Ermias Gete

Whole brain radiotherapy with hippocampal avoidance and simultaneous integrated boost for 1-3 brain metastases: a feasibility study using volumetric modulated arc therapy.

PURPOSE To evaluate the feasibility of using volumetric modulated arc therapy (VMAT) to deliver whole brain radiotherapy (WBRT) with hippocampal avoidance and a simultaneous integrated boost (SIB) for one to three brain metastases. METHODS AND MATERIALS Ten patients previously treated with stereotactic radiosurgery for one to three brain metastases underwent repeat planning using VMAT. The whole brain prescription dose was 32.25 Gy in 15 fractions, and SIB doses to brain metastases were 63 Gy to lesions >or=2.0 cm and 70.8 Gy to lesions <2.0 cm in diameter. The mean dose to the hippocampus was kept at <6 Gy(2). Plans were optimized for conformity and target coverage while minimizing hippocampal and ocular doses. Plans were evaluated on target coverage, prescription isodose to target volume ratio, conformity number, homogeneity index, and maximum dose to prescription dose ratio. RESULTS Ten patients had 18 metastases. Mean values for the brain metastases were as follows: conformity number = 0.73 +/- 0.10, target coverage = 0.98 +/- 0.01, prescription isodose to target volume = 1.34 +/- 0.19, maximum dose to prescription dose ratio = 1.09 +/- 0.02, and homogeneity index = 0.07 +/- 0.02. For the whole brain, the mean target coverage and homogeneity index were 0.960 +/- 0.002 and 0.39 +/- 0.06, respectively. The mean hippocampal dose was 5.23 +/- 0.39 Gy(2). The mean treatment delivery time was 3.6 min (range, 3.3-4.1 min). CONCLUSIONS VMAT was able to achieve adequate whole brain coverage with conformal hippocampal avoidance and radiosurgical quality dose distributions for one to three brain metastases. The mean delivery time was under 4 min.

A Monte Carlo approach to validation of FFF VMAT treatment plans for the TrueBeam linac

PURPOSE To commission and benchmark a vendor-supplied (Varian Medical Systems) Monte Carlo phase-space data for the 6 MV flattening filter free (FFF) energy mode on a TrueBeam linear accelerator for the purpose of quality assurance of clinical volumetric modulated arc therapy (VMAT) treatment plans. A method for rendering the phase-space data compatible with BEAMnrc/DOSXYZnrc simulation software package is presented. METHODS Monte Carlo (MC) simulations were performed to benchmark the TrueBeam 6 MV FFF phase space data that have been released by the Varian MC Research team. The simulations to benchmark the phase space data were done in three steps. First, the original phase space which was created on a cylindrical surface was converted into a format that was compatible with BEAMnrc. Second, BEAMnrc was used to create field size specific phase spaces located underneath the jaws. Third, doses were calculated with DOSXYZnrc in a water phantom for fields ranging from 1 × 1 to 40 × 40 cm(2). Calculated percent depth doses (PDD), transverse profiles, and output factors were compared with measurements for all the fields simulated. After completing the benchmarking study, three stereotactic body radiotherapy (SBRT) VMAT plans created with the Eclipse treatment planning system (TPS) were calculated with Monte Carlo. Ion chamber and film measurements were also performed on these plans. 3D gamma analysis was used to compare Monte Carlo calculation with TPS calculations and with film measurement. RESULTS For the benchmarking study, MC calculated and measured values agreed within 1% and 1.5% for PDDs and in-field transverse profiles, respectively, for field sizes >1 × 1 cm(2). Agreements in the 80%-20% penumbra widths were better than 2 mm for all the fields that were compared. With the exception of the 1 × 1 cm(2) field, the agreement between measured and calculated output factors was within 1%. It is of note that excellent agreement in output factors for all field sizes including highly asymmetric fields was achieved without accounting for backscatter into the beam monitor chamber. For the SBRT VMAT plans, the agreement between Monte Carlo and ion chamber point dose measurements was within 1%. Excellent agreement between Monte Carlo, treatment planning system and Gafchromic film dose distribution was observed with over 99% of the points in the high dose volume passing the 3%, 3 mm gamma test. CONCLUSIONS The authors have presented a method for making the Varian IAEA compliant 6 MV FFF phase space file of the TrueBeam linac compatible with BEAMnrc/DOSXYZnrc. After benchmarking the modified phase space against measurement, they have demonstrated its potential for use in MC based quality assurance of complex delivery techniques.

