than Superflab.

Semiconductor dosimetry in modern external-beam radiation therapy.

Semiconductor dosimeters are ubiquitous in modern external-beam radiation therapy. They possess key features. The response, electronically available in real time, is stable and linear with absorbed dose for given irradiation conditions; the radiation-sensitive volume can be rather small in size, while retaining mechanical strength and high sensitivity. We describe three common semiconductor dosimeters: diodes, metal-oxide-semiconductor field-effect transistors and diamonds. We discuss in detail their operation principles and applications in modern external-beam radiation therapy, primarily with megavoltage photon beams. We also explore their use in proton and heavy ion therapy, and in experimental radiotherapy techniques such as synchrotron-based micro-beam radiation therapy.

A novel add-on collimator for preclinical radiotherapy applications using a standard cell irradiator: design, construction, and validation.

PURPOSE: Preclinical radiotherapy applications require dedicated irradiation systems which are expensive and not widely available. In this work, a clinical dual source 137 Cs cell irradiator was adapted to deliver 1-cm diameter preclinical treatment beams using a lead and stainless steel custom-made collimator to treat one or two mice at a time. METHODS: The dosimetric characteristics of all the different components of the system (including collimator, phantoms, and radiation sources) have been simulated with EGSnrc Monte Carlo methods. The collimator was constructed based on these simulations and the calculated results were verified against dosimetric measurements with MOSKin detectors, GAFchromic films, and dosimetric gels. RESULTS: The comparisons showed an agreement, in terms of full width half maximum values, between the simulated and the measured dose cross profiles at the midline within 4% for both gel dosimetry and GAFchromic films. Out of beam dose, measured in air at the collimator midplane with MOSFET detectors was between 6% and 10% of the beam axis dose. The dimensions of the beam are constant along the vertical axis of the collimator and also the simulated and measured Percentage Depth Dose (PDD) curves show an agreement within 1%. CONCLUSIONS: The collimator design developed in this work allows the creation of a beam with the necessary characteristics for ablative radiotherapy treatments on small animals using a standard clinical cell irradiator. This collimator design will make advanced preclinical studies with ablative beams possible for all those institutions which do not have collimated preclinical irradiators available.

Validation of linear energy transfer computed in a Monte Carlo dose engine of a commercial treatment planning system.

The relative biological effectiveness (RBE) of protons is highly variable and difficult to quantify. However, RBE is related to the local ionization density, which can be related to the physical measurable dose weighted linear energy transfer (LETD). The aim of this study was to validate the LETD calculations for proton therapy beams implemented in a commercially available treatment planning system (TPS) using microdosimetry measurements and independent LETD calculations (Open-MCsquare (MCS)). The TPS (RayStation v6R) was used to generate treatment plans on the CIRS-731-HN anthropomorphic phantom for three anatomical sites (brain, nasopharynx, neck) for a spherical target (Ø = 5 cm) with uniform target dose to calculate the LETD distribution. Measurements were performed at the University Medical Center Groningen proton therapy center (Proteus Plus, IBA) using a µ +-probe utilizing silicon on insulator microdosimeters capable of detecting lineal energies as low as 0.15 keV µm-1 in tissue. Dose averaged mean lineal energy [Formula: see text] depth-profiles were measured for 70 and 130 MeV spots in water and for the three treatment plans in water and an anthropomorphic phantom. The [Formula: see text] measurements were compared to the LETD calculated in the TPS and MCS independent dose calculation engine. D · [Formula: see text] was compared to D · LETD in terms of a gamma-index with a distance-to-agreement criteria of 2 mm and increasing dose difference criteria to determine the criteria for which a 90% pass rate was accomplished. Measurements of D · [Formula: see text] were in good agreement with the D · LETD calculated in the TPS and MCS. The 90% passing rate threshold was reached at different D · LETD difference criteria for single spots (TPS: 1% MCS: 1%), treatment plans in water (TPS: 3% MCS: 6%) and treatment plans in an anthropomorphic phantom (TPS: 6% MCS: 1%). We conclude that D · LETD calculations accuracy in the RayStation TPS and open MCSquare are within 6%, and sufficient for clinical D · LETD evaluation and optimization. These findings remove an important obstacle in the road towards clinical implementation of D · LETD evaluation and optimization of proton therapy treatment plans. Novelty and significance The dose weighed linear energy transfer (LETD) distribution can be calculated for proton therapy treatment plans by Monte Carlo dose engines. The relative biological effectiveness (RBE) of protons is known to vary with the LETD distribution. Therefore, there exists a need for accurate calculation of clinical LETD distributions. Previous LETD validations have focused on general purpose Monte Carlo dose engines which are typically not used clinically. We present the first validation of mean lineal energy [Formula: see text] measurements of the LETD against calculations by the Monte Carlo dose engines of the Raystation treatment planning system and open MCSquare.

Two-dimensional solid-state array detectors: A technique for in vivo dose verification in a variable effective area.

