Daily Setup Research Articles

PO-1896: Implant geometry based PTV margins for prostate patients with daily setup on markers

PO-1896: Implant geometry based PTV margins for prostate patients with daily setup on markers

A study of the dosimetric impact of daily setup variations measured with cone-beam CT on three-dimensional conformal radiotherapy for early-stage breast cancer delivered in the prone position.

PurposeTo evaluate the dosimetric impact of daily positioning variations measured with cone‐beam computed tomography (CBCT) on whole‐breast radiotherapy patients treated in the prone position.MethodsDaily CBCT was prospectively acquired for 30 consecutive patients positioned prone. Treatment for early‐stage (≤II) breast cancer was prescribed with standard dose (50 Gy/25 fractions) or hypofractionation (42.56 Gy/16 fractions) for 13 and 17 patients, respectively. Systematic and random errors were calculated from the translational CBCT shifts and used to determine population‐based setup margins. Mean translations (±one standard deviation) for each patient were used to simulate the dosimetric impact on targets (PTV_eval and lumpectomy cavity), heart, and lung. Paired Student’s t tests at α = 0.01 were used to compare dose metrics after correction for multiple testing (P < 0.002). Significant correlation coefficients were used to identify associations (P < 0.01).ResultsOf 597 total fractions, 20 ± 13% required patient rotation. Mean translations were 0.29 ± 0.27 cm, 0.41 ± 0.34 cm, and 0.48 ± 0.33 cm in the anterior–posterior, superior–inferior, and lateral directions leading to calculated setup margins of 0.63, 0.88, and 1.10 cm, respectively. Average three‐dimensional (3D) shifts correlated with the maximum distance of breast tissue from the sternum (r = 0.62) but not with body‐mass index. Simulated shifts showed significant, but minor, changes in dose metrics for PTV_eval, lung, and heart. For left‐sided treatments (n = 18), mean heart dose increased from 109 ± 75 cGy to 148 ± 115 cGy. Shifts from the original plan caused PTV_eval hotspots (V105%) to increase by 5.2% ± 3.8%, which correlated with the total MU of wedged fields (r = 0.59). No significant change in V95% to the cavity was found.ConclusionsLarge translational variations that occur when positioning prone breast patients had small but significant dosimetric effects on 3DCRT plans. Daily CBCT may still be necessary to correct for rotational variations that occur in 20% of treatments. To maintain planned dose metrics, unintended beam shifts toward the heart and the contribution of wedged fields should be minimized.

Quantitative Analysis of the Treatment Set Up Reliability of a Prone Breast Boost for Early Stage Breast Cancer

Adjuvant whole-breast radiation and a tumor bed boost for early-stage breast cancer can be delivered in the supine or prone position. Delivering the boost in the prone position has the advantage of removing the supine simulation CT and the capability of obtaining the total treatment dose distribution. However, the reliability of daily prone boost set up is largely unknown, and imaging is used for treatment verification. Here we evaluate the reliability of a daily clinical setup protocol for a prone mini-tangent boost, using daily cone beam CT (CBCT) to verify the adequacy of target coverage. We retrospectively reviewed 13 patients treated with a prone mini-tangent boost. The target (CTV) was the tumor bed without further expansion. Distance from CTV to block edge was 1.5cm. The boost prescription was 10Gy in 5 fractions. Coverage goals were the treating physician’s discretion, However, in all cases the planned dose to 90% of the CTV (D90) > 90% of prescription. Patients were positioned daily using a clinical setup with visual verification. Port films were taken to verify day 1 setup and when there was concern about the daily clinical setup. A CBCT was then acquired. If needed, additional shifts were made based on the CBCT, often followed by port films. Retrospectively, the simulation CT was independently registered to the daily CBCTs by a physician, and a daily tumor bed contour was generated. The treatment plan was transferred to the daily CBCT and delivered dose was calculated. To assess the reliability of clinical setup prior to CBCT acquisition, we assessed how often the daily D90 was < 90% of prescription dose (D90 < 90%) and how often the daily volume receiving full prescription dose (V100daily) did not receive the planned V100 (V100plan) by > 10% (V100plan-V100daily > 10%). Twelve patients with full imaging details were analyzed. Of 60 analyzable daily fractions, D90 < 90% for 3/60 fractions (5%), and V100plan-V100daily > 10% for 11/60 fractions (18%). Post-CBCT shifts were made for 1 fraction with D90 < 90% and for 2 fractions with V100plan-V100daily > 10%. Fractions for which coverage did not meet reliability metrics most commonly involved superficial tumor beds that extended into the region of subcutaneous dose build-up. In all such cases, allowing a portion of the target to receive less than prescription dose was deemed acceptable by the treating physician. We found that a daily clinical treatment setup for a prone boost was reliable in most cases. Addition of a CBCT was helpful to identify clinical setup variability. Treatment of a superficial tumor bed may be more sensitive to small variations in daily treatment setup due to the dose gradient across the target. When superficial target coverage is critical, supine boost positioning may be considered.

