Stefanie Ehrbar

Respiratory motion-management in stereotactic body radiation therapy for lung cancer – A dosimetric comparison in an anthropomorphic lung phantom (LuCa)

BACKGROUND AND PURPOSE The objective of this study was to compare the latest respiratory motion-management strategies, namely the internal-target-volume (ITV) concept, the mid-ventilation (MidV) principle, respiratory gating and dynamic couch tracking. MATERIALS AND METHODS An anthropomorphic, deformable and dynamic lung phantom was used for the dosimetric validation of these techniques. Stereotactic treatments were adapted to match the techniques and five distinct respiration patterns, and delivered to the phantom while radiographic film measurements were taken inside the tumor. To report on tumor coverage, these dose distributions were used to calculate mean doses (Dmean), changes in homogeneity indices (ΔH2-98), gamma agreement, and areas covered by the planned minimum dose (A>Dmin). RESULTS All techniques achieved good tumor coverage (A>Dmin>99.0%) and minor changes in Dmean (±3.2%). Gating and tracking strategies showed superior results in gamma agreement and ΔH2-98 compared to ITV and MidV concepts, which seem to be more influenced by the interplay and the gradient effect. For lung, heart and spinal cord, significant dose differences between the four techniques were found (p<0.05), with lowest doses for gating and tracking strategies. CONCLUSION Active motion-management techniques, such as gating or tracking, showed superior tumor dose coverage and better organ dose sparing than the passive techniques based on tumor margins.

Three-dimensional versus four-dimensional dose calculation for volumetric modulated arc therapy of hypofractionated treatments.

PURPOSE Respiratory motion is a non-negligible source of uncertainty in radiotherapy. A common approach is to delineate the target volume in all respiratory phases (ITV) and to calculate a treatment plan using the average reconstruction of the four-dimensional computed tomography (4DCT) scans. In this study the extent of the interplay effect caused by interaction between dynamic dose delivery and respiratory tumor motion, as well as other motion effects were investigated. These effects are often ignored when the ITV concept is used. METHODS AND MATERIALS Nine previously treated patients with in ten abdominal or thoracic cancer lesions (3 liver, 3 adrenal glands and 4 lung lesions) were selected for this planning study. For all patients, phase-sorted respiration-correlated 4DCT scans were taken, and volumetric modulated arc therapy (VMAT) treatments were planned using the ITV concept. Margins from ITV to planning target volume (PTV) of 3-10mm were used. Plans were optimized and dose distributions were calculated on the average reconstruction of the 4DCT. 4D dose distributions were calculated to evaluate motion effects, caused by the interference of dynamic treatment delivery with respiratory tumor motion and inhomogeneously planned target dose. These calculations were performed on the phase-sorted CT series with a respiration-correlated assignment of the treatment plans monitor units (MU) to the respiration phases of the 4DCT. The 4D dose was accumulated with rigid as well as deformable registrations of the CT series and compared to the original 3D dose distribution. Maximum, minimum and mean doses to ITV and PTV, and maximum or mean doses to organs at risk (OAR), were compared after rigid accumulation. The dose variation in the gross tumor volume (GTV) was compared after deformable registration. RESULTS Using rigid registrations, variations in the investigated dose parameters between 3D and 4D dose calculations were found to be within -2.1% to 1.4% for all target volumes and within -0.8% to 1.7% in OAR. Using deformable registrations, dose differences in the GTV were below 3.8% for dose accumulation of lung and adrenal gland patients. For liver patients the used deformable registrations were not considered to be robust enough. It was also shown that a major part of the dose differences originates from the Hounsfield unit differences between 3D and 4D calculations, regardless of the interplay effect. CONCLUSION The evaluated motion effects during VMAT treatments resulted in negligible dose variability. Therefore, the approximation of calculating the dose on the average reconstruction of the 4DCT (3D dose calculation), instead of calculating on the respiration-correlated phase CTs (4D dose calculation) with assignment of the corresponding MUs, gives acceptable results.

