Intensity-modulated Radiosurgery Research Articles

Using machine learning to predict gamma passing rate in volumetric-modulated arc therapy treatment plans.

This study aims to develop an algorithm to predict gamma passing rate (GPR) in the volumetric-modulated arc therapy (VMAT) technique. A total of 118 clinical VMAT plans, including 28 mediastina, 25 head and neck, 40 brains intensity-modulated radiosurgery, and 25 prostate cases, were created in RayStation treatment planning system for Edge and TrueBeam linacs. In-house scripts were developed to compute Modulation indices such as plan-averaged beam area (PA), plan-averaged beam irregularity (PI), total monitor unit (MU), leaf travel/arc length, mean dose rate variation, and mean gantry speed variation. Pretreatment verifications were performed on ArcCHECK phantom with SNC software. GPR was calculated with 3%/2mm and 10% threshold. The dataset was randomly split into a training (70%) and a test (30%) dataset. A random forest regression (RFR) model and support vector regression (SVR) with linear kernel were trained to predict GPR using the complexity metrics as input. The prediction performance was evaluated by calculating the mean absolute error (MAE), R2 , and root mean square error (RMSE). RMSEs at γ 3%/2mm for RFR and SVR were 1.407±0.103 and 1.447±0.121, respectively. MAE was 1.14±0.084 for RFR and 1.101±0.09 for SVR. R2 was equal to 0.703±0.027 and 0.689±0.053 for RFR and SVR, respectively. GPR of 3%/2mm with a 10% threshold can be predicted with an error smaller than 3% for 94% of plans using RFR and SVR models. The most important metrics that had the greatest impact on how accurately GPR can be predicted were determined to be the PA, PI, and total MU. In terms of its prediction values and errors, SVR (linear) appeared to be comparable with RFR for this dataset. Based on our results, the PA, PI, and total MU calculations may be useful in guiding VMAT plan evaluation and ultimately reducing uncertainties in planning and radiation delivery.

Evaluation of parameters affecting gamma passing rate in patient-specific QAs for multiple brain lesions IMRS treatments using ray-station treatment planning system.

PurposeUsing intensity‐modulated radiosurgery (IMRS) with single isocenter for the treatment of multiple brain lesions has gained acceptance in recent years. One of the challenges of this technique is conducting a patient‐specific quality assurance (QA), involving accurate gamma passing rate (GPR) calculations for small and wide spread‐out targets. We evaluated effects of parameters such as dose grid and energy on GPR using our clinical IMRS plans.MethodsTen patients with total of 40 volumetric modulated arc therapy (VMAT) plans were created in Raystation (V.8A) treatment planning system (TPS) for the Varian Edge Linac using 6 and 10 flattening filter‐free (FFF) beams and planned dose grids of 1 mm and 2 mm resulting in four plans with 6–10 targets per patient. All parameters and objectives except dose grid and energy were kept the same in all plans. Next, patient‐specific QAs were measured evaluating GPR with 10% threshold, 3%/3 mm objective, and an acceptance criterion of 95%. Modulation factors (MF) and confidence intervals were calculated. Two modes of measurements, standard density (SD) and high density (HD), were used.ResultsGenerally, plans computed with 1 mm dose grid have higher GPRs than those with 2 mm dose grid for both energies used. The GPRs of 6 FFF plans were higher than those of 10 FFF plans. GPR showed no noticeable difference between HD and SD measurements. Negative correlation between MF and GPR was observed. The HD pass rates fall within the confidence interval of SD.ConclusionCalculated dose grid should be less than or equal to one‐third of distance to agreement, thus 1 mm planned dose grid is recommended to reduce artifacts in gamma calculation. GPR of SD and HD measurement modes is almost the same, which indicates that SD mode is clinically preferable for performing patient‐specific QAs. According to our results, using 6 FFF beams with 1 mm planned dose grid is more accurate and reliable for dose calculation of IMRS plans.

Intensity modulated operating mode of the rotating gamma system.

