Stereotactic Body Radiation Therapy Treatment Plans Research Articles

Novel Hybrid Volumetric Modulated Arc Therapy (VMAT) for Stereotactic Body Radiation Therapy (SBRT) of Lung Tumors in Proximity to the Chest Wall

It can be challenging to optimize SBRT treatment plans of peripheral lung lesions because of the large variation in tissue heterogeneity and close proximity to the chest wall. It has been shown that coplanar volumetric modulated arc therapy (VMAT) is capable of achieving high dose conformity comparable to coplanar static-angle IMRT but with slower dose fall-off than noncoplanar IMRT. To reduce dose to the chest wall and lung, we developed a hybrid VMAT planning strategy to achieve both high dose conformity and steep fall-off beyond the target volume. Seventeen lung lesions in sixteen patients with medically inoperable early stage non-small cell lung cancer or lung oligometastases treated with hybrid VMAT were reviewed. All cases had overlap of the planning target volume (PTV) with the chest wall. Our hybrid VMAT planning approach utilized noncoplanar conformal beams and coplanar intensity modulated arcs. Three to four noncoplanar beams were manually optimized to form the base dose plans, which contribute 25% to 30% of the prescribed dose. The VMAT portion, developed by base plan optimization, consisted of two full rotations. To demonstrate the benefit of the hybrid technique, we compared the hybrid plans with pure VMAT plans. Plan quality was evaluated using: conformity index (CI), ratio of 50% isodose volume to PTV volume (R50%), dose homogeneity, and lung volume receiving 10 and 20 Gy (V10 and V20). All plans in this study were normalized by scaling the 100% isodose line to cover 95% of the PTV with the dose maximum ranging from 124% to 134%. In all cases, the hybrid plans yielded better dose conformity (averaged CI = 1.09; range 1.03 - 1.24) than that of the pure VMAT plans (averaged CI = 1.14; range 1.06 - 1.28). Moreover, the dose gradient was steeper in the hybrid plans: averaged hybrid R50%= 4.90, (range 4.02 - 6.97) vs. pure VMAT 5.36 (range 4.24 - 7.45). Thus the hybrid technique was able to reduce dose to the chest wall and ribs. We noted that R50% is correlated to the size of PTV. No significant difference between the two approaches was found in pulmonary V20 and V10. Treatment delivery time of the hybrid plans ranged from 11 minutes to 14 minutes. At a median follow-up of 6.7 months (range, 1 to 14 months), local control is 95.2% (1 in-field failure in patient with a 4.5 cm mass). One patient experienced grade 2 chest wall pain and 1 patient experienced grade 1 dermatitis, which have resolved. Using the hybrid VMAT technique, we have achieved higher dose conformity with lower dose to the chest wall and ribs compared to a standard coplanar VMAT approach. Longer-term follow up is required to quantify the clinical significance of the hybrid VMAT strategy.

Poster - Thur Eve - 64: Evaluation of SmartArc and RapidArc for lung SBRT treatment planning and delivery.

To evaluate the performance of RapidArc® (Eclipse 10.0.28) and SmartArc® (Pinnacle 9.0) radiotherapy plans for lung stereotactic body radiation therapy (SBRT) in terms of dosimetric plan quality, delivery efficiency, inhomogeneity corrections and accuracy of dose delivery using a custom-built heterogeneity insert for ArcCHECK™ (Sun Nuclear Corp., FL, USA). SmartArc® and RapidArc® plans were generated for 10 patients. The quality of the plans was evaluated in terms of conformity indices (R100 and R50 ) and the dose to the organs at risk. The efficiency was evaluated in terms of the monitor units (MUs) required for a given prescription dose. For dose verification, we designed and manufactured a heterogeneity insert for ArcCHECK™ with densities that simulate soft tissue, lung, bone, and air and having multiple locations for point dose measurements. Accuracy of dose delivery was assessed using gamma analysis. The overall plan quality was similar when comparing SmartArc® with RapidArc®. However, RapidArc® plans required significantly more MUs-up to 72%™compared with SmartArc® plans (p<0.001). ArcCHECK™ measurements in the presence of inhomogeneities showed better agreement for SmartArc® plans. Plan quality for SmartArc® and RapidArc® was comparable. However, SmartArc® plans were more efficient, requiring significantly fewer monitor units, and were delivered more accurately in a non-homogeneous phantom. With the custom-built heterogeneity insert, ArcCHECK™ can be used efficiently to verify inhomogeneity corrections and dose delivery accuracy for lung SBRT plans.