Validation of dynamic treatment-couch tracking for prostate SBRT

Purpose In stereotactic body radiation therapy (SBRT) of prostatic cancer, a high dose per fraction is applied to the target with steep dose gradients. Intrafractional prostate motion can occur unpredictably during the treatment and lead to target miss. This work investigated the dosimetric benefit of motion compensation with dynamic treatment‐couch tracking for prostate SBRT treatments in the presence of prostatic motion. Methods Ten SBRT treatment plans for prostate cancer patients with integrated boosts to their index lesion were prepared. The treatment plans were applied with a TrueBeam linear accelerator to a phantom in (a) static reference position, (b) moved with five prostate motion trajectories without any motion compensation, and (c) with real‐time compensation using transponder‐guided couch tracking. The geometrical position of the electromagnetic transponder was evaluated in the tracked and untracked situation. The dosimetric performance of couch tracking was evaluated, using Gamma agreement indices (GAI) and other dose parameters. These were evaluated within the phantoms biplanar diode array, as well as target‐ and organ‐specific. Results The root‐mean‐square error of the motion traces (range: 0.8–4.4 mm) was drastically reduced with couch tracking (0.2–0.4 mm). Residual motion was mainly observed at abrupt direction changes with steep motion gradients. The phantom measurements showed significantly better GAI1%/1mm with tracked (range: 83.4%–100.0%) than with untracked motion (28.9%–99.7%). Also GAI2%/2mm was significantly superior for the tracked (98.4%–100.0%) than the untracked motion (52.3%–100.0%). The organ‐specific evaluation showed significantly better target coverage with tracking. The dose to the rectum and bladder showed a dependency on the anterior–posterior motion direction. Conclusions Couch tracking clearly improved the dosimetric accuracy of prostate SBRT treatments. The treatment couch was able to compensate the prostatic motion with only some minor residual motion. Therefore, couch tracking combined with electromagnetic position monitoring for prostate SBRT is feasible and improves the accuracy in treatment delivery when prostate motion is present.

ITV, mid-ventilation, gating or couch tracking – A comparison of respiratory motion-management techniques based on 4D dose calculations

PURPOSE Respiratory motion-management techniques (MMT) aim to ensure tumor dose coverage while sparing lung tissue. Dynamic treatment-couch tracking of the moving tumor is a promising new MMT and was compared to the internal-target-volume (ITV) concept, the mid-ventilation (MidV) principle and the gating approach in a planning study based on 4D dose calculations. METHODS For twenty patients with lung lesions, planning target volumes (PTV) were adapted to the MMT and stereotactic body radiotherapy treatments were prepared with the 65%-isodose enclosing the PTV. For tracking, three concepts for target volume definition were considered: Including the gross tumor volume of one phase (single-phase tracking), including deformations between phases (multi-phase tracking) and additionally including tracking latencies of a couch tracking system (reliable couch tracking). The accumulated tumor and lung doses were estimated with 4D dose calculations based on 4D-CT datasets and deformable image registration. RESULTS Single-phase tracking showed the lowest ipsilateral lung Dmean (median: 3.3Gy), followed by multi-phase tracking, gating, reliable couch tracking, MidV and ITV concepts (3.6, 3.8, 4.1, 4.3 and 4.8Gy). The 4D dose calculations showed the MidV and single-phase tracking overestimated the target mean dose (-2.3% and -1.3%), while it was slightly underestimated by the other MMT (<+1%). CONCLUSION The ITV concept ensures tumor coverage, but exposes the lung tissue to a higher dose. The MidV, gating and tracking concepts were shown to reduce the lung dose. Neglecting non-translational changes of the tumor in the target volume definition for tracking results in a slightly reduced target coverage. The slightly inferior dose coverage for MidV should be considered when applying this technique clinically.