The purpose of this work was to explore two novel operation modalities of the rotating gamma systems (RGS) that could expand its clinical application to lesions in close proximity to critical organs at risk (OAR). The approach taken in this study consists of two components. First, a Geant4-based Monte Carlo (MC) simulation toolkit is used to model the dosimetric properties of the RGS Vertex 360™ for the normal, intensity modulated radiosurgery (IMRS), and speed modulated radiosurgery (SMRS) operation modalities. Second, the RGS Vertex 360™ at the Rotating Gamma Institute in Debrecen, Hungary is used to collect experimental data for the normal and IMRS operation modes. An ion chamber is used to record measurements of the absolute dose. The dose profiles are measured using Gafchromic EBT3 films positioned within a spherical water equivalent phantom. A strong dosimetric agreement between the measured and simulated dose profiles and penumbra was found for both the normal and IMRS operation modes for all collimator sizes (4, 8, 14, and 18 mm diameter). The simulated falloff and maximum dose regions agree better with the experimental results for the 4 and 8 mm diameter collimators. Although the falloff regions align well in the 14 and 18 mm collimators, the maximum dose regions have a larger difference. For the IMRS operation mode, the simulated and experimental dose distributions are ellipsoidal, where the short axis aligns with the blocked angles. Similarly, the simulated dose distributions for the SMRS operation mode also adopt an ellipsoidal shape, where the short axis aligns with the angles where the orbital speed is highest. For both modalities, the dose distribution is highly constrained with a sharper penumbra along the short axes. Dose modulation of the RGS can be achieved with the IMRS and SMRS modes. By providing a highly constrained dose distribution with a sharp penumbra, both modes could be clinically applicable for the treatment of lesions in close proximity to critical OARs.

Targeting accuracy of single-isocenter intensity-modulated radiosurgery for multiple lesions

To investigate the targeting accuracy of intensity-modulated SRS (IMRS) plans designed to simultaneously treat multiple brain metastases with a single isocenter. A home-made acrylic phantom able to support a film (EBT3) in its coronal plane was used. The phantom was CT scanned and three coplanar small targets (a central and two peripheral) were outlined in the Eclipse system. Peripheral targets were 6 cm apart from the central one. A reference IMRS plan was designed to simultaneously treat the three targets, but only a single isocenter located at the center of the central target was used. After positioning the phantom on the linac using the room lasers, a CBCT scan was acquired and the reference plan were mapped on it, by placing the planned isocenter at the intersection of the landmarks used in the film showing the linac isocenter. The mapped plan was then recalculated and delivered. The film dose distribution was derived using a cloud computing application (www.radiochromic.com) that uses a triple-channel dosimetry algorithm. Comparison of dose distributions using the gamma index (5%/1 mm) were performed over a 5 × 5 cm2 region centered over each target. 2D shifts required to get the best gamma passing rates on the peripheral target regions were compared with the reported ones for the central target. The experiment was repeated ten times in different sessions. Average 2D shifts required to achieve optimal gamma passing rates (99%, 97%, 99%) were 0.7 mm (SD: 0.3 mm), 0.8 mm (SD: 0.4 mm) and 0.8 mm (SD: 0.3 mm), for the central and the two peripheral targets, respectively. No statistical differences (p > 0.05) were found for targeting accuracy between the central and the two peripheral targets. The study revealed a targeting accuracy within 1 mm for off-isocenter targets within 6 cm of the linac isocenter, when a single-isocenter IMRS plan is designed.

Evaluation of Dose Distribution in Intensity Modulated Radiosurgery for Lung Cancer under Condition of Respiratory Motion.

The dose of a real tumor target volume and surrounding organs at risk (OARs) under the effect of respiratory motion was calculated for a lung tumor plan, based on the target volume covering the whole tumor motion range for intensity modulated radiosurgery (IMRS). Two types of IMRS plans based on simulated respiratory motion were designed using humanoid and dynamic phantoms. Delivery quality assurance (DQA) was performed using ArcCHECK and MapCHECK2 for several moving conditions of the tumor and the real dose inside the humanoid phantom was evaluated using the 3DVH program. This evaluated dose in the tumor target and OAR using the 3DVH program was higher than the calculated dose in the plan, and a greater difference was seen for the RapidArc treatment than for the standard intensity modulated radiation therapy (IMRT) with fixed gantry angle beams. The results of this study show that for IMRS plans based on target volume, including the whole tumor motion range, tighter constraints of the OAR should be considered in the optimization process. The method devised in this study can be applied effectively to analyze the dose distribution in the real volume of tumor target and OARs in IMRT plans targeting the whole tumor motion range.

EP-1656: Feasibility of an “off-target isocenter” technique for cranial intensity-modulated radiosurgery

EP-1656: Feasibility of an “off-target isocenter” technique for cranial intensity-modulated radiosurgery

Dosimetric comparison of intensity modulated radiosurgery with dynamic conformal arc radiosurgery for small cranial lesions.