Poster - Thur Eve - 63: Dosimetric impact of breathing motion in lung SBRT: Dual vs single volumetric modulated arc therapy.

Volumetric modulated arc therapy (VMAT) is a time efficient treatment delivery platform capable of producing highly conformal dose distributions with a single 360° arc. However, additional arcs can be used to further improve the conformal dose distribution. For these reasons, VMAT is often used for stereotactic body radiation therapy (SBRT) in which the treatment deliveries are hypofractionated. The dosimetric impact of tumour motion, especially in lung SBRT where tumour motion is most significant and treatments are hypofractionated, has always been a clinical concern. Through the use of 4-dimensional computed tomography (4D-CT), 4D dose distributions can be calculated that account for dosimetric errors due to motion and temporal variation in lung density that are not accounted for in clinical treatment plans. The purpose of this study was to quantify the dosimetric differences that arise due to tumour motion and variations in lung density between single and dual VMAT SBRT treatment plans. Six patients previously treated for stage I/II non-small-cell lung cancer with SBRT were included in this retrospective study. 3D and 4D dose distributions were calculated for both single and dual arc plans for each of the six patients. Dose-volume histogram metrics are reported for the target and critical structures. The results show significant differences (p ≤ 0.05) between the 3D and 4D dose distributions for the ratio of the prescription isodose volume to the primary target volume (PTV). This result was consistent for both single and dual arc VMAT plans.

Sci-Thur PM: YIS - 03: Comparing 4D-VMAT, Gated-VMAT and 3D-VMAT in SBRT treatment of lung cancer.

To evaluate the treatment plan qualities of 4D-VMAT, gated-VMAT and 3D-VMAT in the treatment of non-small cell lung cancer (NSCLC) in stereotactic body radiation therapy (SBRT). 4D-VMAT is a motion compensation strategy that aims to exploit relative target and OAR motion to increase OAR sparing over 3D-VMAT without the long treatment times associated with gated-VMAT. The 4D-VMAT algorithm incorporates the entire patient respiratory cycle and 4D-CT in the optimization process. Resulting treatment plans synchronize the delivery of each MLC aperture to a specific phase of the target motion. Using software developed in Matlab™, SBRT treatment plans for 4D-VMAT, gated-VMAT and 3D-VMAT were generated on 3 patients with NSCLC. Tumour motion ranged from 1.4-3.4 cm. The fractionation scheme was 48Gy in 4 fractions with the GTV receiving 100% of the prescribed dose. For gated-VMAT, the treatment window constrained residual tumour motion to 3 mm or less corresponding to duty cycles of 40-60%. In 3D-VMAT, the ITV was generated by merging the GTV from all phases. A b-spline transformation model was used to register the 4D-CT images and DVHs were calculated from total dose accumulated on the max expiration phase. For the majority of OARs, gated-VMAT provided the greatest radiation sparing but significantly extended treatment times (25-35 gantry interruptions/arc). For 3D-VMAT, only 2 patients had clinically acceptable plans that met all the strict dose limits. OAR sparing in 4D-VMAT was comparable to gated-VMAT but with significantly improved delivery efficiency.

SU-E-T-511: Dose Variations Related to Tumor Size and Location for Pencil Beam and Anisotropic Analytical Algorithms.

