Stereotactic Treatment Planning Research Articles

TU‐A‐108‐09: Reduction of Treatment Times for Breath‐Hold Patients in a Spot‐Scanning Proton Machine Through the Use of Mini‐Ridge Filters

Purpose: Study the feasibility of synchrotron‐based spot‐scanning proton therapy for breath‐hold patients by reducing treatment time through the use of a Mini‐Ridge Filter (MRF) to increase the width of the Bragg peaks. Methods: Five spot‐scanning nozzles were simulated in TOPAS[1]: four with MRFs of varying maximal thicknesses (6.15mm‐24.6mm) and one without an MRF. The MRF ridges were aligned transverse to the beam, with the number of ridges (4–16) proportional to MRF thickness. The material thickness of these ridges approximately followed a Gaussian distribution. Using these simulations, Monte Carlo data was generated for Eclipse machine commissioning. For each nozzle, standard and stereotactic (SR) lung phantom treatment plans were created and assessed for delivery time and plan quality. Results: Use of an MRF resulted in a reduction of the number of energy layers needed in treatment plans, thereby decreasing the number of synchrotron spills needed and hence treatment time. For normal plans, treatment time without an MRF was 65 seconds per field, whereas all four of the MRF plans had treatment times of less than 30 seconds per field‐sufficiently low for breath‐hold. For SR plans, the shortest treatment time achieved was 47 seconds per field, compared to 94 seconds without an MRF. The time reduction had diminishing gains as the MRF thickness increased. PTV coverage and dose uniformity was comparable across all plans; however, the average dose to the surrounding lung increased from 45% of the prescription dose with no MRF up to 54% (worst case with MRF). Conclusion: An MRF can be used to reduce treatment time to aid breath‐hold treatments without compromising plan quality. To balance treatment time and normal tissue dose, the ideal choice was shown to be the thinnest MRF which achieves the timing necessary for breath‐hold.

SU‐E‐T‐672: Comparisons of Single and Multiple Isocenter Stereotactic IMRT Treatment Planning for Multiple Brain Metastases Treatment

Purpose: To compare quality of single and multiple isocenter stereotactic IMRT treatment plans for multiple intracranial targets. Methods: Five patients (two with three and three with two brain metastatic tumors) treated with multiple isocenters using 9 to 12 non‐coplanar beams per lesion were replanned for single isocenter 10 to 12 non‐coplanar beams. Identical contour sets and dose‐volume constraints were applied. Plans were developed using BrainLab iPlan system for delivery with Varian TrueBeam STx machine. The prescribed dose to each target was 25 Gy in five fractions and minimum 95% dose was required to cover > 95% of target volume. Results: All plans satisfied the dose‐volume constraints, with no significant differences in dose conformity between single and multiple isocenter plans. The mean Paddick conformity index (CI) for two target single and multiple isocenter plans was 0.76 and 0.75 (p = 0.93), and for three target single and multiple isocenter plans was 0.73 and 0.70 (p = 0.37), respectively. Normal brain tissue doses were higher for single isocenter plans compared to multiple isocenter plans. The average normal brain volumes receiving 15 and 10 Gy for two target plans (single vs. multiple isocenter) were: (4.8% vs. 2.1%) and (8.9% vs. 4.5%), respectively. For three target plans these volumes were: (2.0% vs. 1.5%) and (4.6% vs. 3.4%). The estimated total treatment time (patient set‐up, imaging‐verification, beam‐on time at 600 MU/min, etc.) were approximately 1.75 and 2.5 times lower with two and three target single isocenter delivery, respectively. Conclusion: Single isocenter plans were capable of producing quality treatment plans similar to multiple isocenter plans. Treatment delivery times were much higher and the normal brain tissue doses were only slightly lower at 10 and 15 Gy dose levels for multiple isocenter plans compared to single isocenter plans. Clinical consequences of these normal tissue doses are however unknown.