To dosimetrically compare the fixed gantry intensity modulated radiosurgery (IMRS) with dynamic conformal arc radiosurgery (DCARS) for cranial lesions. This study investigates whether IMRS can be an adequate dosimetric alternative to DCARS for cranial stereotactic radiosurgery (SRS). Forty-five SRS procedures for solitary brain metastasis (range: 0.44-29.18 cm 3) performed at our institution were selected for this study. Two plans were generated per patient: One IMRS plan using a multileaf collimation (MLC) of 5 mm, and one DCARS plan designed with a 3 mm micro-MLC. Dosimetric comparison metrics include the target coverage (Cov), conformity index (CI), homogeneity index (HI), gradient index (GI), and volume of the normal brain tissue receiving ≥12 Gy (V12). In addition, maximum doses to organs at risk (OAR) (brainstem, optic apparatus and cochlea) were compared for both techniques. Compared to DCARS, IMRS improved mean CI (IMRS: 0.81 vs. 0.63, P < 0.001), with no significant difference in target Cov (IMRS: 0.99 vs. 0.99, P > 0.05), HI (IMRS: 1.22 vs. 1.24, P > 0.05), GI (IMRS: 5.44 vs. 5.44, P > 0.05). A weak significant difference in V12 (IMRS: 4.6 cm 3 vs. 5.2 cm 3, P = 0.033) was obtained. Subgroup analysis per target volume (small: <1 cm 3, intermediate: ≤1 cm 3 and <5 cm 3 and large: ≥5 cm 3) only revealed the statistically difference for CI metric (P < 0.001). No significant differences were found for maximum dose to the OAR. We have shown that IMRS provides the dosimetric advantages compared with DCARS. Based on the dosimetric findings in this study, fixed gantry IMRS technique can be adopted as a standard procedure for cranial SRS when micro-MLC technology is not available on the linear accelerator.

Optimal technique of linear accelerator–based stereotactic radiosurgery for tumors adjacent to brainstem

Stereotactic radiosurgery (SRS) is a well-established technique that is replacing whole-brain irradiation in the treatment of intracranial lesions, which leads to better preservation of brain functions, and therefore a better quality of life for the patient. There are several available forms of linear accelerator (LINAC)–based SRS, and the goal of the present study is to identify which of these techniques is best (as evaluated by dosimetric outcomes statistically) when the target is located adjacent to brainstem. We collected the records of 17 patients with lesions close to the brainstem who had previously been treated with single-fraction radiosurgery. In all, 5 different lesion catalogs were collected, and the patients were divided into 2 distance groups—1 consisting of 7 patients with a target-to-brainstem distance of less than 0.5cm, and the other of 10 patients with a target-to-brainstem distance of ≥ 0.5 and < 1cm. Comparison was then made among the following 3 types of LINAC-based radiosurgery: dynamic conformal arcs (DCA), intensity-modulated radiosurgery (IMRS), and volumetric modulated arc radiotherapy (VMAT). All techniques included multiple noncoplanar beams or arcs with or without intensity-modulated delivery. The volume of gross tumor volume (GTV) ranged from 0.2cm3 to 21.9cm3. Regarding the dose homogeneity index (HIICRU) and conformity index (CIICRU) were without significant difference between techniques statistically. However, the average CIICRU = 1.09 ± 0.56 achieved by VMAT was the best of the 3 techniques. Moreover, notable improvement in gradient index (GI) was observed when VMAT was used (0.74 ± 0.13), and this result was significantly better than those achieved by the 2 other techniques (p < 0.05). For V4Gy of brainstem, both VMAT (2.5%) and IMRS (2.7%) were significantly lower than DCA (4.9%), both at the p < 0.05 level. Regarding V2Gy of normal brain, VMAT plans had attained 6.4 ± 5%; this was significantly better (p < 0.05) than either DCA or IMRS plans, at 9.2 ± 7% and 8.2 ± 6%, respectively. Owing to the multiple arc or beam planning designs of IMRS and VMAT, both of these techniques required higher MU delivery than DCA, with the averages being twice as high (p < 0.05). If linear accelerator is only 1 modality can to establish for SRS treatment. Based on statistical evidence retrospectively, we recommend VMAT as the optimal technique for delivering treatment to tumors adjacent to brainstem.

Treatment Plan Technique and Quality for Single-Isocenter Stereotactic Ablative Radiotherapy of Multiple Lung Lesions with Volumetric-Modulated Arc Therapy or Intensity-Modulated Radiosurgery.