To investigate the tumor geometric relationship to the dose variations of Anisotropic Analytical Algorithm (AAA), pencil beam convolution algorithm with/without modified Bath Power Law(PBMPL)/PB in stereotactic body radiation therapy(SBRT) treatment plans of patients presenting with a solitary primary lung cancer. Treatment plans of 14 patients (7 upper lobe, 7 lower lobe) were used for this study. The planning target volume (PTV) size ranges from 3.9c.c. to 156.7c.c. The SBRT treatment plans were composed of 10-12 non- coplanar photon beams as per RTOG guidelines. The prescription dose for this study were (i) 4×12Gy, (ii) 5×10Gy, and (iii) 5×11Gy. The Varian Eclipse treatment planning system Eclipse v. 8.9 (Palo Alto, CA) was used for this study. Four-dimensional CT (4D CT) data were used to define the integral target volume (ITV) on maximum intensity projection. An 5mm circumferential margin was used to create PTV from ITV. Plans were generated with three algorithms. a). For small lesions (PTV occupy less than 1% of the ipsilateral lung volume), the PBMPL plans had overestimated the dose by average 10% compared to AAA. But the PB without any heterogeneity correction agrees well with AAA. b). For big lesions (PTV occupy more than 1% of the ipsilateral lung volume), the PBMPL plans had agreed well with AAA. But the PB without any heterogeneity correction underestimate the dose by average 15% compared to AAA. c). The tumor location (Group1: within 1cm from the lung wall;Group2: 2cm away from the lung wall Group3: in between zone of 1 and 2cm from the lung wall; Group 4:Spread from the lung wall to the 2cm away zone) seems to relate with dose calculation variations among different algorithms. Prescription adjustment is not necessary for PTV less than 1% of ipsilateral lung volume as the recent suggestion by the quality assurance working group of phase III Rosel study of prescription dose reduction of 10% from 60Gy to 54Gy when utilizing AAA instead of PBC.

SU-E-T-648: Comparison of VMAT Vs Arc Treatment Plans for Patients Undergoing SBRT of Early-Stage Non-Small Cell Lung Cancer (NSCLC).

To evaluate the dosimetric implications of using VMAT (Volume Modulated Arc Therapy) treatment planning techniques compared to traditional Arc therapy methods for patients undergoing SBRT (Stereotactic Body Radiation Therapy) for early-stage Non-Small Cell Lung Cancer (NSCLC). Ten NSCLC cancer patients are planned with both VMAT and Arc techniques. The SBRT treatment plans comparison was quantified by several Dose-Volume Histogram (DVH) indicators including mean, maximum and minimum doses for GTV, ITV, PTV, OAR (Organs At Risk), and V95 (volume receiving at least 95% of the prescribed dose) for PTV. On average VMAT plans require for treatment delivery 16.6 ± 20.2 % more monitor units (MU) than the traditional Arc plans. The average PTV minimum, maximum and mean doses as a percentage of prescribed dose are 94.5 ± 3.9 %, 114.1 ± 3.3 % and 106.6 ± 1.6 % for VMAT vs 91.6 ± 4.4 %, 119.5 ± 5.3 % and 109.5 ± 2.5 % for the Arc technique. The V95 PTV coverage for VMAT plans range from 99.4 % to 100 % with a mean of 99.7 %, compared with a range of 96.8 % to 100 % with a mean of 99 % for the Arc plans. The maximum dose received by the lungs, spinal cord and chest wall show on average significant increases for Arc plans as opposed to VMAT plans (5.7 ± 6 % increase for lungs, 4.4 ± 9.2 % for cord and 2.4 ± 6.3 % for chest wall). The average mean doses and minimum doses for the OAR are similar for both techniques. The comparison of VMAT vs Arc plans for SBRT of NSCLC patients is subject to many variables, including GTV and PTV volume sizes, shape and their proximity relative to the OAR.