SU‐E‐T‐414: 4D Vs 3D Dose Calculations for Non‐Small‐Lung‐Cancer: A Comparison Using Tumor Control Probability (TCP)

Purpose: To investigate the utility of four dimensional (4D) dose calculations for design and optimization of stereotactic lung radiotherapy treatment plans. Methods: Radiotherapy plans for stage I–II lung cancer were created for 8 patients. Clinically acceptable treatment plans were created with dose calculated on the end‐exhale 4D‐CT phase, using a Monte Carlo algorithm. Dose was then projected onto the remaining 9 phases of 4D‐CT using the Monte Carlo algorithm and accumulated onto the end‐exhale phase using commercially available deformable registration software. The resulting DVHs of the gross tumor volume (GTV), planning tumor volume (PTV), and PTVsetup were compared according to target coverage and dose. The PTVsetup was defined as a volume including the GTV and a margin for setup uncertainties but not for respiratory motion. Tumor control probabilities (TCP) resulting from these DVHs were estimated using a wide range of alphas, betas, and tumor cell densities. Results: Differences of up to 5 Gy were observed in dosimetric indices related to the tumor between 3D and 4D calculations. When the TCP was calculated using the resulting DVHs for fractionation schedules typically used in stereotactic body radiation therapy (SBRT), the TCP differed at most by 5% between 4D and 3D cases, and in most cases differed less than 1%. Conclusion: In some cases moderate dosimetric differences may be obtained between 3D and 4D dose calculations. Even when dosimetric differences exist between 3D and 4D calculations, the resulting TCPs do not differ greatly for commonly‐used SBRT dose prescriptions.

SU‐E‐T‐410: Initial Results From Multiple Irradiations of An Anthropomorphic Liver Phantom

Purpose: To describe the Radiological Physics Center's new anthropomorphic liver phantom and to report the results from the initial irradiations. Methods: A new imaging and dosimetry insert was created for the RPC's pelvic phantom that includes non‐coplanar targets. The polystyrene insert represents the liver and includes two Solid WaterTM targets (PTV1 and PTV2) that mimic liver metastases. PTV1, located in the superior‐left‐posterior region of the insert, is an ovoid 2 cm in diameter and 2.5 cm long. PTV2, located in the inferior‐right‐anterior region of the insert is a 3 cm diameter sphere. The insert houses one TLD and 3 planes of radiochromic film in each PTV. The phantom, with a motion table to simulate respiratory motion, was sent to institutions planning to participate in an upcoming RTOG SBRT liver protocol. Institutions were instructed to design and deliver a stereotactic treatment plan that delivered 6 Gy to >=95% of each PTV. The maximum motion of the phantom on the motion table was 1 cm in the superior‐inferior direction. Results: Irradiations from 8 institutions have been analyzed. The acceptance criteria, based on the results of the 8 irradiations, were set at ±7% for the TLD and 85% of the pixels in a region surrounding each PTV passing a ±7%/4 mm global gamma analysis. Five of the 8 irradiations passed these criteria. The three irradiations that did not meet the criteria failed to meet only the film criteria. Breath hold, ITV, gating, and tracking techniques were used to account for motion. Pinnacle, Eclipse, and MultiPlan treatment planning systems were used for planning. Conclusion: The new liver insert along with a motion table tests the ability of the institution's ability to deliver dose to multiple moving targets and is a useful credentialing tool for clinical trials using SBRT. Supported by PHS grant CA10953 and CA081647 from the NCI, DHHS.

SU-E-T-702: Performance of Multiple Conformity, Homogeneity and Dose Gradient Indices in SRS and SBRT Treatment Planning of the Spinal, Pelvic and Lung Lesions

Purpose: Presently, various forms of quality indices (QI) are used to compare and evaluate stereotactic treatment plans with no single QI accepted as universal. In this work we evaluate and compare the performance of 11 previously reported QI: 5 conformity (CI), 5 homogeneity (HI), and 1 dose gradient (DGI) indices as a function of target size, site, and mode of treatment. A novel unified quality function (UQF) which combines conformity, homogeneity and dose gradient is presented and investigated. Methods: 27 spine SRS, 8 pelvic SBRT and 34 lung SBRT cases were analyzed using an in-house wxPython program which imports treatment plan data and calculates various quality indices, dose volume tables, displays 3D dose/CT images, cumulative and differential DVHs. Results: Mean spine, pelvis and lung coverages were 0.91, 0.93, 0.85 respectively. Mean maximum target dose was best for pelvis (108%), followed by the lung (115%) and the spine (118%). Mean minimum target dose was best for the lung (95%), followed by pelvis (94%) and spine (88%). Mean DGI were 4.6, 5.2 and 7.54 for the pelvis, spine and lung. UQF scores the best for pelvis (0.2), followed by spine (0.28) and the lung (0.42). IMRT spine plans had better mean coverages (0.94), and lower maximum target dose (114%), while VMAT plans had better minimum target dose (89%). Conclusion: Mean CI, HI and DGI show various sensitivities to the treatment site, size and the delivery type (VMAT or IMRT). All CI except coverage show large variations with volume < 130 cc while all 5 HI show little dependence on the PTV volume. DGI values fall sharply for the lung (from 15 to 6) and spine (from 10 to 5) as the PTV increases to about 100 cc, while pelvic DGI stays about 5 for all PTV sizes.