MR-Guided Liver Stereotactic Body Radiotherapy (SBRT): To Adapt, or Not to Adapt?

Long-term disease-free survival is possible in liver oligometastatic cancer with ablative local treatment such as SBRT, however in some studies, long term local control remains an issue. Planning target volume (PTV) for tumors accounts for positional uncertainty, but not intra-fractional changes in tumor size, which are assumed to be negligible. Magnetic resonance imaging (MRI)-guided radiotherapy (MRgRT) to the liver using gadoxetate contrast allows for radiation oncologists to evaluate daily fractional dose to liver metastases and to adapt for anatomical variation. The purpose of this study is to determine the extent of gross tumor volume (GTV) changes between simulation and treatment start, as well as during each fraction of SBRT. Patients who underwent 5-fraction maximum inspiratory breath hold MR-guided liver SBRT for metastases were retrospectively screened for use of gadoxetate during simulation and at least the first treatment. An experienced radiation oncologist contoured the GTV on the MR planning scan and subsequent daily setup MRIs. Sørensen–Dice coefficients (DC) were calculated. Wilcoxon signed rank test was used to test significance of the median change in tumor volume using SPSS version 26.0. A total of 37 patients were treated with MR-guided SBRT for metastatic liver lesions, 20 of which had gadoxetate enhanced MRs for simulation and at least the first fraction of treatment. SBRT doses ranged from 40-80 Gy over 5 fractions. 17 out of 20 patients had all treatments using MRgRT (91 evaluable fractions). Median simulation GTV was 5.4 ml (R: 0.29-90.47 ml). From simulation to fraction 1, the mean DC was 0.78 (R: 0.55-.877). Mean DC for tumors treated solely using MRgRT for fractions 1 to 2, 2 to 3, 3 to 4, and 4 to 5 were 0.83, 0.82, 0.82, 0.79, 0.82, respectively. Following simulation, GTV volume was found to increase peaking at fraction 2 with a median 37% increase over simulation (95% CI: 20% to 59% increase, p<0.001) with a median absolute increase of 1.9 ml. Thereafter, volume decreased to a median of 17% (95% CI: -1%-25%) greater than simulation by the final fraction (p<0.01). At the time of the second fraction, 70% of tumors had increased by 20% over simulation volumes with 35% having a volume that was ≥ 50% larger than at the time of simulation. At the time of treatment completion, tumors had begun to regress and only 43% measured ≥ 20% larger than at the time of simulation. Volumes of GTVs visualized on initial gadoxetate enhanced MRI simulation demonstrated notable tumor volume changes throughout SBRT treatment. Potential causes for GTV changes include tumor growth, inflammation, treatment response, or radiation effects on gadoxetate distribution. These changes suggest there may be a role for daily adaptive radiation planning to improve plan coverage using MRgRT. In the absence of MR guidance, this study suggests the importance of minimizing the time to treatment and/or using a larger PTV margin to account for GTV changes.