The aim of this study is to provide a practical approach to the planning technique and evaluation of plan quality for the multi-lesion, single-isocenter stereotactic ablative radiotherapy (SABR) of the lung. Eleven patients with two or more lung lesions underwent single-isocenter volumetric-modulated arc therapy (VMAT) radiosurgery or IMRS. All plans were normalized to the target maximum dose. For each plan, all targets were treated to the same dose. Plan conformity and dose gradient were maximized with dose-control tuning structures surrounding targets. For comparison, multi-isocenter plans were retrospectively created for four patients. Conformity index (CI), homogeneity index (HI), gradient index (GI), and gradient distance (GD) were calculated for each plan. V5, V10, and V20 of the lung and organs at risk (OARs) were collected. Treatment time and total monitor units (MUs) were also recorded. One patient had four lesions and the remainder had two lesions. Six patients received VMAT and five patients received intensity-modulated radiosurgery (IMRS). For those treated with VMAT, two patients received 3-arc VMAT and four received 2-arc VMAT. For those treated with IMRS, two patients were treated with 10 and 11 beams, respectively, and the rest received 12 beams. Prescription doses ranged from 30 to 54 Gy in three to five fractions. The median prescribed isodose line was 84% (range: 80–86%). The median maximum dose was 57.1 Gy (range: 35.7–65.1 Gy). The mean combined PTV was 49.57 cm3 (range: 14.90–87.38 cm3). For single-isocenter plans, the median CI was 1.15 (range: 0.97–1.53). The median HI was 1.19 (range: 1.16–1.28). The median GI was 4.60 (range: 4.16–7.37). The median maximum radiation dose (Dmax) to total lung was 55.6 Gy (range: 35.7–62.0 Gy). The median mean radiation dose to the lung (Dmean) was 4.2 Gy (range: 1.1–9.3 Gy). The median lung V5 was 18.7% (range: 3.8–41.3%). There was no significant difference in CI, HI, GI, GD, V5, V10, and V20 (lung, heart, trachea, esophagus, and spinal cord) between single-isocenter and multi-isocenter plans. This multi-lesion, single-isocenter lung SABR planning technique demonstrated excellent plan quality and clinical efficiency and is recommended for radiosurgical treatment of two or more lung targets for well-suited patients.

Heterogeneity correction for intensity-modulated frameless SRS in pituitary and cavernous sinus tumors: a retrospective study.

BackgroundFrameless immobilization allows for planning and quality assurance of intensity-modulated radiosurgery (IM-SRS) plans. We tested the hypothesis that IM-SRS planning with uniform tissue density corrections results in dose inaccuracy compared to heterogeneity-corrected algorithms.MethodsFifteen patients with tumors of the pituitary or cavernous sinus underwent frameless IM-SRS. Treatment planning CT and MRI scans were obtained and fused to delineate the tumor, optic nerves, chiasm, and brainstem. The plan was developed with static gantry IM-SRS fields using a pencil beam (PB), analytical anisotropic (AAA), and Acuros XB (AXB) algorithms. We evaluated measures of target coverage as well as doses to organs at risk (OAR) for each algorithm. We compared the results of each algorithm in the cases where PTV overlapped OAR (n = 10) to cases without overlapping OAR with PTV (n = 5). Utilizing film dosimetry, we measured the dose distribution for each algorithm through a uniform density target to a rando phantom with non-uniform density of air, tissue, and bone.ResultsThere was no difference in target coverage measured by DMaxPTV, DMinPTV, D95%PTV, or the isodose surface (IDS) covering 95 % of the PTV regardless of algorithm. However, there were differences in dose to OAR. PB predicted higher (p < 0.05) Dmax for the brainstem, chiasm, right optic nerve, and left optic nerve. In cases of PTV overlapping an optic nerve (n = 7), PB was unable to limit dose to 8Gy while achieving PTV coverage (PB 855 cGy vs. AAA 769 cGy, p = 0.05 vs. AXB 658 cGy, p = 0.03). Within the rando phantom, the PB and AAA algorithms over-estimated the dose delivered in the bone-tissue-air interface of the sinus (+17 %), while the AXB algorithm closely predicted the actual dose delivered through the inhomogeneous tissue (+/- 1 % max, p < 0.05).ConclusionsPatients undergoing frameless SRS benefit from heterogeneity corrected dose plans when the lesion lies in areas of widely varying tissue density and near critical normal structures such as the skull base. Film dosimetry confirms that the AXB dose calculation algorithm more accurately predicts actual dose delivered though tissues of varying densities than PB or AAA dose calculation algorithms.