Usefulness of Double Dose Contrast-enhanced Magnetic Resonance Imaging for Clear Delineation of Gross Tumor Volume in Stereotactic Radiotherapy Treatment Planning of Metastatic Brain Tumors: A Dose Comparison Study

To compare the size and clearness of gross tumor volumes (GTV) of metastatic brain tumors on T1-weighted magnetic resonance (MR) images between a single dose contrast administration protocol and a double dose contrast administration protocol to determine the optimum dose of contrast-enhancement for clear delineation of GTV in stereotactic radiotherapy (SRT).

4D VMAT, gated VMAT, and 3D VMAT for stereotactic body radiation therapy in lung

Four-dimensional volumetric modulated arc therapy (4D VMAT) is a treatment strategy for lung cancers that aims to exploit relative target and tissue motion to improve organ at risk (OAR) sparing. The algorithm incorporates the entire patient respiratory cycle using 4D CT data into the optimization process. Resulting treatment plans synchronize the delivery of each beam aperture to a specific phase of target motion. Stereotactic body radiation therapy treatment plans for 4D VMAT, gated VMAT, and 3D VMAT were generated on three patients with non-small cell lung cancer. Tumour motion ranged from 1.4–3.4 cm. The dose and fractionation scheme was 48 Gy in four fractions. A B-spline transformation model registered the 4D CT images. 4D dose volume histograms (4D DVH) were calculated from total dose accumulated at the maximum exhalation. For the majority of OARs, gated VMAT achieved the most radiation sparing but treatment times were 77–148% longer than 3D VMAT. 4D VMAT plan qualities were comparable to gated VMAT, but treatment times were only 11–25% longer than 3D VMAT. 4D VMAT's improvement of healthy tissue sparing can allow for further dose escalation. Future study could potentially adapt 4D VMAT to irregular patient breathing patterns.

Usefulness of double dose contrast-enhanced magnetic resonance imaging for clear delineation of gross tumor volume in stereotactic radiotherapy treatment planning of metastatic brain tumors: a dose comparison study

The purpose of this study was to compare the size and clearness of gross tumor volumes (GTVs) of metastatic brain tumors on T1-weighted magnetic resonance images between a single dose contrast administration protocol and a double dose contrast administration protocol to determine the optimum dose of contrast-enhancement for clear delineation of GTV in stereotactic radiotherapy (SRT). A total of 28 small metastatic brain tumors were evaluated in 13 patients by intra-individual comparison of GTV measurements using single dose and double dose contrast-enhanced thin-slice (1-mm) magnetic resonance imaging (MRI). All patients had confirmed histological types of primary tumors and had undergone hypo-fractionated SRT for metastatic brain tumors. The mean tumor diameter with single dose and double dose contrast-enhancement was 12.0 ± 1.1 mm and 13.2 ± 1.1 mm respectively (P < 0.001). The mean incremental ratio (MIR) obtained by comparing mean tumor diameters was 11.2 ± 0.02 %. The mean volume of GTV-1 (single dose contrast-enhancement) and GTV-2 (double dose contrast-enhancement) was 1.38 ± 0.41 ml and 1.59 ± 0.45 ml respectively (P < 0.01). The MIR by comparing mean tumor volumes was 32.3 ± 0.4 %. The MIR of GTV-1 with < 1ml volume and GTV-1 with > 1ml volume was 41.8 ± 0.05 % and 12.4 ± 0.03 % respectively (P < 0.001). We conclude that double dose contrast-enhanced thin-slice MRI is a more useful technique than single dose contrast-enhanced thin-slice MRI, especially for clear delineation of GTVs of small metastatic brain tumors in treatment planning of highly precise SRT.

