G. Hugo

Population and patient-specific target margins for 4D adaptive radiotherapy to account for intra- and inter-fraction variation in lung tumour position.

In this work, five 4D image-guidance strategies (two population, an offline adaptive and two online strategies) were evaluated that compensated for both inter- and intra-fraction variability such as changes to the baseline tumour position and respiratory pattern. None of the strategies required active motion compensation such as gating or tracking; all strategies simulated a free-breathing-based treatment technique. Online kilovoltage fluoroscopy was acquired for eight patients with lung tumours, and used to construct inter- and intra-fraction tumour position variability models. Planning was performed on a mid-ventilation image acquired from a respiration-correlated CT scan. The blurring effect of tumour position variability was included in the dose calculation by convolution. CTV to PTV margins were calculated for variability in the cranio-caudal direction. A population margin of 9.0 +/- 0.7 mm was required to account for setup error and respiration in the study population without the use of image-guidance. The greatest mean margin reduction was introduced by the offline adaptive strategy. A daily online correction strategy produced a small reduction (1.6 mm) in the mean margin from the offline strategy. Adaptively correcting for an inter-fraction change in the respiratory pattern had little effect on margin size due to most patients having only small daily changes in the respiratory pattern. A daily online correction strategy would be useful for patients who exhibit large variations in the daily mean tumour position, while an offline adaptive strategy is more applicable to patients with less variation.

A simplified method of four-dimensional dose accumulation using the mean patient density representation

The purpose of this work was to demonstrate, both in phantom and patient, the feasibility of using an average 4DCT image set (AVG-CT) for 4D cumulative dose estimation. A series of 4DCT numerical phantoms and corresponding AVG-CTs were generated. For full 4D dose summation, static dose was calculated on each phase and cumulative dose was determined by combining each phases static dose distribution with known tumor displacement. The AVG-CT cumulative dose was calculated similarly, although the same AVG-CT static dose distribution was used for all phases (i.e., tumor displacements). Four lung cancer cases were also evaluated for stereotactic body radiotherapy and conformal treatments; however, deformable image registration of the 4DCTs was used to generate the displacement vector fields (DVFs) describing patient-specific motion. Dose discrepancy between full 4D summation and AVG-CT approach was calculated and compared. For all phantoms, AVG-CT approximation yielded slightly higher cumulative doses compared to full 4D summation, with dose discrepancy increasing with increased tumor excursion. In vivo, using the AVG-CT coupled with deformable registration yielded clinically insignificant differences for all GTV parameters including the minimum, mean, maximum, dose to 99% of target, and dose to 1% of target. Furthermore, analysis of the spinal cord, esophagus, and heart revealed negligible differences in major dosimetric indices and dose coverage between the two dose calculation techniques. Simplifying 4D dose accumulation via the AVG-CT, while fully accounting for tumor deformation due to respiratory motion, has been validated, thereby, introducing the potential to streamline the use of 4D dose calculations in clinical practice, particularly for adaptive planning purposes.

Dosimetric Advantages of Four-Dimensional Adaptive Image-Guided Radiotherapy for Lung Tumors Using Online Cone-Beam Computed Tomography

PURPOSEnThis study compares multiple planning techniques designed to improve accuracy while allowing reduced planning target volume (PTV) margins though image-guided radiotherapy (IGRT) with four-dimensional (4D) cone-beam computed tomography (CBCT).nnnMETHODS AND MATERIALSnFree-breathing planning and 4D-CBCT scans were obtained in 8 patients with lung tumors. Four plans were generated for each patient: 3D-conformal, 4D-union, 4D-offline adaptive with a single correction (offline ART), and 4D-online adaptive with daily correction (online ART). For the 4D-union plan, the union of gross tumor volumes from all phases of the 4D-CBCT was created with a 5-mm expansion applied for setup uncertainty. For offline and online ART, the gross tumor volume was delineated at the mean position of tumor motion from the 4D-CBCT. The PTV margins were calculated from the random components of tumor motion and setup uncertainty.nnnRESULTSnAdaptive IGRT techniques provided better PTV coverage with less irradiated normal tissues. Compared with 3D plans, mean relative decreases in PTV volumes were 15%, 39%, and 44% using 4D-union, offline ART, and online ART planning techniques, respectively. This resulted in mean lung volume receiving > or = 20Gy (V20) relative decreases of 21%, 23%, and 31% and mean lung dose relative decreases of 16%, 26%, and 31% for the 4D-union, 4D-offline ART, and 4D-online ART, respectively.nnnCONCLUSIONSnAdaptive IGRT using CBCT is feasible for the treatment of patients with lung tumors and significantly decreases PTV volume and dose to normal tissues, allowing for the possibility of dose escalation. All analyzed 4D planning strategies resulted in improvements over 3D plans, with 4D-online ART appearing optimal.