A novel approach to embed eye shields in customized bolus on nasal dorsum treatment for electron radiotherapy

We aim to demonstrate the unique use of embedded lead eye shields in an electron wax bolus when treating the nasal dorsum. A patient presented to the clinic with squamous cell carcinoma of the nasal dorsum requiring treatment with en face electrons. A 3D customized wax bolus was designed and imported into the treatment planning system (TPS) to calculate the dose distribution. Due to high lens dose, the bolus was customized further to create 2 milled open slots in the wax, over the lens of eye, to allow lead sheets totaling 4 mm to be slid into the wax. The patient was brought back to the clinic to be scanned with the wax bolus fitting snugly over the nose, eyes, and cheek regions. The 3D milled insert holes were contoured on the CT in the TPS, assigned HU of 2758, to mimic the lead insertion. The lens dose with lead inserts was compared to the plan without lead insert. To further confirm the lens dose, EBT3 films were placed on the right and left eye under the bolus, and nose dorsum on the first day of treatment. The maximum dose of right lens, as calculated in the TPS with the simulated lead shields in place, decreased from 989.5cGy to 457cGy. The maximum dose of left lens decreased from 1085.4cGy to 501cGy. The dose readings from EBT3 films were in good agreement with the TPS, with deviation of 3.32%, 0.26%, and 3.44% for right lens, left lens, and nose, respectively. Daily positioning deviations compared to the plan were 0.65 ± 0.16cm and 0.63 ± 0.29cm for right eye and left eye, respectively. This novel device demonstrated the feasibility, in terms of dose calculation accuracy in the TPS and fabrication, of using customized bolus with lead inserts to conveniently shield the lens of the eyes in an electron treatment for the nose, enabling a streamlined daily setup.

Development of a Tongue Immobilization Device Using a 3D Printer for Intensity Modulated Radiation Therapy of Nasopharyngeal Cancer Patients.

PurposeThis study aimed to reduce radiation doses to the tongue, a patient-specific semi-customized tongue immobilization device (SCTID) was developed using a 3D printer for helical tomotherapy (HT) of nasopharyngeal cancer (NPCa). Dosimetric characteristics and setup stability of the SCTID were compared with those of a standard mouthpiece (SMP).Materials and MethodsFor displacement and robust immobilization of the tongue, the SCTID consists of four parts: upper and lower tooth stoppers, tongue guider, tongue-tip position guide bar, and connectors. With the SCTID and SMP, two sets of planning computed tomography and HT plans were obtained for 10 NPCa patients. Dosimetric and geometric characteristics were compared. Position reproducibility of the tongue with SCTID was evaluated by comparing with planned dose and adaptive accumulated dose of the tongue and base of the tongue based on daily setup mega-voltage computed tomography.ResultsUsing the SCTID, the tongue was effectively displaced from the planning target volume compared to the SMP. The median mucosa of the tongue (M-tongue) dose was significantly reduced (20.7 Gy vs. 27.8 Gy). The volumes of the M-tongue receiving a dose of 15 Gy, 30 Gy, and 45 Gy and the volumes of the mucosa of oral cavity and oropharynx (M-OC/OP) receiving a dose of 45 Gy and 60 Gy were significantly lower than using the SMP. No significant differences was observed between the planned dose and the accumulated adaptive dose in any dosimetric characteristics of the tongue and base of tongue.ConclusionSCTID can not only reduce the dose to the M-tongue and M-OC/OP dramatically, when compared to SMP, but also provide excellent reproducibility and easy visual verification.

Novel Use of Biphasic Cuirass Ventilation During Definitive Radiation Therapy: A Technical Report

The use of biphasic cuirass ventilator supported radiation therapy has never been documented. We present the first technical report here.A 57-year-old man with obstructive sleep apnea presented with a T0N1M0 right sided, human papillomavirus related head and neck cancer diagnosed on excisional lymph node biopsy. On further workup, the cancer was found to have originated in the right tonsil and was staged as T1N1. The patient started definitive treatment with concurrent chemo-radiation therapy, but after 5 treatments was no longer able to lay in a supine position for treatment.Diagnostic imaging workup eventually revealed an idiopathic right sided hemi-diaphragm eventration. After consultation with cardiology, pulmonology, and head and neck surgery, recommendation was made for tracheostomy to tolerate supine radiotherapy position, but the patient refused.Instead, computed tomography simulation for radiotherapy replanning was performed using a combination of biphasic cuirass ventilation, home continuous positive airway pressure and oxygen. The patient then tolerated definitive treatment to a dose of 69.96 Gray in 33 fractions with concurrent chemotherapy and experienced no unexpected side effects. Although complex, daily treatment setup was consistent. Daily onboard imaging was precise and accurate. The patient continues to follow up with radiation oncology, medical oncology, and pulmonology.This is the first use of biphasic cuirass ventilator supported radiotherapy reported in the scientific literature. Although daily treatment setup is complex, its use could be considered in patients unable to tolerate radiation therapy treatment positioning as an alternative to tracheostomy.