Dosimetric feasibility of an “off-target isocenter” technique for cranial intensity-modulated radiosurgery

To evaluate the dosimetric effect of placing the isocenter away from the planning target volume (PTV) on intensity-modulated radiosurgery (IMRS) plans to treat brain lesions. A total of 15 patients who received cranial IMRS at our institution were randomly selected. Each patient was treated with an IMRS plan designed with the isocenter located at the target center (plan A). A second off-target isocenter plan (plan B) was generated for each case. In all the plans,100% of the prescription dose covered 99% of the target volume. The plans A and B were compared for the target dosage (conformity index [CI] and homogeneity index) and organs-at-risk (OAR) dose sparing. Peripheral dose falloff was compared by using the metrics volume of normal brain receiving more than 12-Gy dose (V12) and CI at the level of the 50% of the prescription dose (CI 50%). The values found for each metric (plan B vs plan A) were (mean ± standard deviation [SD]) as follows—CI: 1.28 ± 0.15 vs 1.28 ± 0.15, p = 0.978; homogeneity index (HI): 1.29 ± 0.14 vs 1.34 ± 0.17, p = 0.079; maximum dose to the brainstem: 2.95 ± 2.11 vs 2.89 ± 1.88Gy, p = 0.813; maximum dose to the optical pathway: 2.65 ± 4.18 vs 2.44 ± 4.03Gy, p = 0.195; and maximum dose to the eye lens: 0.33 ± 0.73 vs 0.33 ± 0.53Gy, p = 0.970. The values of the peripheral dose falloff were (plan B vs plan A) as follows—V12: 5.98 ± 4.95 vs 6.06 ± 4.92cm3, p = 0.622, and CI 50%: 6.08 ± 2.77 vs 6.28 ± 3.01, p = 0.119. The off-target isocenter solution resulted in dosimetrically comparable plans as the center-target isocenter technique, by avoiding the risk of gantry-couch collision during the cone beam computed tomography (CBCT) acquisition.

SU‐E‐T‐450: Dosimetric Impact of Rotational Error On Multiple‐Target Intensity‐Modulated Radiosurgery (IMRS) with Single‐Isocenter

Purpose:Evaluating the dosimetric‐impact on multiple‐targets placed away from the isocenter‐target with varying rotational‐error introduced by initial setup uncertainty and/or intrafractional‐movementMethods:CyberKnife‐Phantom was scanned with the Intracranial SRS‐protocol of 1.25mm slice‐thickness and the multiple‐targets(GTV) of 1mm and 10mm in diameter were contoured on the Eclipse. PTV for distal‐target only was drawn with 1mm expansion around the GTV to find out how much margin is needed to compensate for the rotational‐error. The separation between the isocenter‐target and distal‐target was varied from 3cm to 7cm. RapidArc‐based IMRS plans of 16Gy single‐fraction were generated with five non‐coplanar arcs by using Varian TrueBeam‐STx equipped with high resolution MLC leaves of 2.5mm at center and with dose‐rate of 1400MU/min at 6MV for flatteringfilter‐ free(FFF). An identical CT image with intentionally introduced 1° rotational‐error was registered with the planning CT image, and the isodose distribution and Dose‐Volume‐Histogram(DVH) were compared with the original plans. Additionally, the dosimetric‐impact of rotational error was evaluated with that of 6X photon energy which was generated with the same target‐coverage.Results:For the 1mm‐target with 6X‐FFF, PTV‐coverage(D100) of the distal‐target with 1° rotational‐error decreased from 1.00 to 0.35 as the separation between isocenter‐target and distal‐target increased from 3cm to 7cm. However, GTV‐coverage(D100) was 1.0 except that of 7cm‐separation(0.55), which resulted from the 1mm‐margin around the distal‐target. For 6X photon, GTV‐coverage remained at 1.0 regardless of the separation of targets, showing that the dosimetric‐impact of rotational error depends on the degree of rotational‐error, separation of targets, and dose distribution around targets. For 10mm‐target, PTV‐coverage of distaltarget located 3cm‐away was better than that of 1mm‐target(0.93 versus 0.7) and GTV‐coverage was 1.0 regardless of 6X‐FFF/6X photon beams.Conclusion:RapidArc‐based Multiple‐target IMRS may Resultin a compromised outcome due to the setup‐error for distal‐targets. With 1mm‐margin, GTV‐coverage of distal‐target was 1.0 although the separation of targets was up to 5cm.

Critical neurological structure sparing radiosurgery of vestibular schwannoma: dosimetric comparison of different techniques and dose prescription methods.