Planning Study Comparison of Real-Time Target Tracking and Four-Dimensional Inverse Planning for Managing Patient Respiratory Motion

PURPOSEnReal-time target tracking (RT-TT) and four-dimensional inverse planning (4D-IP) are two potential methods to manage respiratory target motion. In this study, we evaluated each method using the cumulative dose-volume criteria in lung cancer radiotherapy.nnnMETHODS AND MATERIALSnRespiration-correlated computed tomography scans were acquired for 4 patients. Deformable image registration was applied to generate a displacement mapping for each phase image of the respiration-correlated computed tomography images. First, the dose distribution for the organs of interest obtained from an idealized RT-TT technique was evaluated, assuming perfect knowledge of organ motion and beam tracking. Inverse planning was performed on each phase image separately. The treatment dose to the organs of interest was then accumulated from the optimized plans. Second, 4D-IP was performed using the probability density function of respiratory motion. The beam arrangement, prescription dose, and objectives were consistent in both planning methods. The dose-volume and equivalent uniform dose in the target volume, lung, heart, and spinal cord were used for the evaluation.nnnRESULTSnThe cumulative dose in the target was similar for both techniques. The equivalent uniform dose of the lung, heart, and spinal cord was 4.6 +/- 2.2, 11 +/- 4.4, and 11 +/- 6.6 Gy for RT-TT with a 0-mm target margin, 5.2 +/- 3.1, 12 +/- 5.9, and 12 +/- 7.8 Gy for RT-TT with a 2-mm target margin, and 5.3 +/- 2.3, 11.9 +/- 5.0, and 12 +/- 5.6 Gy for 4D-IP, respectively.nnnCONCLUSIONnThe results of our study have shown that 4D-IP can achieve plans similar to those achieved by RT-TT. Considering clinical implementation, 4D-IP could be a more reliable and practical method to manage patient respiration-induced motion.

Cumulative lung dose for several motion management strategies as a function of pretreatment patient parameters.

PURPOSEnTo evaluate patient parameters that may predict for relative differences in cumulative four-dimensional (4D) lung dose among several motion management strategies.nnnMETHODS AND MATERIALSnDeformable image registration and dose accumulation were used to generate 4D treatment plans for 18 patients with 4D computed tomography scans. Three plans were generated to simulate breath hold at normal inspiration, target tracking with the beam aperture, and mid-ventilation aperture (control of the target at the mean daily position and application of an iteratively computed margin to compensate for respiration). The relative reduction in mean lung dose (MLD) between breath hold and mid-ventilation aperture (DeltaMLD(BH)) and between target tracking and mid-ventilation aperture (DeltaMLD(TT)) was calculated. Associations between these two variables and parameters of the lesion (excursion, size, location, and deformation) and dose distribution (local dose gradient near the target) were also calculated.nnnRESULTSnThe largest absolute and percentage differences in MLD were 1.0 Gy and 21.5% between breath hold and mid-ventilation aperture. DeltaMLD(BH) was significantly associated (p < 0.05) with tumor excursion. The DeltaMLD(TT) was significantly associated with excursion, deformation, and local dose gradient. A linear model was constructed to represent DeltaMLD vs. excursion. For each 5 mm of excursion, target tracking reduced the MLD by 4% compared with the results of a mid-ventilation aperture plan. For breath hold, the reduction was 5% per 5 mm of excursion.nnnCONCLUSIONSnThe relative difference in MLD among different motion management strategies varied with patient and tumor characteristics for a given dosimetric target coverage. Tumor excursion is useful to aid in stratifying patients according to appropriate motion management strategies.