Long-Term Outcomes of Fractionated Stereotactic Radiation Therapy for Pituitary Adenomas at the BC Cancer Agency

PURPOSE To assess the long-term disease control and toxicity outcomes of fractionated stereotactic radiation therapy (FSRT) in patients with pituitary adenomas treated at the BC Cancer Agency. METHODS AND MATERIALS To ensure a minimum of 5 years of clinical follow-up, this study identified a cohort of 76 patients treated consecutively with FSRT between 1998 and 2007 for pituitary adenomas: 71% (54/76) had nonfunctioning and 29% (22/76) had functioning adenomas (15 adrenocorticotrophic hormone-secreting, 5 growth hormone-secreting, and 2 prolactin-secreting). Surgery was used before FSRT in 96% (73/76) of patients. A median isocenter dose of 50.4 Gy was delivered in 28 fractions, with 100% of the planning target volume covered by the 90% isodose. Patients were followed up clinically by endocrinologists, ophthalmologists, and radiation oncologists. Serial magnetic resonance imaging was used to assess tumor response. RESULTS With a median follow-up time of 6.8 years (range, 0.6 - 13.1 years), the 7-year progression-free survival was 97.1% and disease-specific survival was 100%. Of the 2 patients with tumor progression, both had disease control after salvage surgery. Of the 22 patients with functioning adenomas, 50% (11/22) had complete and 9% (2/22) had partial responses after FSRT. Of the patients with normal pituitary function at baseline, 48% (14/29) experienced 1 or more hormone deficiencies after FSRT. Although 79% (60/76) of optic chiasms were at least partially within the planning target volumes, no patient experienced radiation-induced optic neuropathy. No patient experienced radionecrosis. No secondary malignancy occurred during follow-up. CONCLUSION In this study of long-term follow-up of patients treated for pituitary adenomas, FSRT was safe and effective.

Optimization of Stereotactic Radiotherapy Treatment Delivery Technique for Base-Of-Skull Meningiomas

This study compares static conformal field (CF), intensity modulated radiotherapy (IMRT), and dynamic arcs (DA) for the stereotactic radiotherapy of base-of-skull meningiomas. Twenty-one cases of base-of-skull meningioma (median planning target volume [PTV] = 21.3 cm3) previously treated with stereotactic radiotherapy were replanned with each technique. The plans were compared for Radiation Therapy Oncology Group conformity index (CI) and homogeneity index (HI), and doses to normal structures at 6 dose values from 50.4 Gy to 5.6 Gy. The mean CI was 1.75 (CF), 1.75 (DA), and 1.66 (IMRT) (p < 0.05 when comparing IMRT to either CF or DA plans). The CI (IMRT) was inversely proportional to the size of the PTV (Spearmans rho = -0.53, p = 0.01) and at PTV sizes above 25 cm3, the CI (IMRT) was always superior to CI (DA) and CI (CF). At PTV sizes below 25 cm3, there was no significant difference in CI between each technique. There was no significant difference in HI between plans. The total volume of normal tissue receiving 50.4, 44.8, and 5.6 Gy was significantly lower when comparing IMRT to CF and DA plans (p < 0.05). There was significantly improved dose sparing for the brain stem and ipsilateral temporal lobe with IMRT but no significant difference for the optic chiasm or pituitary gland. These results demonstrate that stereotactic IMRT should be considered to treat base-of-skull meningiomas with a PTV larger than 25 cm3, due to improved conformity and normal tissue sparing, in particular for the brain stem and ipsilateral temporal lobe.