To investigate potential sparing of critical neurological structures (CNSs) during radiosurgery of vestibular schwannoma (VS) employing different techniques and dose prescription methods. Fused CT and MRI datasets of eight patients with unilateral VS representing a wide range of target volume (0.48 to 12.08 cc; mean = 3.56 cc), shape and proximity to CNSs such as cochlea, trigeminal nerve and brainstem were re-planned employing static conformal field (SCF), dynamic conformal arc (DCA) and intensity modulated radiosurgery (IMRS) techniques. For every patient, five plans were created for a fixed margin dose of 12 Gy prescribed at 80% in three plans (SCF_80%, DCA_80%, and IMRS_80%) and 50% in another two plans (SCF_50% and DCA_50%). All plans were compared using standard dosimetric indices. Primary goal of every plan to cover ≥99% of target volume with 12 Gy was fulfilled for all patients with minimum significant dose to target (D₉₉) ≥11.99 Gy. Best conformity index (CI Paddick = 0.62 ± 0.12) was observed in SCF_80% and DCA_80% plans whereas; sharpest dose gradient index of 3.40 ± 0.40 was resulted from DCA_50%. All five plans resulted similar maximum dose to brainstem (11.04 ± 2.23 to 11.53 ± 1.10 Gy), cochlea (9.02 ± 1.79 to 10.15 ± 1.26 Gy) and trigeminal nerve (11.55 ± 1.38 to 12.19 ± 2.12 Gy). Among 80% prescription plans, IMRS_80% reduces mean and D₅ (P < 0.05) to all CNSs. Prescription of dose at 50% isodose sharpened the dose gradient and significantly (P < 0.05) reduced mean dose and D₅ to all CNSs at the cost of target conformity (P = 0.01). Mean dose to cochlea and trigeminal nerve were least at 4.53 ± 0.86 and 6.95 ± 2.02 Gy from SCF_50% and highest at 6.65 ± 0.70 and 8.40 ± 2.11 Gy from DCA_80% plans respectively. This dosimetric data provides a guideline for choosing optimum treatment option and scope of inter institutional dosimetric comparison for further improvement in radiosurgery of Vestibular Schwannoma (VS).

A dosimetric evaluation of the Eclipse AAA algorithm and Millennium 120 MLC for cranial intensity-modulated radiosurgery

The aim of this study is to assess the accuracy of a convolution-based algorithm (anisotropic analytical algorithm [AAA]) implemented in the Eclipse planning system for intensity-modulated radiosurgery (IMRS) planning of small cranial targets by using a 5-mm leaf-width multileaf collimator (MLC). Overall, 24 patient-based IMRS plans for cranial lesions of variable size (0.3 to 15.1cc) were planned (Eclipse, AAA, version 10.0.28) using fixed field-based IMRS produced by a Varian linear accelerator equipped with a 120 MLC (5-mm width on central leaves). Plan accuracy was evaluated according to phantom-based measurements performed with radiochromic film (EBT2, ISP, Wayne, NJ). Film 2D dose distributions were performed with the FilmQA Pro software (version 2011, Ashland, OH) by using the triple-channel dosimetry method. Comparison between computed and measured 2D dose distributions was performed using the gamma method (3%/1mm). Performance of the MLC was checked by inspection of the DynaLog files created by the linear accelerator during the delivery of each dynamic field. The absolute difference between the calculated and measured isocenter doses for all the IMRS plans was 2.5% ± 2.1%. The gamma evaluation method resulted in high average passing rates of 98.9% ± 1.4% (red channel) and 98.9% ± 1.5% (blue and green channels). DynaLog file analysis revealed a maximum root mean square error of 0.46mm. According to our results, we conclude that the Eclipse/AAA algorithm provides accurate cranial IMRS dose distributions that may be accurately delivered by a Varian linac equipped with a Millennium 120 MLC.

A dual resolution measurement based Monte Carlo simulation technique for detailed dose analysis of small volume organs in the skull base region