Online-adaptive versus robust IMPT for prostate cancer: How much can we gain?

Background/purposeIntensity-modulated proton therapy (IMPT) is highly sensitive to anatomical variations which can cause inadequate target coverage during treatment. Available mitigation techniques include robust treatment planning and online-adaptive IMPT. This study compares a robust planning strategy to two online-adaptive IMPT strategies to determine the benefit of online adaptation. Materials/methodsWe derived the robustness settings and safety margins needed to yield adequate target coverage (V95%≥98%) for >90% of 11 patients in a prostate cancer cohort (88 repeat CTs). For each patient, we also adapted a non-robust prior plan using a simple restoration and a full adaptation method. The restoration uses energy-adaptation followed by a fast spot-intensity re-optimization. The full adaptation uses energy-adaptation followed by the addition of new spots and a range-robust spot-intensity optimization.Dose was prescribed as 55 Gy(RBE) to the low-dose target (lymph nodes and seminal vesicles) with a boost to 74 Gy(RBE) to the high-dose target (prostate). Daily patient set-up was simulated using implanted intra-prostatic markers. ResultsMargins of 4 and 8 mm around the high- and low-dose target regions, a 6 mm setup error and a 3% range error were found to obtain adequate target coverage for all repeat CTs of 10/11 patients (94.3% of all 88 repeat CTs).Both online-adaptive strategies yielded V95%≥98% and better OAR sparing in 11/11 patients. Median OAR improvements up to 11%-point and 16%-point were observed when moving from robust planning to respectively restoration and full adaption. ConclusionBoth full plan adaptation and simple dose restoration can increase OAR sparing besides better conforming to the target criteria compared to robust treatment planning.

Evaluating the positional uncertainty of intrafraction, adjacent fields, and daily setup with the BrainLAB ExacTrac system in patients who are receiving craniospinal irradiation.

PurposeTo investigate the daily setup, interfraction motion, variability in the junction areas, and dosimetric effect in craniospinal irradiation (CSI) patients.MethodsFifteen CSI patients who had undergone split‐field IMRT were followed in the study. Previous, middle, and posttreatment, each target volume position was evaluated using the ExacTrac system. Interfraction and intrafraction motions, the margin of the junction in adjacent targets volumes, and the dosimetric effect of the longitudinal residual error were analyzed.ResultsThe lowest attainment rate within the tolerance of the initial setup error was 66.79% in six directions. The values of the initial error were within 15 mm (SD 4.5 mm) in the translation direction and 5° (SD 1.3°) in the rotation direction after the transposition of the treatment isocenter. With the guidance of the ExacTrac system, the interfraction and intrafraction residual errors were almost within the tolerance after correction, the margin of CTV‐to‐PCTV was in the range of target expansion criteria. The residual longitudinal errors resulted in only slight changes in the mean doses of PGTV and PCTV, while the maximum dose of the spinal cord increased by 16.1%. The patients did not exhibit any side‐effects by the overall treatment during the follow‐up period.ConclusionsPosition correction is necessary after setup and the transposition of the treatment isocenter. Intra‐fraction motion in the lateral direction should be monitored throughout treatment. The position errors in junction areas are almost within the tolerance after correction. The patients did not exhibit any side‐effects by the overall treatment.

Predictive value of 0.35T magnetic resonance imaging radiomic features in stereotactic ablative body radiotherapy of pancreatic cancer: A pilot study.