Modeling and Performance Evaluation of a Robotic Treatment Couch for Tumor Tracking

Abstract Tumor motion during radiation therapy increases the irradiation of healthy tissue. However, this problem may be mitigated by moving the patient via the treatment couch such that the tumor motion relative to the beam is minimized. The treatment couch poses limitations to the potential mitigation, thus the performance of the Protura (CIVCO) treatment couch was characterized and numerically modeled. The unknown parameters were identified using chirp signals and verified with one-dimensional tumor tracking. The Protura tracked chirp signals well up to 0.2 Hz in both longitudinal and vertical directions. If only the vertical or only the longitudinal direction was tracked, the Protura tracked well up to 0.3 Hz. However, there was unintentional yet substantial lateral motion in the former case. And during vertical motion, the extension caused rotation of the Protura around the lateral axis. The numerical model matched the Protura up to 0.3 Hz. Even though the Protura was designed for static positioning, it was able to reduce the tumor motion by 69% (median). The correlation coefficient between the tumor motion reductions of the Protura and the model was 0.99. Therefore, the model allows tumor-tracking results of the Protura to be predicted.

Comparison of multi-leaf collimator tracking and treatment-couch tracking during stereotactic body radiation therapy of prostate cancer

PURPOSE AND BACKGROUND Motion mitigation during prostate stereotactic body radiation therapy (SBRT) ensures optimal target coverage while reducing the risk of overdosage of nearby organs. The geometrical and dosimetrical performance of motion mitigation with the multileaf-collimator (MLC tracking) or the treatment couch (couch tracking) were compared. MATERIAL AND METHODS For ten prostate patients, SBRT treatment plans with integrated boosts were prepared using volumetric modulated arc technique. For the geometrical evaluation, a lead sphere at the beam isocenter was moved according to five prostate motion curves (i) without mitigation, (ii) with MLC tracking or (iii) with couch tracking. During irradiation, MV images were taken and the over-/underexposed areas were evaluated. For the dosimetrical evaluation, the plans were applied to a dosimetric phantom. Dose distributions with and without mitigation were evaluated inside the target structure and organs at risk. RESULTS The median over-/underexposed area was reduced significantly from 2.02cm2 without mitigation to 1.00cm2 and 0.45cm2 with MLC and couch tracking. Closest dosimetrical agreement to the static references was achieved with couch tracking. CONCLUSIONS MLC and couch tracking at a conventional linear accelerator significantly improved the accuracy of prostate SBRT in the presence of motion, whereby couch tracking showed slightly better performance than MLC tracking.

Modelling the immunosuppressive effect of liver SBRT by simulating the dose to circulating lymphocytes: an in-silico planning study

BackgroundTumor immune-evasion and associated failure of immunotherapy can potentially be overcome by radiotherapy, which however also has detrimental effects on tumor-infiltrating and circulating lymphocytes (CL). We therefore established a model to simulate the radiation-dose delivered to CL.MethodsA MATLAB-model was established to quantify the CL-dose during SBRT of liver metastases by considering the factors: hepatic blood-flow, −velocity and transition-time of individual hepatic segments, as well as probability-based recirculation. The effects of intra-hepatic tumor-location and size, fractionation and treatment planning parameters (VMAT, 3DCRT, photon-energy, dose-rate and beam-on-time) were analyzed. A threshold dose ≥0.5Gy was considered inactivating CL and CL0.5 (%) is the proportion of inactivated CL.ResultsMean liver dose was mostly influenced by treatment-modality, whereas CL0.5 was mostly influenced by beam-on-time. 3DCRT and VMAT (10MV-FFF) resulted in lowest CL0.5 values of 16 and 19%. Metastasis location influenced CL0.5, with a mean of 19% for both apical and basal and 31% for the central location. PTV-volume significantly increased CL0.5 from 27 to 67% (10MV-FFF) and from 31 to 98% (6MV-FFF) for PTV-volumes ranging from 14cm3 to 268cm3.ConclusionA simulation-model was established, quantifying the strong effects of treatment-technique, tumor-location and tumor-volume on dose to CL with potential implications for immune-optimized treatment-planning in the future.