The purpose of this study was to examine dose distribution of a skull base tumor and surrounding critical structures in response to high dose intensity-modulated radiosurgery (IMRS) with Monte Carlo (MC) simulation using a dual resolution sandwich phantom. The measurement-based Monte Carlo (MBMC) method (Lin et al., 2009) was adopted for the study. The major components of the MBMC technique involve (1) the BEAMnrc code for beam transport through the treatment head of a Varian 21EX linear accelerator, (2) the DOSXYZnrc code for patient dose simulation and (3) an EPID-measured efficiency map which describes non-uniform fluence distribution of the IMRS treatment beam. For the simulated case, five isocentric 6MV photon beams were designed to deliver a total dose of 1200cGy in two fractions to the skull base tumor. A sandwich phantom for the MBMC simulation was created based on the patient's CT scan of a skull base tumor [gross tumor volume (GTV)=8.4cm3] near the right 8th cranial nerve. The phantom, consisted of a 1.2-cm thick skull base region, had a voxel resolution of 0.05×0.05×0.1cm3 and was sandwiched in between 0.05×0.05×0.3cm3 slices of a head phantom. A coarser 0.2×0.2×0.3cm3 single resolution (SR) phantom was also created for comparison with the sandwich phantom. A particle history of 3×108 for each beam was used for simulations of both the SR and the sandwich phantoms to achieve a statistical uncertainty of <2%. Our study showed that the planning target volume (PTV) receiving at least 95% of the prescribed dose (VPTV95) was 96.9%, 96.7% and 99.9% for the TPS, SR, and sandwich phantom, respectively. The maximum and mean doses to large organs such as the PTV, brain stem, and parotid gland for the TPS, SR and sandwich MC simulations did not show any significant difference; however, significant dose differences were observed for very small structures like the right 8th cranial nerve, right cochlea, right malleus and right semicircular canal. Dose volume histogram (DVH) analyses revealed much smoother DVH curves for the dual resolution sandwich phantom when compared to the SR phantom. In conclusion, MBMC simulations using a dual resolution sandwich phantom improved simulation spatial resolution for skull base IMRS therapy. More detailed dose analyses for small critical structures can be made available to help in clinical judgment.

SU-E-T-45: Maximizing the Spatial Resolution of the MatriXX Device for Radiosurgery Plan QA

Purpose: The MatriXX array detector array has been shown to provide inadequate sampling and absolute dose measurements for small, highly modulated fields. These devices generally fail due to spatial frequency contributions exceeding the sampling rate as defined by the Nyquist criteria and maximum point doses falling between chamber locations. This study attempts to demonstrate the feasibility of using an extended source detector distance (ESDD) measurement combined with array shifting to maximize MatriXX resolution for the quality assurance of radiosurgery fields. Methods: Measurements were taken using a Novalis TX with HD-MLCs at an extended 202 cm SDD with 3.8 mm shifts in the radial and transverse directions. A VBA code was developed to sum the four shifted images into a single dose plane. A line pair pattern with decreasing MLC openings from 1.25 to 0.25 cm was used to quantify the MatriXX resolution using conventional and shifted ESDD techniques. Next, the quality assurance of an intensity modulated radiosurgery (IMRS) plan was measured conventionally with the MatriXX and portal imaging device, and then compared to the shifted ESDD technique using a 3%, 3 mm gamma analysis and point dose comparison. Results: The ESDD technique increased the effective sampling frequency from 1.3 to 5.6 cm−1 allowing for the accurate representation of frequency contributions up to 2 cm−1 corresponding to dose fluctuations from single HD-MLC openings. The gamma passing rate of the ESDD measured IMRS fields averaged 95.4% compared to 86.4% for conventional SDD and 99.5% for portal dosimetry. Absolute point doses differences improved on average from 4.3% and 3.3%, for conventional SDD and portal respectively, to 2.7% for the ESSD technique. Conclusion: This study demonstrates that the resolution of the MatriXX can be improved by a factor of four with extended SDD and array shifting techniques allowing for improved QA of radiosurgery plans.

Feasibility of an online adaptive replanning method for cranial frameless intensity-modulated radiosurgery

To introduce an approach for online adaptive replanning (i.e., dose-guided radiosurgery) in frameless stereotactic radiosurgery, when a 6-dimensional (6D) robotic couch is not available in the linear accelerator (linac). Cranial radiosurgical treatments are planned in our department using intensity-modulated technique. Patients are immobilized using thermoplastic mask. A cone-beam computed tomography (CBCT) scan is acquired after the initial laser-based patient setup (CBCTsetup). The online adaptive replanning procedure we propose consists of a 6D registration-based mapping of the reference plan onto actual CBCTsetup, followed by a reoptimization of the beam fluences (“6D plan”) to achieve similar dosage as originally was intended, while the patient is lying in the linac couch and the original beam arrangement is kept. The goodness of the online adaptive method proposed was retrospectively analyzed for 16 patients with 35 targets treated with CBCT-based frameless intensity modulated technique. Simulation of reference plan onto actual CBCTsetup, according to the 4 degrees of freedom, supported by linac couch was also generated for each case (4D plan). Target coverage (D99%) and conformity index values of 6D and 4D plans were compared with the corresponding values of the reference plans. Although the 4D-based approach does not always assure the target coverage (D99% between 72% and 103%), the proposed online adaptive method gave a perfect coverage in all cases analyzed as well as a similar conformity index value as was planned. Dose-guided radiosurgery approach is effective to assure the dose coverage and conformity of an intracranial target volume, avoiding resetting the patient inside the mask in a “trial and error” way so as to remove the pitch and roll errors when a robotic table is not available.