CT‐only planning for Gamma Knife radiosurgery in the treatment of trigeminal neuralgia: Methodology and outcomes from a single institution

Gamma Knife radiosurgery (GKRS) has been established as a safe and effective treatment option for trigeminal neuralgia. Some patients have contraindications to magnetic resonance imaging (MRI), the standard stereotactic imaging used for GKRS treatment planning. Computerized tomography (CT) imaging may be used as an alternative in this scenario. We sought to evaluate the outcomes of our patients treated using this technique. Between August 2001 and November 2009, 19 patients with trigeminal neuralgia were treated with GKRS using CT-only planning. The course of the trigeminal nerve was determined based upon anatomical landmarks when the nerve was not directly visualized on the treatment-planning CT. Median dose used was 90 Gy (range 85-90 Gy). Follow-up data based on Barrow Neurological Institute (BNI) pain score and toxicity were obtained using electronic medical records and by telephone interview. With median follow-up time of 18 months (range 4-36 months), improvement in quality of life after GKRS was reported in 17 of 19 patients. Freedom from BNI IV-V pain relapse was 82% at 24 months. By 3 months post-GKRS, 50% of patients were able to discontinue medications completely. Three patients reported numbness after GKRS; none of these patients described bothersome numbness. Use of contrast did not affect treatment outcome (P = 0.31). Stereotactic CT-only treatment planning of GKRS for the treatment of trigeminal neuralgia is feasible and safe. Further studies are necessary to determine if the long-term durability of pain relief is comparable to that of MRI-based GKRS planning.

Variable Circular Collimator in Robotic Radiosurgery: A Time-Efficient Alternative to a Mini-Multileaf Collimator?

Compared with many small circular beams used in CyberKnife treatments, beam's eye view-shaped fields are generally more time-efficient for dose delivery. However, beam's eye view-shaping devices, such as a mini-multileaf collimator (mMLC), are not presently available for CyberKnife, although a variable-aperture collimator (Iris, 12 field diameters; 5-60 mm) is available. We investigated whether the Iris can mimic noncoplanar mMLC treatments using a limited set of principal beam orientations (nodes) to produce time-efficient treatment plans. The data from 10 lung cancer patients and the beam-orientation optimization algorithm "Cycle" were used to generate stereotactic treatment plans (3 × 20 Gy) for a CyberKnife virtually equipped with a mMLC. Typically, 10-16 favorable beam orientations were selected from 117 available robot node positions using beam's eye view-shaped fields with uniform fluence. Second, intensity-modulated Iris plans were generated by inverse optimization of nonisocentric circular candidate beams targeted from the same nodes selected in the mMLC plans. The plans were evaluated using the mean lung dose, lung volume receiving ≥20 Gy, conformality index, number of nodes, beams, and monitor units, and estimated treatment time. The mMLC plans contained an average of 12 nodes and 11,690 monitor units. For a comparable mean lung dose, the Iris plans contained 12 nodes, 64 beams, and 21,990 monitor units. The estimated fraction duration was 12.2 min (range, 10.8-13.5) for the mMLC plans and 18.4 min (range, 12.9-28.5) for the Iris plans. In contrast to the mMLC plans, the treatment time for the Iris plans increased with an increasing target volume. The Iris plans were, on average, 40% longer than the corresponding mMLC plans for small targets (<80 cm(3)) and ≤121% longer for larger targets. For a comparable conformality index, similar results were obtained. For stereotactic lung irradiation, time-efficient and high-quality plans were obtained for robotic-controlled noncoplanar treatments using a mMLC. Iris is a time-efficient alternative for small targets, with similar or better plan quality.

Evaluation of pencil beam convolution and anisotropic analytical algorithms in stereotactic lung irradiation

The aim of this study was to evaluate differences in dose distributions in stereotactic body radiation therapy treatment plans for lung tumors calculated with pencil beam convolution (PBC) algorithm with modified Batho power law (MBPL) versus heterogeneity corrected anisotropic analytical algorithm (AAA) of the Varian Eclipse treatment planning system. The four-dimensional computed tomography images from 20 patients with lung cancer were used to create treatment plans. Plans used five to seven nonopposing coplanar 6 MV beams. Plans generated with the PBC algorithm and MBPL for tissue heterogeneity corrections were optimized to deliver 60 Gy in three fractions to at least 95% of the planned target volume, and the normal tissue doses for spinal cord, esophagus, heart, and ipsilateral bronchus were restricted to less than 18, 27, 30, and 30 Gy, respectively. Plans were recalculated with AAA, retaining identical beam arrangements, photon beam fluences, and monitor units. The pencil beam plans, designed to deliver 60 Gy, delivered on average 51.6 Gy when re-calculated with the AAA, suggesting a reduction of at least 10% to prescription dose is appropriate when calculating with the AAA.