Stereotactic Radiosurgery for Gynecologic Cancer

Stereotactic body radiotherapy (SBRT) distinguishes itself by necessitating more rigid patient immobilization, accounting for respiratory motion, intricate treatment planning, on-board imaging, and reduced number of ablative radiation doses to cancer targets usually refractory to chemotherapy and conventional radiation. Steep SBRT radiation dose drop-off permits narrow 'pencil beam' treatment fields to be used for ablative radiation treatment condensed into 1 to 3 treatments.Treating physicians must appreciate that SBRT comes at a bigger danger of normal tissue injury and chance of geographic tumor miss. Both must be tackled by immobilization of cancer targets and by high-precision treatment delivery. Cancer target immobilization has been achieved through use of indexed customized Styrofoam casts, evacuated bean bags, or body-fix molds with patient-independent abdominal compression.1-3 Intrafraction motion of cancer targets due to breathing now can be reduced by patient-responsive breath hold techniques,4 patient mouthpiece active breathing coordination,5 respiration-correlated computed tomography,6 or image-guided tracking of fiducials implanted within and around a moving tumor.7-9 The Cyberknife system (Accuray [Sunnyvale, CA]) utilizes a radiation linear accelerator mounted on a industrial robotic arm that accurately follows patient respiratory motion by a camera-tracked set of light-emitting diodes (LED) impregnated on a vest fitted to a patient.10 Substantial reductions in radiation therapy margins can be achieved by motion tracking, ultimately rendering a smaller planning target volumes that are irradiated with submillimeter accuracy.11-13Cancer targets treated by SBRT are irradiated by converging, tightly collimated beams. Resultant radiation dose to cancer target volume histograms have a more pronounced radiation "shoulder" indicating high percentage target coverage and a small high-dose radiation "tail." Thus, increased target conformality comes at the expense of decreased dose uniformity in the SBRT cancer target. This may have implications for both subsequent tumor control in the SBRT target and normal tissue tolerance of organs at-risk. Due to the sharp dose falloff in SBRT, the possibility of occult disease escaping ablative radiation dose occurs when cancer targets are not fully recognized and inadequate SBRT dose margins are applied. Clinical target volume (CTV) expansion by 0.5 cm, resulting in a larger planning target volume (PTV), is associated with increased target control without undue normal tissue injury.7,8 Further reduction in the probability of geographic miss may be achieved by incorporation of 2-[18F]fluoro-2-deoxy-D-glucose (18F-FDG) positron emission tomography (PET).8 Use of 18F-FDG PET/CT in SBRT treatment planning is only the beginning of attempts to discover new imaging target molecular signatures for gynecologic cancers.

Stereotactic Radiosurgery for Gynecologic Cancer

Stereotactic body radiotherapy (SBRT) distinguishes itself by necessitating more rigid patient immobilization, accounting for respiratory motion, intricate treatment planning, on-board imaging, and reduced number of ablative radiation doses to cancer targets usually refractory to chemotherapy and conventional radiation. Steep SBRT radiation dose drop-off permits narrow 'pencil beam' treatment fields to be used for ablative radiation treatment condensed into 1 to 3 treatments. Treating physicians must appreciate that SBRT comes at a bigger danger of normal tissue injury and chance of geographic tumor miss. Both must be tackled by immobilization of cancer targets and by high-precision treatment delivery. Cancer target immobilization has been achieved through use of indexed customized Styrofoam casts, evacuated bean bags, or body-fix molds with patient-independent abdominal compression.1-3 Intrafraction motion of cancer targets due to breathing now can be reduced by patient-responsive breath hold techniques,4 patient mouthpiece active breathing coordination,5 respiration-correlated computed tomography,6 or image-guided tracking of fiducials implanted within and around a moving tumor.7-9 The Cyberknife system (Accuray [Sunnyvale, CA]) utilizes a radiation linear accelerator mounted on a industrial robotic arm that accurately follows patient respiratory motion by a camera-tracked set of light-emitting diodes (LED) impregnated on a vest fitted to a patient.10 Substantial reductions in radiation therapy margins can be achieved by motion tracking, ultimately rendering a smaller planning target volumes that are irradiated with submillimeter accuracy.11-13 Cancer targets treated by SBRT are irradiated by converging, tightly collimated beams. Resultant radiation dose to cancer target volume histograms have a more pronounced radiation "shoulder" indicating high percentage target coverage and a small high-dose radiation "tail." Thus, increased target conformality comes at the expense of decreased dose uniformity in the SBRT cancer target. This may have implications for both subsequent tumor control in the SBRT target and normal tissue tolerance of organs at-risk. Due to the sharp dose falloff in SBRT, the possibility of occult disease escaping ablative radiation dose occurs when cancer targets are not fully recognized and inadequate SBRT dose margins are applied. Clinical target volume (CTV) expansion by 0.5 cm, resulting in a larger planning target volume (PTV), is associated with increased target control without undue normal tissue injury.7,8 Further reduction in the probability of geographic miss may be achieved by incorporation of 2-[18F]fluoro-2-deoxy-D-glucose (18F-FDG) positron emission tomography (PET).8 Use of 18F-FDG PET/CT in SBRT treatment planning is only the beginning of attempts to discover new imaging target molecular signatures for gynecologic cancers.