Quality and accuracy of cone beam computed tomography gated by active breathing control

The purpose of this study was to evaluate the quality and accuracy of cone beam computed tomography (CBCT) gated by active breathing control (ABC), which may be useful for image guidance in the presence of respiration. Comparisons were made between conventional ABC-CBCT (stop and go), fast ABC-CBCT (a method to speed up the acquisition by slowing the gantry instead of stopping during free breathing), and free breathing respiration correlated CBCT. Image quality was assessed in phantom. Accuracy of reconstructed voxel intensity, uniformity, and root mean square error were evaluated. Registration accuracy (bony and soft tissue) was quantified with both an anthropomorphic and a quality assurance phantom. Gantry angle accuracy was measured with respect to gantry speed modulation. Conventional ABC-CBCT scan time ranged from 2.3 to 5.8 min. Fast ABC-CBCT scan time ranged from 1.4 to 1.8 min, and respiratory correlated CBCT scans took 2.1 min to complete. Voxel intensity value for ABC gated scans was accurate relative to a normal clinical scan with all projections. Uniformity and root mean square error performance degraded as the number of projections used in the reconstruction of the fast ABC-CBCT scans decreased (shortest breath hold, longest free breathing segment). Registration accuracy for small, large, and rotational corrections was within 1 mm and 1 degrees. Gantry angle accuracy was within 1 degrees for all scans. For high-contrast targets, performance for image-guidance purposes was similar for fast and conventional ABC-CBCT scans and respiration correlated CBCT.

Optical-CT imaging of complex 3D dose distributions

The limitations of conventional dosimeters restrict the comprehensiveness of verification that can be performed for advanced radiation treatments presenting an immediate and substantial problem for clinics attempting to implement these techniques. In essence, the rapid advances in the technology of radiation delivery have not been paralleled by corresponding advances in the ability to verify these treatments. Optical-CT gel-dosimetry is a relatively new technique with potential to address this imbalance by providing high resolution 3D dose maps in polymer and radiochromic gel dosimeters. We have constructed a 1st generation optical-CT scanner capable of high resolution 3D dosimetry and applied it to a number of simple and increasingly complex dose distributions including intensity-modulated-radiation-therapy (IMRT). Prior to application to IMRT, the robustness of optical-CT gel dosimetry was investigated on geometry and variable attenuation phantoms. Physical techniques and image processing methods were developed to minimize deleterious effects of refraction, reflection, and scattered laser light. Here we present results of investigations into achieving accurate high-resolution 3D dosimetry with optical-CT, and show clinical examples of 3D IMRT dosimetry verification. In conclusion, optical-CT gel dosimetry can provide high resolution 3D dose maps that greatly facilitate comprehensive verification of complex 3D radiation treatments. Good agreement was observed at high dose levels (>50%) between planned and measured dose distributions. Some systematic discrepancies were observed however (rms discrepancy 3% at high dose levels) indicating further work is required to eliminate confounding factors presently compromising the accuracy of optical-CT 3D gel-dosimetry.

SU‐GG‐J‐154: Quality and Accuracy of Cone Beam Computed Tomography Gated by Active Breathing Control.

Purpose: To evaluate the quality and accuracy of cone beam computed tomography(CBCT) gated by active breathing control (ABC) for image guidance. Method and Materials: Comparisons were made between conventional ABC‐CBCT (stop and go), modified ABC‐CBCT (a method to speed up the acquisition by slowing the gantry instead of stopping during free breathing), and free breathing respiration correlated CBCT. All CBCTimages were acquired with an Elekta LINAC equipped with an on board KV imager with two gantry speed settings. Speed changes for the modified ABC‐CBCT were triggered by the ABC. Breath hold and free breathing patterns used were: (breath hold seconds / free breathing seconds) 3 / 15, 5 / 10, and 10 / 15. Image quality was assessed with a CATPHAN with high contrast, low contrast, and uniformity inserts. CT number linearity was evaluated. Registration assessment (bony and soft tissue) was quantified with both an anthropomorphic and a quality assurance phantom. A calibration phantom was used to assess gantry angle accuracy with respect to gantry speed modulation. Results: Conventional ABC‐CBCT scan time for the breath hold patterns assessed ranged from 2.3 to 5.8 minutes. Modified ABC‐CBCT scan time ranged from 1.4 to 1.8 minutes, and respiratory correlated CBCT scans took 2.1 minutes to complete. CT number linearity for ABC gated scans was comparable to a normal clinical scan with all projections. Registration accuracy for small, large, and rotational corrections was within 1 mm and 1. Conclusion: Modified ABC‐CBCT (slow gantry during breath hold) scans can potentially improve the efficiency of conventional ABC‐CBCT (stop and go), or replace respiration correlated CBCT, without degradation of performance for image guidance purposes. Conflict of Interest: Research sponsored by Elekta, Inc.