Comparison of linac-based fractionated stereotactic radiotherapy and tomotherapy treatment plans for intra-cranial tumors

This study compares and analyzes stereotactic radiotherapy using tomotherapy and linac-based fractionated stereotactic radiotherapy in the treatment of intra-cranial tumors, according to some cases. In this study, linac-based fractionated stereotactic radiotherapy and tomotherapy treatment were administered to five patients diagnosed with intra-cranial cancer in which the dose of 18–20 Gy was applied on 3–5 separate occasions. The tumor dosing was decided by evaluating the inhomogeneous index (II) and conformity index (CI). Also, the radiation-sensitive tissue was evaluated using low dose factors V1, V2, V3, V4, V5, and V10, as well as the non-irradiation ratio volume (NIV). The values of the II for each prescription dose in the linac-based non-coplanar radiotherapy plan and tomotherapy treatment plan were (0.125±0.113) and (0.090±0.180), respectively, and the values of the CI were (0.899±0.149) and (0.917±0.114), respectively. The low dose areas, V1, V2, V3, V4, V5, and V10, in radiation-sensitive tissues in the linac-based non-coplanar radiotherapy plan fell into the ranges 0.3%−95.6%, 0.1%−87.6%, 0.1%−78.8%, 38.8%-69.9%, 26.6%-65.2%, and 4.2%−39.7%, respectively, and the tomotherapy treatment plan had ranges of 13.6%−100%, 3.5%−100%, 0.4%−94.9%, 0.2%−82.2%, 0.1%−78.5%, and 0.3%−46.3%, respectively. Regarding the NIV for each organ, it is possible to obtain similar values except for the irradiation area of the brain stem. The percentages of NIV 10%, NIV20%, and NIV30%for the brain stem in each patient were 15%−99.8%, 33.4%−100%, and 39.8%−100%, respectively, in the fractionated stereotactic treatment plan and 44.2%-96.5%, 77.7%-99.8%, and 87.8%−100%, respectively, in the tomotherapy treatment plan. In order to achieve higher-quality treatment of intra-cranial tumors, treatment plans should be tailored according to the isodose target volume, inhomogeneous index, conformity index, position of the tumor upon fractionated stereotactic radiosurgery, and radiation dosage for radiation-sensitive tissues.

SU‐GG‐J‐73: Investigate the Equivalence of Cone Beam CT (CBCT) versus 4‐Dimensinoal (4D) MIP Images for Internal Target Volume (ITV) Definition

Purpose: Maximum intensity projection (MIP) reconstructed 4‐dimensional (4D) computed tomography (CT) reflects the range of target motion and thus is generally used for internal target volume (ITV) definition in lung stereotactic body radiotherapy treatment planning. During treatment delivery, CBCT is used for image guidance by aligning the visualized target in the CBCT with the MIP‐based ITV. This work investigates whether these two image modalities are equivalent in defining an ITV. Materials and Methods: A ball‐shaped polystyrene phantom with different built‐in objects (cube, cone, and sphere) was attached to a motor‐driven platform, which simulates a sinusoidal movement with changeable motion amplitude and frequency. The motion of the platform was set along patient superior‐inferior direction with 1‐cm peak‐to‐peak amplitude. The Varian on‐board Exact Arms kV CBCT system and the GE LightSpeed 4‐slice CT integrated with Respiratory Position Management 4DCT scanner were used to scan the phantom that moved with three frequencies (e.g. 3.4, 4.5, and 5.8 seconds). MIP images were produced. The objects were contoured in the MIP and CBCT images, respectively, and the volumes of these objects were compared between the two sets of images. Results: It was found that the MIP‐based volumes of these objects increase with the increase of the motion period while the CBCT‐based volumes do not change with the frequency. Even for the fast motion (3.4 sec), the volumes contoured in the MIP images, which are smallest among the volumes of three frequencies, are generally 10% larger than those delineated from CBCT. Conclusions: The delineated volume of an ITV changes as the frequency of target motion changes in the MTP reconstructed 4DCT. The delineated volume of a moving target is not affect by the frequency, but by motion magnitude, in CBCT. In general, CBCT and MIP images are not equivalent in defining an ITV.