Potential dosimetric benefits of adaptive tumor tracking over the internal target volume concept for stereotactic body radiation therapy of pancreatic cancer

BackgroundRadiotherapy for pancreatic cancer has two major challenges: (I) the tumor is adjacent to several critical organs and, (II) the mobility of both, the tumor and its surrounding organs at risk (OARs). A treatment planning study simulating stereotactic body radiation therapy (SBRT) for pancreatic tumors with both the internal target volume (ITV) concept and the tumor tracking approach was performed. The two respiratory motion-management techniques were compared in terms of doses to the target volume and organs at risk.Methods and MaterialsTwo volumetric-modulated arc therapy (VMAT) treatment plans (5 × 5 Gy) were created for each of the 12 previously treated pancreatic cancer patients, one using the ITV concept and one the tumor tracking approach. To better evaluate the overall dose delivered to the moving tumor volume, 4D dose calculations were performed on four-dimensional computed tomography (4DCT) scans. The resulting planning target volume (PTV) size for each technique was analyzed. Target and OAR dose parameters were reported and analyzed for both 3D and 4D dose calculation.ResultsTumor motion ranged from 1.3 to 11.2 mm. Tracking led to a reduction of PTV size (max. 39.2%) accompanied with significant better tumor coverage (p<0.05, paired Wilcoxon signed rank test) both in 3D and 4D dose calculations and improved organ at risk sparing. Especially for duodenum, stomach and liver, the mean dose was significantly reduced (p<0.05) with tracking for 3D and 4D dose calculations.ConclusionsBy using an adaptive tumor tracking approach for respiratory-induced pancreatic motion management, a significant reduction in PTV size can be achieved, which subsequently facilitates treatment planning, and improves organ dose sparing. The dosimetric benefit of tumor tracking is organ and patient-specific.

Unconscious physiological response of healthy volunteers to dynamic respiration-synchronized couch motion

BackgroundIntrafractional motion can be a substantial uncertainty in precision radiotherapy. Conventionally, the target volume is expanded to account for the motion. Couch-tracking is an alternative, where the patient is moved to compensate for the tumor motion. However, the couch motion may influence the patient’s stress and respiration behavior decreasing the couch-tracking effectiveness.MethodsIn total, 100 volunteers were positioned supine on a robotic couch, which moved dynamically and respiration synchronized. During the measurement, the skin conductivity, the heartrate, and the gaze location were measured indicating the volunteer’s stress. Volunteers rated the subjective motion sickness using a questionnaire. The measurement alternated between static and tracking segments (three cycles), each 1 min long.ResultsThe respiration amplitude showed no significant difference between tracking and static segments, but decreased significantly from the first to the last tracking segment (p < 0.0001). The respiration frequency differed significantly between tracking and static segments (p < 0.0001), but not between the first and the last tracking segment. The physiological parameters and the questionnaire showed mild signals of stress and motion sickness.ConclusionGenerally, people tolerated the couch motions. The interaction between couch motion and the patient’s breathing pattern should be considered for a clinical implementation.Trial registrationThe study was registered at ClinicalTrials.gov (NCT02820532) and the Swiss national clinical trials portal (SNCTP000001878) on June 20, 2016.

EP-1748: An experimental comparison of advanced respiratory motion management techniques

ESTRO 35 2016 _____________________________________________________________________________________________________ 0.63 mAs) were acquired at 1 Hz. For stereoscopic localization, the intersection of the ray lines connecting the detected image locations with the corresponding sources was found, whereas monoscopic localization first computed a prostate position probability density function (PDF) based on previously published motion covariances, and then finds the maximum likelihood position along the ray line passing through this PDF. Stereoand monoscopic localization results were compared to the ground truth provided by the linac log file.