The aim of this study was to evaluate the potential and feasibility of radiomic features extracted from low field strength (0.35T) magnetic resonance images (MRIs) in predicting treatment response for patients with pancreatic cancer undergoing stereotactic body radiotherapy (SBRT). Twenty patients with unresected, non-metastatic pancreatic ductal adenocarcinoma (PDAC) were enrolled, all of whom received neoadjuvant chemotherapy followed by five-fraction MR-guided SBRT with a radiation dose range of 33-50Gy. For each patient, five daily setup scans were acquired from a hybrid 0.35T MRI/radiotherapy unit. Tumor heterogeneity quantified with radiomic features extracted from the gross tumor volume (GTV) was averaged over the course of treatment. Random forest (RF) and adaptive least absolute shrinkage and selection operator (LASSO) classification models were constructed to identify radiomics features predictive of treatment response. Predictive capability of the top-performing features was then evaluated using the receiver operating characteristic area under curve (AUC) obtained using leave-one-out cross-validation. Half of the 20 patients showed response to treatment, defined by tumor regression on histopathology or tumor response on follow-up dynamic contrast-enhanced computed tomography (CT). The most predictive features selected by the RF method were GLCM energy and GLSZM gray-level variance. The RF-based model achieved an AUC=0.81 with a 95% confidence interval of [0.594 to 1] The LASSO algorithm selected GLCM energy as the only predictive feature, achieving an AUC=0.81 with 95% confidence interval of [0.596 to 1]. The findings of this study suggest that radiomic features extracted during MR-guided SBRT may contain predictive information about response of PDAC patients to treatment. Using the images acquired during treatment of PDAC patients supports continued expansion of radiomic analysis based on low field strength MR images and may hold the potential for providing timely indications of response to treatment.

Technical Note: Design and characterization of a large diameter parallel plate ionization chamber for accurate integral depth dose measurements with proton beams.

The goal was to develop and test a large diameter parallel plate ionization chamber capable of intercepting at least 98% of the proton beamlets tested with the system. A commercial synchrotron proton therapy system was used for the study (Hitachi, Ltd, Hitachi City, Japan; Model: Probeat-V). The energies investigated were in the range of 100 to 192MeV. Three beam spot options available from the system were used. A PTW Bragg peak IC of diameter 84mm (BP84) (Model PTW34070) was employed for comparison in a scanning water phantom. A prototype of 150mm diameter was produced (PTW, Freiburg, Germany; model: T34089) and used for the testing. Monte Carlo calculations were also performed with FLUKA to guide the BP150 design and for comparison to the radiological measurements. For comparison, a 40cm diameter ideal virtual detector was included in the Monte Carlo model. The measured proton range R90 agrees between the BP84 and BP150 ionization chambers within +0.06/-0.27mm across the energies 100-192MeV, which is less than the daily experimental setup uncertainty of 0.4mm. The differences in the absolute integral depth dose curves (IDDs) between the BP84 and BP150 ranged from 0.3% to 1.0% for the spot sizes and beam energies tested. As predicted by the Monte Carlo modeling, the greatest differences were found in the plateau region of the IDDs. Also, the IDDs measured with the BP150 were very similar to those of the ideal 40cm diameter detector Monte Carlo simulations. We conclude that the BP150 offers a small, but a useful reduction in uncertainty from the nuclear halo effect for the system under test.

Initial Experience with MR-Guided Adaptive Radiotherapy for Head and Neck Cancers: Daily Setup and Dosimetric Variability on an MR-Linac

Initial Experience with MR-Guided Adaptive Radiotherapy for Head and Neck Cancers: Daily Setup and Dosimetric Variability on an MR-Linac

Daily localization of partial breast irradiation patients with three-dimensional ultrasound imaging.

PurposeAccurate localization of the lumpectomy cavity during accelerated partial breast radiation (APBR) is essential for daily setup to ensure the prescribed dose encompasses the target and avoids unnecessary irradiation to surrounding normal tissues. Three-dimensional ultrasound (3D-US) allows direct visualization of the lumpectomy cavity without additional radiation exposure. The purpose of this study was to evaluate the feasibility of 3D-US in daily target localization for APBR.Materials and methods: Forty-seven patients with stage I breast cancer who underwent breast conserving surgery were treated with a 2-week course of APBR. Patients with visible lumpectomy cavities on high quality 3D-US images were included in this analysis. Prior to each treatment, X-ray and 3D-US images were acquired and compared to images from simulation to confirm accurate position and determine shifts. Volume change of the lumpectomy cavity was determined daily with 3D-US.ResultsA total of 118 images of each modality from 12 eligible patients were analyzed. The average change in cavity volume was 7.8% (range, -24.1% to 14.4%) on 3D-US from simulation to the end-of-treatment. Based on 3D-US, significantly larger shifts were necessary compared to portal films in all three dimensions: anterior/posterior (p = 7E-11), left/right (p = 0.002), and superior/inferior (p = 0.004).ConclusionGiven that the lumpectomy cavity is not directly visible via X-ray images, accurate positioning may not be fully achieved by X-ray images. Therefore, when the lumpectomy cavity is visible on US, 3D-US can be considered as an alternative to X-ray imaging during daily positioning for selected patients treated with APBR, thus avoiding additional exposure to ionizing radiation.