SU‐E‐T‐880: An Investigation of Kernel‐Based Dynamic Dose Painting Treatment Approach

Purpose: Kernel‐based dynamic dose painting was introduced at the 2010 AAPM meeting. The purpose of this study was to investigate a practical treatment planning and delivery approach with current technology and potential dosimetric advantages on 3D patient anatomy for such an approachMethods: For a dynamic dose painting delivery, a dose kernel (i.e., paintbrush) is first formed from a high number of converging beams, and then such a kernel transverses (i.e. paint) continuously throughout a target volume to create a desired dose distribution. In this study, we employed fixed ball‐shaped kernels modeled from the beam geometry similar to those of commercial Gamma Knife (GK) or body GK units such as Gyroknife, and dose painting was realized by shifting the patient (i.e., target) via a dynamic couch with six degrees of freedom. In addition, the size of the kernel was also made changeable along its painting path. We tested such an approach on treatment planning of prostate stereotactic body radiotherapy (SBRT) cases. Results: Conformal prostate SBRT treatment plans were feasibly developed. At the posterior prostate target and the rectum junction area, separation in the isodose lines between 100% and 75% of the prescription dose was less than 2 mm, yielding a dose fall‐off gradient of approximately 13% per mm or higher. Under a nominal dose rate of 600 MU/min, the beam‐on time for such a delivery was approximately 30 minutes for a dose of 10‐12 Gy to the target periphery. The effective number of the beams for such treatment however reached approximately 8,500. Conclusions: Kernel‐based dynamic dose painting offers a new level of dose conformity and normal tissue sparing surrounding a target.

SU-E-T-583: QA Verifications of RapidArc Type SBRT Treatment Plans

Purpose: RapidArc treatments combine features of IMRT and arc treatments. It may be a superior treatment technique for stereotactic body radiation therapy(SBRT) in both improved dose distribution as well as improved patient comfort and reduction in motion due to rapid treatmentdelivery. This is a study of the QA verification results on 15 spine and 9 lung patients treated with RapidArc SBRT since August 2009. Methods: Treatements were delivered on a Varian Trilogy with RapidArc upgrade. Plans generated used 6 MV photons utilizing Varian Eclipse software. Most plans consisted of a 2 arc technique. The 3D diode array is the Delta4 system from Scandidos. Delta4 has 1069 P type Si diodes on three wings, which are inserted to two planes in a cylindrical PMMA phantom. The three dimensional dose is transferred to the Delta4 by DICOM RT. To minimize the MLC leakage, and to reduce the finite leaf width (0.5 cm) effects, we used collimator angles of 155 and 215 (Varian scale). Fraction dose ranged from 400 cGy to 1800 cGy. For each plan, the planned dose distribution and measured dose distribution are compared by using 2D isodose display, profile comparisons, percentage dose deviation, and DTA and Gamma index. Results: The average percentage of diode with Gamma index (3% dose and 3 mm) 1 or less is 95.5% with standard deviation of 3.4% for spine SBRT plans, and average percentage of diode with Gamma index (3% dose and 3 mm) 1 or less is 97.4% with standard deviation of 2.0% for lungSBRT plans, Conclusions: All measurement results agree well with the dose distribution of all SBRT plans. Delta4 system is a good QA tool for RapidArc type SBRTtreatments.