Monte Carlo modeling of HD120 multileaf collimator on Varian TrueBeam linear accelerator for verification of 6X and 6X FFF VMAT SABR treatment plans

A Monte Carlo (MC) validation of the vendor‐supplied Varian TrueBeam 6 MV flattened (6X) phase‐space file and the first implementation of the Siebers‐Keall MC MLC model as applied to the HD120 MLC (for 6X flat and 6X flattening filterfree (6X FFF) beams) are described. The MC model is validated in the context of VMAT patient‐specific quality assurance. The Monte Carlo commissioning process involves: 1) validating the calculated open‐field percentage depth doses (PDDs), profiles, and output factors (OF), 2) adapting the Siebers‐Keall MLC model to match the new HD120‐MLC geometry and material composition, 3) determining the absolute dose conversion factor for the MC calculation, and 4) validating this entire linac/MLC in the context of dose calculation verification for clinical VMAT plans. MC PDDs for the 6X beams agree with the measured data to within 2.0% for field sizes ranging from 2 × 2 to 40 × 40 cm2. Measured and MC profiles show agreement in the 50% field width and the 80%‐20% penumbra region to within 1.3 mm for all square field sizes. MC OFs for the 2 to 40 cm2 square fields agree with measurement to within 1.6%. Verification of VMAT SABR lung, liver, and vertebra plans demonstrate that measured and MC ion chamber doses agree within 0.6% for the 6X beam and within 2.0% for the 6X FFF beam. A 3D gamma factor analysis demonstrates that for the 6X beam, > 99% of voxels meet the pass criteria (3%/3 mm). For the 6X FFF beam, > 94% of voxels meet this criteria. The TrueBeam accelerator delivering 6X and 6X FFF beams with the HD120 MLC can be modeled in Monte Carlo to provide an independent 3D dose calculation for clinical VMAT plans. This quality assurance tool has been used clinically to verify over 140 6X and 16 6X FFF TrueBeam treatment plans. PACS number: 87.55.K‐

A simple and robust trajectory‐based stereotactic radiosurgery treatment

Introduction: We present a Trajectory‐based Volumetric Modulated Arc Therapy (TVMAT) technique for Stereotactic Radiosurgery (SRS) that takes advantage of a modern linacs ability to modulate dose rate and move the couch dynamically. In addition, we investigate the quality of the developed TVMAT method and the dosimetric accuracy of the technique. Methods: The main feature of the TVMAT technique is a standard beam trajectory formed by dynamic motion of the treatment couch and the linac gantry. The couch rotates slowly through 180 degrees while the gantry delivers radiation through continuous sweeps of the gantry. The number of partial arcs that constitute the trajectory can be varied between two and eight and as the number of partial arcs increases, the trajectory more finely samples 4π geometry. Along these trajectories, the multi‐leaf collimator (MLC) and dose rate are optimized through an inverse planning framework. The TVMAT method was tested on ten cranial SRS patients who were previously treated with the Dynamic Conformal Arc (DCA) technique. The plans were compared with the DCA and a four‐ arc VMAT technique with regards to dose to the OAR, dose falloff, V12Gy, and V4Gy. Validation measurements were performed using ion‐chamber and Gafchromic film. In addition, the trajectory‐log files were analyzed and compared with the treatment plan beam data. Results: The TVMAT treatment plans were successfully delivered with a treatment time between 3–8 min which mostly depended on total cumulated dose. Ion chamber measurements had an average measured error of 1.1 ± 0.6% and a maximum value of 2.2% of the delivered dose. The 2%, 2 mm gamma pass rates for the film measurements were 96% or greater. In a preliminary comparison of ten patients who underwent SRS treatments with the DCA technique, the TVMAT and VMAT techniques were able to produce plans with comparable dose falloff and OAR doses, while achieving better dose conformality, V4Gy and V12Gy when compared to the original DCA plans. The improvement of the TVMAT plans were as follows (mean % improvement ± standard err): Conformity (10 ± 2%), V4 (20 ± 20%), V12 (27 ± 10%), volume weighted mean dose to organs at risk (13 ± 13%), homogeneity index (2 ± 2%) and falloff (4 ± 2%). Conclusion: We have developed and validated a trajectory‐based dose delivery method which has dose distribution improvements while having a treatment time of 3–8 min. In addition, it has the potential for a simpler planning experience while maintaining an accurate delivery on the Varian Truebeam Linac.