TU‐C‐351‐04: Cone Beam CT Acquisition During Volumetric Arc Radiotherapy Delivery: Correction for Induced Artifacts

Purpose: To evaluate the feasibility of kV cone beam CT(CBCT) acquisition simultaneously with volumetric modulated arc therapy (VMAT) delivery, and to test a method to correct for degradation of image quality due to VMAT delivery.Method and Materials: A commercial CBCT system was modified to enable simultaneous CBCT acquisition with VMAT delivery.CBCT scans of an image quality phantom were acquired during VMAT delivery while varying the VMAT parameters. Dose rate, energy, and field size of the VMAT delivery, and phantom size, were varied to evaluate the effect on image quality. The mean and standard deviation of the signal in a known location was quantified both in the raw 2D projection images and also in the reconstructed 3D CBCTimages. A nonlinear filter was tested to remove structural artifact and noise. An analytical scatter correction model was developed and used to remove scatter generated by the VMAT beam. Results: Structural artifact was reduced in the CBCT projections with a nonlinear filter. Scatter generated from the VMAT delivery varied with field size and dose rate, and minimally with phantom size. An analytical scatter model was constructed based on the VMAT fluence (field area times dose rate) for each CBCT projection, and applied to reduce scatter per projection. Applying the model improved uniformity from 7.9% to 3.0% and improved the contrast to noise ratio from 0.97 to 1.84. Conclusion: Megavoltage scatter, and its per projection variation, is the largest component contributing to degradation of CBCTimage quality during VMAT delivery. The degradation was reduced with a scatter model based on the VMAT delivery. A secondary component was structural artifact related to the repetition rate of the megavoltage beam and the readout mechanism of the kV detector. Conflict of Interest: Research sponsored by Elekta, Inc.

WE‐E‐AUD A‐05: A Simplified Method of Dose Accumulation in 4DCT

Purpose: To demonstrate, both in phantom and patient, the feasibility of approximating the subjects density distribution via an average 4DCT image (AVG‐CT), calculate cumulative dose delivered during respiration with this technique, and evaluate the results with a full 4D dose summation. Method and Materials: A series of 4DCT numerical phantoms (9 phases, lungtumor excursions 2, 3, and 4 cm in S‐I direction) and their AVG‐CT images were generated. For full 4D dose summation, static dose was calculated on each phase, each dose matrix was sampled with known displacement, and dose was accumulated over all phases. Using the same clinical plan, the AVG‐CT cumulative dose was calculated by combining the static AVG‐CT dose with known tumor displacement, and assuming the dose distribution was the same for all phases. Four lungcancer cases were also evaluated for stereotactic body radiotherapy and conformal treatments. Here, deformable image registration was used to generate the patient‐specific motion model from 4DCT. Dose accumulation was analogous to phantom, however, each phases dose matrix was sampled using the displacement vector field.Dose discrepancy (D) between full 4D summation and AVG‐CT approach was calculated and compared. Results: For all phantoms, AVG‐CT approximation yielded slightly higher cumulative doses compared to full 4D summation, with dose discrepancy increasing with increased tumor excursion. In vivo, using the AVG‐CT coupled with deformable registration yielded a modest increase in cumulative dose relative to full 4D dose summation and not at clinically applicable levels (D < 2%). Even for a patient with substantial tumor motion near the diaphragm, dose discrepancy was within 4%. Conclusion: Simplifying 4D dose accumulation via the AVG‐CT, while fully accounting for tumor deformation due to respiratory motion, has been validated, thereby introducing the potential to streamline the use of 4D dose calculations in clinical practice.