SU-E-T-521: Is Cone Beam CT (CBCT) Equivalent to 4-Dimensinoal (4D) MIP Images to Account for Target Motion in SBRT Lung Treatment?

Purpose: Maximum intensity projection (MIP) reconstructed 4D CT reflects the range of target motion and thus is generally used for internal target volume (ITV) definition in lung stereotactic body radiotherapytreatment planning. During treatment delivery, CBCT is used for image guidance by aligning the target in the CBCT within the MIP‐based ITV. This work investigates whether these two image modalities are equivalent in determining the range of the target motion. Methods: A ball‐shaped polystyrene phantom with built‐in cube and sphere was attached to a motor‐ driven platform, which simulates a sinusoidal movement with changeable motion amplitude and frequency. Target motion was simulated both one‐ and two‐dimensionally by adjusting the platform alignment angle with the patient superior‐inferior (S‐I) direction. 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 four motion periods and varying peak‐to‐peak amplitudes. MIP images were produced. The maximum dimensions of the moving objects visualized in the two sets of images were measured on axial, coronal, and sagital views and compared, respectively. Results: When the motion is along the S‐I direction, the measured dimensions along the right‐ left and AP directions agree with the objects' dimensions with small uncertainties. The measured dimension along the S‐I direction in the MIP and the CBCT differ by a maximum of 10.2%, with a larger uncertainty associated with CBCT‐based measurements. We also found that the measured dimensions and the centroid positions of each object do not change with motion period, but with motion amplitude. Conclusions: The measured maximum dimensions of the objects, which are not affected by the motion frequency but by motion amplitude, are comparable between the two image modalities. Therefore, in general, CBCT and MIP images are equivalent in determining target motion.

Helical Tomotherapy-Based STAT Stereotactic Body Radiation Therapy: Dosimetric Evaluation for a Real-Time SBRT Treatment Planning and Delivery Program

Stereotactic body radiation therapy (SBRT) treatments have high-dose gradients and even slight patient misalignment from the simulation to treatment could lead to target underdosing or organ at risk (OAR) overdosing. Daily real-time SBRT treatment planning could minimize the risk of geographic miss. As an initial step toward determining the clinical feasibility of developing real-time SBRT treatment planning, we determined the calculation time of helical TomoTherapy–based STAT radiation therapy (RT) treatment plans for simple liver, lung, and spine SBRT treatments to assess whether the planning process was fast enough for practical clinical implementation. Representative SBRT planning target volumes for hypothetical liver, peripheral lung, and thoracic spine lesions and adjacent OARs were contoured onto a planning computed tomography scan (CT) of an anthropomorphic phantom. Treatment plans were generated using both STAT RT “full scatter” and conventional helical TomoTherapy “beamlet” algorithms. Optimized plans were compared with respect to conformality index (CI), heterogeneity index (HI), and maximum dose to regional OARs to determine clinical equivalence and the number of required STAT RT optimization iterations and calculation times were determined. The liver and lung dosimetry for the STAT RT and standard planning algorithms were clinically and statistically equivalent. For the liver lesions, “full scatter” and “beamlet” algorithms showed a CI of 1.04 and 1.04 and HI of 1.03 and 1.03, respectively. For the lung lesions, “full scatter” and “beamlet” algorithms showed a CI of 1.05 and 1.03 and HI of 1.05and 1.05, respectively. For spine lesions, “full scatter” and “beamlet” algorithms showed a CI of 1.15 and 1.14 and HI of 1.22 and 1.14, respectively. There was no difference between treatment algorithms with respect to maximum doses to the OARs. The STAT RT iteration time with current treatment planning systems is 45 sec, and the treatment planning required 3 iterations or 135 sec for STAT RT liver and lung SBRT plans and 7 iterations or 315 sec for STAT RT spine SBRT plans. Helical TomoTherapy–based STAT RT treatment planning with the “full scatter” algorithm provides levels of dosimetric conformality, heterogeneity, and OAR avoidance for SBRT treatments that are clinically equivalent to those generated with the Helical TomoTherapy “beamlet” algorithm. STAT RT calculation times for simple SBRT treatments are fast enough to warrant further investigation into their potential incorporation into an SBRT program with daily real-time planning. Development of methods for accurate target and OAR determination on megavoltage computed tomography scans incorporating high-resolution diagnostic image co-registration software and CT detector–based exit dose measurement for quality assurance are necessary to build a real-time SBRT planning and delivery program.