Factors predictive of obliteration after arteriovenous malformation radiosurgery.

OBJECTIVE To investigate predictive factors of complete obliteration following treatment with linac-based stereotactic radiosurgery for intracerebral arteriovenous malformations. METHODS Archived plans for 48 patients treated at the British Columbia Cancer Agency and who underwent post-treatment digital subtraction angiography to assess obliteration were studied. Actuarial estimates of obliteration were calculated using the Kaplan-Meier method. Univariate and multivariate Cox proportional hazards models were used for analysis of incidence of obliteration. Log-rank test was used to search for parameters associated with obliteration. RESULTS Complete nidus obliteration was achieved in 38/48 patients (79.2%). Actuarial rate of obliteration was 75.9% at 4 years (95% confidence interval 63.1%-88.6%). On univariate analysis, prescribed dose to the margin (p=0.002) and dose to isocentre (p=0.022) showed statistical significance. No parameters were significant in a multivariate model. According to the log-rank test, prescribed dose to the margin of >20 Gy (p=0.004) and dose to the isocentre of >25 Gy (p=0.004) were associated with obliteration. CONCLUSION Reported series in the literature suggest a number of different factors are predictive of complete obliteration of arteriovenous malformations following radiosurgery. However, differing definitions of volume and complete obliteration makes direct comparison between series difficult. This study demonstrates that complete obliteration of the nidus following linear accelerator-based stereotactic radiosurgery for arteriovenous malformations appears to be most closely related to the prescribed marginal dose. In particular, a marginal dose of >20 Gy is strongly associated with obtaining complete obliteration of the nidus.

Monte Carlo validation of the TrueBeam 10XFFF phase–space files for applications in lung SABR

PURPOSE To establish the clinical acceptability of universal Monte Carlo phase-space data for the 10XFFF (flattening filter free) photon beam on the Varian TrueBeam Linac, including previously unreported data for small fields, output factors, and inhomogeneous media. The study was particularly aimed at confirming the suitability for use in simulations of lung stereotactic ablative radiotherapy treatment plans. METHODS Monte Carlo calculated percent depth doses (PDDs), transverse profiles, and output factors for the TrueBeam 10 MV FFF beam using generic phase-space data that have been released by the Varian MC research team were compared with in-house measurements and published data from multiple institutions (ten Linacs from eight different institutions). BEAMnrc was used to create field size specific phase-spaces located underneath the jaws. Doses were calculated with DOSXYZnrc in a water phantom for fields ranging from 1 × 1 to 40 × 40 cm(2). Particular attention was paid to small fields (down to 1 × 1 cm(2)) and dose per pulse effects on dosimeter response for high dose rate 10XFFF beams. Ion chamber measurements were corrected for changes in ion collection efficiency (P(ion)) with increasing dose per pulse. MC and ECLIPSE ANISOTROPIC ANALYTICAL ALGORITHM (AAA) calculated PDDs were compared to Gafchromic film measurement in inhomogeneous media (water, bone, lung). RESULTS Measured data from all machines agreed with Monte Carlo simulations within 1.0% and 1.5% for PDDs and in-field transverse profiles, respectively, for field sizes >1 × 1 cm(2) in a homogeneous water phantom. Agreements in the 80%-20% penumbra widths were better than 2 mm for all the fields that were compared. For all the field sizes considered, the agreement between their measured and calculated output factors was within 1.1%. Monte Carlo results for dose to water at water/bone, bone/lung, and lung/water interfaces as well as within lung agree with film measurements to within 2.8% for 10 × 10 and 3 × 3 cm(2) field sizes. This represents a significant improvement over the performance of the ECLIPSE AAA. CONCLUSIONS The 10XFFF phase-space data offered by the Varian Monte Carlo research team have been validated for clinical use using measured, interinstitutional beam data in water and with film dosimetry in inhomogeneous media.