Dose recalculation in megavoltage cone-beam CT for treatment evaluation: Removal of cupping and truncation artefacts in scans of the thorax and abdomen

Purpose To correct megavoltage cone-beam CT (MVCBCT) images of the thorax and abdomen for cupping and truncation artefacts to reconstruct the 3D-delivered dose distribution for treatment evaluation. Materials and methods MVCBCT scans of three phantoms, three lung and two rectal cancer patients were acquired. The cone-beam projection images were iteratively corrected for cupping and truncation artefacts and the resulting primary transmission was used for cone-beam reconstruction. The reconstructed scans were merged into the planning CT scan (MVCBCT+). Dose distributions of clinical IMRT, stereotactic and conformal treatment plans were recalculated on the uncorrected and corrected MVCBCT+ scans using the treatment planning system and compared to the planned dose distribution. Results The dose distributions on the corrected MVCBCT+ of the phantoms were accurate for 99% of the voxels within 2% or 2 mm. Using this method the errors in mean GTV dose reduced from about 10% to 1% for the patients. Conclusions The method corrects cupping and truncation artefacts in cone-beam scans of the thorax and abdomen in addition to head-and-neck (demonstrated previously). The corrected scans can be used to calculate the influence of anatomical changes on the 3D-delivered dose distribution.

Impact of tissue heterogeneity corrections in stereotactic body radiation therapy treatment plans for lung cancer

This study aims at evaluating the impact of tissue heterogeneity corrections on dosimetry of stereotactic body radiation therapy treatment plans. Four-dimensional computed tomography data from 15 low stage non-small cell lung cancer patients was used. Treatment planning and dose calculations were done using pencil beam convolution algorithm of Varian Eclipse system with Modified Batho Power Law for tissue heterogeneity. Patient plans were generated with 6 MV co-planar non-opposing four to six field beams optimized with tissue heterogeneity corrections to deliver a prescribed dose of 60 Gy in three fractions to at least 95% of the planning target volume, keeping spinal cord dose <10 Gy. The same plans were then regenerated without heterogeneity correction by recalculating previously optimized treatment plans keeping identical beam arrangements, field fluences and monitor units. Compared with heterogeneity corrected plans, the non-corrected plans had lower average minimum, mean, and maximum tumor doses by 13%, 8%, and 6% respectively. The results indicate that tissue heterogeneity is an important determinant of dosimetric optimization of SBRT plans.

SU-FF-T-633: Impact of Tissue Heterogeneity Corrections in Stereotactic Body Radiation Therapy Treatment Plans for Non Small Cell Lung Cancer Patients

Purpose: To study the effect of tissue heterogeneity corrections on the dosimetry of stereotactic body radiation therapy treatment plans for patients with non-small cell lung cancer. Materials and Methods: Data from 15 stage I non-small cell lung patients was used. Treatment planning was done with 6 MV non-opposing coplanar beams using Varian Eclipse TPS (PBC Algorithm with Modified Batho Power Law). Prescription dose was 60 Gy in 3 fractions with tissue heterogeneity corrections. Plans were normalized to deliver prescription dose to 95% of the planning target volume (PTV) maintaining the spinal cord dose under 10 Gy. Separate plans were generated by recalculating the optimized treatment plans without heterogeneity correction while keeping identical beam arrangements, monitor units, and fluences. Results: Compared with the heterogeneity corrected plans, the uncorrected plans had lower minimum, mean, and maximum tumor doses by at least 12.5%, 7.7%, and 6.0%. The PTV receiving less than prescription dose was significantly higher (>25%) in the uncorrected plans. The percentage of uninvolved lung volume receiving minimum doses of 10 Gy, 15 Gy, 20 Gy, and 30 Gy was ⩽ 10% for both treatment plans. However, the uncorrected plans calculated slightly less dose to normal lung. The average maximum spinal cord dose was 8.56 Gy and 9.73 Gy for corrected and uncorrected plans respectively. Conclusions: Treatment plans optimized with tissue heterogeneity corrections were generated to satisfy planning goals and constraints. Re-calculating such plans without tissue heterogeneity corrections and keeping identical conditions of beam and monitor units resulted in target underdosing because the planning system assumed that more tissue attenuation was present than is actually the case. By extension, failure to use tissue heterogeneity correction to plan complex beam arrangements to treat tumors within the lung will lead to delivery of higher doses than are prescribed to less than optimum dose distributions.