Analysis of Radiation Pneumonitis (RP) Incidence in a Phase I Stereotactic Body Radiotherapy (SBRT) Dose Escalation Study for Multiple Metastases

Analysis of Radiation Pneumonitis (RP) Incidence in a Phase I Stereotactic Body Radiotherapy (SBRT) Dose Escalation Study for Multiple Metastases

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.

WE‐D‐BRA‐01: Stereotactic Body Radiotherapy (SBRT): Technical and Clinical Considerations

Stereotactic Body Radiotherapy (SBRT) has now been applied clinically for more than a decade. The accumulation of published technical and clinical studies currently available has offered insights into both the efficacy and special concerns associated with the delivery of SBRT to different sites. The safe and effective delivery of SBRT requires careful attention to tumor motion management, special dosimetric and planning challenges, and commissioning of dedicated equipment and quality assurance. This session will provide an overview of these important technical considerations as well as a survey of clinical outcomes reported based on the use of SBRT in the treatment of different tumor sites.Learning Objectives:1. Describe available methods for motion management and the rational selection of planning margins2. Discuss techniques for SBRT treatment planning and the impact of dose calculation algorithm3. Describe key elements of commissioning, patient‐specific QA, and routine periodic machine performance QA4. Review clinical observations reported after SBRT for lung, liver, pancreas, spine, head &amp; neck, and prostate

SU-GG-T-591: Daily Imaging and Dose Deformation for Assessment of Stereotactic Body Radiation Therapy (SBRT) of Lung Cancer

Purpose: The clinical application of lung SBRT requires extremely accurate targeting during treatment since its dose regimen is so potent. Although abdominal compression may be used to reduce respiratory motion, tumor motion is not eliminated entirely and it is still necessary to evaluate the delivered dose to the target during treatment. This study was designed to explore a framework for accurate tracking of delivered dose using daily CBCT data. Method and Materials: Ten lung SBRT patients were used for this study. The GTV-PTV margins were 5 mm. The prescription was 20 Gy per fraction for three fractions for a total of 60 Gy. For each fraction, an onboard CBCT system was used to verify patient position during treatment. The Pinnacle Treatment Planning System, research version 8.1 y was used to calculate SBRT treatment plans and generate GTV volumes for each fraction based on CBCT intensity thresholds, as well as to perform deformation dose processing based on these volumes. The gEUD was used to evaluate each fraction and the three-fraction composite delivered dose to the GTV. Results: Results show that in eight of ten patients, the GTV gEUDs of the composite plan were higher than that of the original plan, indicating that the GTV received the intended dose. Two of the patients had a lower GTV gEUD for the three-fraction composite plan when compared to the original plan. This indicated the same partial volume of the GTV received a lower dose in multiple fractions. Conclusions: This study demonstrates that this technique can be used to assess the dose actually delivered in lung SBRT on a daily basis. It will be used to investigate strategies for further improvement in treatment delivery. This method may be useful for correcting an underdose in an early fraction by modifying subsequent fractions.