SU‐D‐105‐03: Developing QA Procedures for Gated VMAT SABR Treatments

PURPOSE To develop a QA procedure for gated VMAT SABR liver cancer treatments and investigate the gating parameters for acceptable plan delivery in terms of the dose to a moving volume and treatment delivery time. METHODS 10 patient plans for VMAT SABR liver were created using the Eclipse™ TPS. The verification plans were then transferred to a CT-scanned Quasar™ phantom and delivered on a TrueBeam™ linac using a 10FFF beam and Varians RPM system for respiratory gating. Two kinds of breathing patterns were used: free breathing (FB) and an interrupted (∼5s pause) end of exhale coached breathing (CB) pattern. Ion chamber and Gafchromic™ film measurements were acquired for a gated delivery while the phantom moved under the described breathing patterns and a non-gated, stationary phantom delivery. The gate window was set to obtain a range of residual target motion from 2-10 mm. RESULTS Preliminary chamber measurements indicate that the dose to the center of the PTV can vary considerably under gated delivery compared to the static case. The effect can be significant for free breathing; ∼4-12% over the selected range of residual target motion. The agreement was more consistent with CB pattern at ∼1-4%. Gamma analysis (3%, 3mm) showed an agreement above 99.74% for all gated deliveries compared to the static delivery. The treatment time with a gate width of 2 mm was ∼265s for the CB pattern compared to ∼740s under a typical FB pattern. A non-gated delivery of the same plan took ∼100s. CONCLUSION Gated VMAT treatments have been delivered successfully to a motorized phantom. FB patterns contain considerable variability and it is difficult to achieve acceptable results even with very small gate windows. However, a CB pattern combined with a sufficiently small gate, resulted in acceptable dose distributions that can be delivered in a reasonable amount of time. Francis Viel received funding from the Natural Sciences and Engineering Research Council of Canada. This work has been supported by the Varian Research Collaborations Program.

Machine‐specific quality assurance procedure for stereotactic treatments with dynamic couch rotations

Objective: We present a method in which the treatment couchs accuracy is measured using the electronic portal imaging device (EPID) and a phantom of our own construction. Using this phantom, we were able to quantify the treatment couch walkout, and the rotation angle accuracy for both static and dynamic couch treatments. These measurements were used to provide an accurate measure of the treatment couch isocenter as well as to verify the couch rotation angle recorded in the trajectory log. Methods: The phantom was constructed using a polystyrene slab in which five ball bearings of 4 mm diameter are placed on the same plane at varying radii (0, 2.8, 4.4, 5.6, and 6.7 cm). The couch was rotated through its full extent (‐90, 90 degrees) while MV images were acquired continuously. The couch rotational accuracy was calculated using a least squares minimization which fit the locations of the BBs to their expected locations relative to reference setup conditions. Using this approach, rotation angle and isocenter walkout was calculated in three dimensions. These measurements were used to quantify the accuracy of the couch as well as to validate the Varian TrueBeam trajectory logs. Additionally, a method for an EPID‐based couch star‐shot measurement was developed and compared with the traditional film‐based method. Results: The measured couch center of rotation consisted of a cloud of points clustered around the room isocenter within 0.7 mm distance. The trajectory log couch angle values agreed with those recorded in the DICOM header of the EPID images to the third significant digit and the couch rotation angles recorded in the trajectory log and DICOM header agreed with the calculated values to 0.08 degrees. Comparison of couch star‐shot measurement developed in this study with film‐based star‐shot measurements gave an agreement to within 0.2 mm. Conclusion: We have developed a quality assurance method for the treatment couch which is simple, accurate, and enables the user to access a multitude of consistent data with a single measurement. Using this method, we have shown that the treatment couch is accurate for both static and dynamic stereotactic deliveries.