SU-FF-T-570: A Dosimetric Comparison as a Function of Respiration and Tumor Location in Stereotactic Body Radiation Therapy Treatment Plans For Patients with Non Small Cell Lung Cancer

Purpose: To calculate and compare doses to the tumor and normal tissues at full respiratory cycle, inhalation, and exhalation as a function of the tumor site and location within the thorax. Materials and Methods: 20 patients were scanned using 4D CT techniques. Treatment plans were created with five to seven 6 MV non-opposing coplanar beams using Varian Eclipse Treatment Planning System with tissue heterogeneity corrections for three respiratory phase combinations. A 7 mm margin was given to the internal target volume to create the planning target volume. The prescribed dose was 60 Gy in 3 fractions. Optimization goals and normal tissue constraints were the same as in the RTOG 0236 criteria. Statistical analysis used a mixed effect ANOVA model, and a 2-sided 0.05 alpha level was used for hypothesis testing. Results: Lower peripheral tumors had three times the displacement of the upper peripheral tumors. Central tumors received lower minimum doses than the peripheral tumors. When tumors were in the peripheral region, the uninvolved lung mean dose was significantly different for all phases of respiration (p=0.008), specially, when the tumors were in the upper peripheral region (p=0.04), and the doses were higher (>26%) in the lower peripheral region compared to the upper one for all phases of respiration. The V15, V20, and V30 for uninvolved lung were below 10%. Patients with central tumors received higher maximum doses in the esophagus than patients with peripheral tumors. For the ipsilateral bronchus, patients with central tumors received higher mean and maximum doses than patients with peripheral tumors. There were no statistical differences in the mean and maximum doses to the heart or to the spinal cord. Conclusions: Lesions in the lower part of the lung are more impacted by respiratory motion. Central lesions receive less tumor dose to preserve normal tissue dose limits.

Implementation and experimental validation of the high dose rate stereotactic treatment mode at Varian accelerators.

Background. Treating small tumours in the brain requires usage of small fields. However, it is a challenge for all treatment planning systems (TPS) to modulate small fields (≤3×3 cm2) for which lateral electronic equilibrium does not exits. Material and methods. Open field beam data was measured for the stereotactic mode at our Varian Trilogy accelerator and was implemented in the Eclipse TPS for both the Pencil Beam (PB) and the Anisotropic Analytical Algorithm (AAA). Additional output factors, profiles and percentage depth dose curves were measured in a water phantom for asymmetric fields and fields defined by the Varian 120 MLC and the BrainLAB m3 MLC. Dose distributions in transverse sections were measured with gafchromic films in a homogeneous phantom. These were compared to those calculated by the Eclipse TPS for 11 stereotactic treatment plans. The plans included non-coplanar, dynamic and arc fields. All comparisons for the film measurements in the phantom were made with a Gamma criterion of 2 mm in distance and 3% in dose. Results. The AAA and the PB algorithms broadened the penumbra of the profiles by approximately 1.5 mm and 2.0 mm, respectively. For static fields defined by the Varian 120 MLC, the measured field size was 1.0 mm broader than the calculated. The output factors were typically within 2% for field sizes ≥ 2×2 cm2. More than 95% of the points passed the Gamma criterion. Up to 50% of the points in the low dose regions failed the criterion, but overall the match was satisfactory. Conclusion. The accuracy of calculations performed by the Eclipse TPS for the SRS mode was shown to be within a few percent for most clinical fields. AAA was found to be superior to the PB algorithm in modelling the dose in the penumbra region.

Impact of Individualized Target Volumes on Stereotactic Body Radiotherapy Treatment Planning (SBRT): Volumetric Analysis

Impact of Individualized Target Volumes on Stereotactic Body Radiotherapy Treatment Planning (SBRT): Volumetric Analysis

Interfraction Variations in Respiratory Phase Offset between Tumor and Xyphoid: Dosimetric Consequences during Respiratory Gated Stereotactic Radiotherapy for Lung Cancer

To assess the potential dosimetric impact of variations in interfraction phase offset of tumor vs. xyphoid process (surrogate for chest wall) on target coverage in stereotactic treatment plans utilizing respiratory gating.