PURPOSE: We introduce a technique that employs a 2D detector in transmission mode (TM) to verify dose maps at a depth of dmax in Solid Water. TM measurements, when taken at a different surface-to-detector distance (SDD), allow for the area at dmax (in which the dose map is calculated) to be adjusted. METHODS: We considered the detector prototype "MP512" (an array of 512 diode-sensitive volumes, 2 mm spatial resolution). Measurements in transmission mode were taken at SDDs in the range from 0.3 to 24 cm. Dose mode (DM) measurements were made at dmax in Solid Water. We considered radiation fields in the range from 2 × 2 cm2 to 10 × 10 cm2 , produced by 6 MV flattened photon beams; we derived a relationship between DM and TM measurements as a function of SDD and field size. The relationship was used to calculate, from TM measurements at 4 and 24 cm SDD, dose maps at dmax in fields of 1 × 1 cm2 and 4 × 4 cm2 , and in IMRT fields. Calculations were cross-checked (gamma analysis) with the treatment planning system and with measurements (MP512, films, ionization chamber). RESULTS: In the square fields, calculations agreed with measurements to within ±2.36%. In the IMRT fields, using acceptance criteria of 3%/3 mm, 2%/2 mm, 1%/1 mm, calculations had respective gamma passing rates greater than 96.89%, 90.50%, 62.20% (for a 4 cm SSD); and greater than 97.22%, 93.80%, 59.00% (for a 24 cm SSD). Lower rates (1%/1 mm criterion) can be explained by submillimeter misalignments, dose averaging in calculations, noise artifacts in film dosimetry. CONCLUSIONS: It is possible to perform TM measurements at the SSD which produces the best fit between the area at dmax in which the dose map is calculated and the size of the monitored target.

Quality assurance of Cyberknife robotic stereotactic radiosurgery using an angularly independent silicon detector.

PURPOSE: The aim of this work was to evaluate the use of an angularly independent silicon detector (edgeless diodes) developed for dosimetry in megavoltage radiotherapy for Cyberknife in a phantom and for patient quality assurance (QA). METHOD: The characterization of the edgeless diodes has been performed on Cyberknife with fixed and IRIS collimators. The edgeless diode probes were tested in terms of basic QA parameters such as measurements of tissue-phantom ratio (TPR), output factor and off-axis ratio. The measurements were performed in both water and water-equivalent phantoms. In addition, three patient-specific plans have been delivered to a lung phantom with and without motion and dose measurements have been performed to verify the ability of the diodes to work as patient-specific QA devices. The data obtained by the edgeless diodes have been compared to PTW 60016, SN edge, PinPoint ionization chamber, Gafchromic EBT3 film, and treatment planning system (TPS). RESULTS: The TPR measurement performed by the edgeless diodes show agreement within 2.2% with data obtained with PTW 60016 diode for all the field sizes. Output factor agrees within 2.6% with that measured by SN EDGE diodes corrected for their field size dependence. The beam profiles' measurements of edgeless diodes match SN EDGE diodes with a measured full width half maximum (FWHM) within 2.3% and penumbra widths within 0.148 mm. Patient-specific QA measurements demonstrate an agreement within 4.72% in comparison with TPS. CONCLUSION: The edgeless diodes have been proved to be an excellent candidate for machine and patient QA for Cyberknife reproducing commercial dosimetry device measurements without need of angular dependence corrections. However, further investigation is required to evaluate the effect of their dose rate dependence on complex brain cancer dose verification.

CyberKnife® fixed cone and Iris™ defined small radiation fields: Assessment with a high-resolution solid-state detector array.

PURPOSE: The challenges of accurate dosimetry for stereotactic radiotherapy (SRT) with small unflattened radiation fields have been widely reported in the literature. In this case, suitable dosimeters would have to offer a submillimeter spatial resolution. The CyberKnife® (Accuray Inc., Sunnyvale, CA, USA) is an SRT-dedicated linear accelerator (linac), which can deliver treatments with submillimeter positional accuracy using circular fields. Beams are delivered with the desired field size using fixed cones, the InCise™ multileaf collimator or a dynamic variable-aperture Iris™ collimator. The latter, allowing for field sizes to be varied during treatment delivery, has the potential to decrease treatment time, but its reproducibility in terms of output factors (OFs) and dose profiles (DPs) needs to be verified. METHODS: A 2D monolithic silicon array detector, the "Octa", was evaluated for dosimetric quality assurance (QA) for a CyberKnife system. OFs, DPs, percentage depth-dose (PDD) and tissue maximum ratio (TMR) were investigated, and results were benchmarked against the PTW SRS diode. Cross-plane, in-plane and 2 diagonal dose profiles were measured simultaneously with high spatial resolution (0.3 mm). Monte Carlo (MC) simulations with a GEANT4 (GEometry ANd Tracking 4) tool-kit were added to the study to support the experimental characterization of the detector response. RESULTS: For fixed cones and the Iris, for all field sizes investigated in the range between 5 and 60 mm diameter, OFs, PDDs, TMRs, and DPs in terms of FWHM measured by the Octa were accurate within 3% when benchmarked against the SRS diode and MC calculations. CONCLUSIONS: The Octa was shown to be an accurate dosimeter for measurements with a 6 MV FFF beam delivered with a CyberKnife system. The detector enabled real-time dosimetric verification for the variable aperture Iris collimator, yielding OFs and DPs consistent with those obtained with alternative methods.