SU-FF-T-563: Dosimetric Impact of Five Tumor Delineation Strategies in Stereotactic Body Radiation Therapy for Lung Cancer

Purpose: The goal of this study was to examine the dosimetric impact of five different tumor delineation strategies in stereotactic body radiation therapy(SBRT) for lungcancer patients. Methods and Materials: Seven patients who had previously undergone SBRT for lungcancer were retrospectively investigated. For each patient, a free breathing (FB) CT and a 4DCT were acquired. Based on the 4DCT scans, three post‐processing CTimages were reconstructed: maximum intensity projection (MIP), average intensity projection (AIP), and slow‐CT (SCT) images. The gross target volumes (GTVs) were delineated on the following CTimagedata sets: GTVFB on FB CT, GTV0%, GTV10%, …, GTV90% on 4DCT, GTVMIP on MIP CT, GTVAIP on AIP CT, and GTVSCT on SCT. The GTVs delineated on the 4DCT were combined to create the internal target volume (ITV). Five SBRT treatment plans were created on the FB CTimage based on the tumor delineated above: GTVFB, ITV, GTVMIP, GTVAIP, and GTVSCT. For each plan, the 4D dose was calculated using deformable‐image registration. Results: On average the tumor D100 (minimum dose received by 100% of the tumor) of the ITV and GTVMIP based plans is 3.0±4.0Gy (p=0.09) and 0.9±4.5Gy (p=0.61) respectively above that of the GTVFB based plan. While the tumor D100 of the GTVAIP and GTVSCT based plans is 2.8±6.0Gy (p=0.26) and 0.8±4.1Gy (p=0.61) below that of the GTVFB based plan respectively. Compared with the GTVFB based plan, total lung V20 of the ITV based plan is 0.4±1.0% (p=0.36) absolute higher, while that of the GTVMIP, GTVAIP, and GTVSCT based plans is 0.5±0.7% (p=0.09), 0.4±0.7% (p=0.17), and 0.2±0.5% (p=0.44) absolutely lower respectively. Conclusions: All plans can deliver equally well dose coverage to the tumor. The difference in lungdose among the five plans is also significantly small.

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

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Hypofractionated stereotactic body radiotherapy(SBRT) in the treatment of lung tumors delivers high dose fractions with radioablative potential. Consequenlty, accurate dosimetric representation is crucial in treatment planning. To evaluate the treatment uncertainties introduced due to homogeneous dose calculation and manual beam aperture design for lung cancer patients treated with SBRT. Twenty-one manually designed, homogeneously-calculated SBRT plans(PLAN1), with coplanar and noncoplanar beam arrangements, were generated from CT planning data sets of clinical stage I non-small cell lung cancer patients. If those plans met all dosimetric parameters described in RTOG 0236, three additional plans with identical beam arrangements were designed. PLAN2 had identical beam design as PLAN1, but was recalculated with heterogeneity correction. PLAN3 and PLAN4 were designed using heterogeneity correction from the start, allowing changes in beam characteristics to achieve adequate target volume coverage. Beam weights and aperture were optimized manually(PLAN3) or automatically(PLAN4) by using the Dynamic Machine Parameter Optimization(DMPO) algorithm in Pinnacle. PLAN1 and PLAN4 served as the bases of comparison for PLAN2 and PLAN3, respectively. Dosimetric parameters of the four plans were evaluated and compared. Six cases failed to meet all dosimetric criteria and were excluded from further planning. Of the remaining 15 cases that passed, only 3 retained adequate PTV V60/V54 coverage (mean absolute reduction: 10.6%/5.6%, respectively) when recalculated with heterogeneity correction(PLAN2). One homogeneously-calculated case that passed all critical normal tissue constraints failed after it was recalculated with heterogeneity correction. All manually(PLAN3) and DMPO(PLAN4)-designed, heterogeneity-corrected plans met all dosimetric criteria. DMPO-designed plans exhibited the best PTV coverage without compromising critical normal tissue constraints. The active time for planning was significantly shorter using DMPO, compared to manual planning. Failing to apply heterogeneity correction to hypofractionated SBRT treatment plans results in reduced target volume coverage with the potential for overdosing critical normal tissues. Heterogeneity-corrected plans displayed superior PTV coverage with only minor increases in lung dose-volume parameters. The automated beam weighting/aperture optimization (DMPO) algorithm with heterogeneity correction resulted in the most optimal plans and the shortest planning time.