Verification of the 3D dose distribution in spinal radiosurgery by using a BANG3® polymer gel dosimeter

In intensity-modulated radiosurgery (IMRS) treatment, radiation delivery techniques require the ability to accurately verify complex three-dimensional (3D) dose distributions. This study was designed to evaluate and verify dosimetry generated from gels, films, and treatment planning systems. In this study, commercially available BANG3® polymer gel was used to confirm the accuracy of the treatment plan and to assess the dosimetric uncertainty of the radiosurgery procedure. BANG3® gels that are read with R2 magnetic resonance (MR) imaging mapping are useful options. The gel is a tissue equivalent, and the relaxation ratio measured using MR imaging is proportional to the dose absorbed in the gel. A cylindrical container (5″ deep, 7″ high) filled with BANG3® gel was mounted in a cubic phantom (The EASY CUBEr, Euromechanics, Schwarzenbruck, Germany). We then carried out the same process using the gel and gafchromic film as would be used for a patient with metastatic T-spine cancer by using a Novalis Radiosurgery system (Brain LAB Inc., Germany). Our experimental results provided the dose distribution and the radiation delivery precision. Comparisons of the measured and the calculated relative dose distributions showed good agreement in the high-dose region with differences of 2 mm. BANG3® polymer gel dosimetry can be useful for the verification of clinical treatment radiosurgery plans.

Intensity-modulated radiosurgery with rapidarc for multiple brain metastases and comparison with static approach

Rotational RapidArc (RA) and static intensity-modulated radiosurgery (IMRS) have been used for brain radiosurgery. This study compares the 2 techniques from beam delivery parameters and dosimetry aspects for multiple brain metastases. Twelve patients with 2–12 brain lesions treated with IMRS were replanned using RA. For each patient, an optimal 2-arc RA plan from several trials was chosen for comparison with IMRS. Homogeneity, conformity, and gradient indexes have been calculated. The mean dose to normal brain and maximal dose to other critical organs were evaluated. It was found that monitor unit (MU) reduction by RA is more pronounced for cases with larger number of brain lesions. The MU-ratio of RA and IMRS is reduced from 104% to 39% when lesions increase from 2 to 12. The dose homogeneities are comparable in both techniques and the conformity and gradient indexes and critical organ doses are higher in RA. Treatment time is greatly reduced by RA in intracranial radiosurgery, because RA uses fewer MUs, fewer beams, and fewer couch angles.

Intensity modulated radiosurgery for the spine: Dosimetric impact of beamlet size variation in the leaf travel direction

Background and purposeTo investigate the dosimetric impact of beamlet size in the leaf travel direction for the spinal treatment using intensity-modulated radiosurgery (IMRS). Materials and methodsThe IMRS plans of ten patients (11 lesions – 6 thoracic, 2 cervical, 3 lumbar) were re-planned using four different beamlet sizes (1, 2, 5, and 10mm) – in the leaf travel direction, while keeping the Y-dimension by multi-leaf collimator (MLC) width fixed, and compared to the reference plan with beamlet size of 3mm. To evaluate the beamlet size effect, target volumes (coverage, conformity, and size effect), organ at risks (OARS) (doses to the spinal cord, lung and kidneys), and integral dose, and monitor units (MUs) were calculated. ResultsTarget coverage and dose conformity for planning target volume (PTV) were not correlated with beamlet size. Maximum (p=0.000) and mean (p=0.000) spinal cord doses decreased by 4.0% and 3.4% from 23.4% and 28.6% as beamlet size decreased from 10 to 1mm. The integral doses, MUs and doses to other organs increased at smaller beamlet sizes. MUs for a beamlet size of 10mm decreased by 31.4%, as compared with that at the reference beamlet size. ConclusionsDespite no dosimetric benefits with respect to target volume and an MU increase, a definite dose reduction was observed at the spinal cord for smaller beamlet sizes. Treatment with IMRS planning for the spine will benefit from the use of a beamlet size between 2 and 4mm.