Imaging and Dosimetry Study of Inter-fraction Setup Error in a Murine Xenograft Flank Tumor Radiation Model.

Modern small animal irradiation platforms provide for accurate delivery of radiation under 3D image guidance. However, leveraging these improvements currently comes at the cost of lower-throughput experimentation. Herein, we characterized setup accuracy and dosimetric robustness for mock/sham irradiation of a murine xenograft flank tumor model using the X-RAD SmART+ with the vendor-supplied Monte Carlo (MC) treatment planning system (SmART ATP). The chosen beam arrangement was parallel-opposing using a 20 mm square collimator, aligned off-axis for ipsilateral lung sparing. Using a cohort of five mice imaged with cone beam computed tomography (CBCT) over five consecutive mock-irradiation fractions, we compared inter-fraction setup variability resulting from a vendor-supplied multi-purpose bed with anesthesia nose cone with a more complicated immobilization solution with an integrated bite block with nose cone and Styrofoam platform. A hypothetical "high-throughput" image-guidance scenario was investigated, wherein the day 1 stage coordinates (resulting from CBCT guidance) were applied on days 2-5. Daily inter-fraction setup errors were evaluated per specimen (days 2-5) using CBCT-derived offsets from day 1 stage coordinates. Using the CBCT images and Monte Carlo dose calculation, 3D dosimetric plan robustness was evaluated for the vendor-supplied immobilization solution, for both the high-throughput guidance scenario as well as for use of daily CBCT-based alignment. Inter-fraction setup offset magnitude was 3.6 (±1.5) mm for the vendor-supplied immobilization compared to 3.3 (±1.8) mm for the more complicated solution. For the vendor-supplied immobilization, we found that daily CBCT was needed to adequately cover the flank tumors dosimetrically, given our chosen treatment approach.

Coping with interfraction time trends in tumor setup.

PurposeInterfraction tumor setup variations in radiotherapy are often reduced with image guidance procedures. Clinical target volume (CTV)–planning target volume (PTV) margins are then used to deal with residual errors. We have investigated characterization of setup errors in patient populations with explicit modelling of occurring interfraction time trends.MethodsThe core of a “trendline characterization” of observed setup errors in a population is a distribution of trendlines, each obtained by fitting a straight line through a patient's daily setup errors. Random errors are defined as daily deviations from the trendline. Monte Carlo simulations were performed to predict the impact of offline setup correction protocols on residual setup errors in patient populations with time trends. A novel CTV‐PTV margin recipe was derived that assumes that systematic underdosing of tumor edges in multiple consecutive fractions, as caused by trend motion, should preferentially be avoided. Similar to the well‐known approach by van Herk et al. for conventional error characterization (no explicit modelling of trends), only a predefined percentage of patients (generally 10%) was allowed to have nonrandom (systematic + trend) setup errors outside the margin. Additionally, a method was proposed to avoid erroneous results in Monte Carlo simulations with setup errors, related to decoupling of error sources in characterizations. The investigations were based on a database of daily measured setup errors in 835 prostate cancer patients that were treated with 39 fractions, and on Monte Carlo–generated patient populations with time trends.ResultsWith conventional characterization of setup errors in patient populations with time trends, predicted standard deviations of residual systematic errors (Σres) after application of an offline correction protocol could be underestimated by more than 50%, potentially resulting in application of too small margins. With the new trendline characterization this was avoided. With the novel CTV‐PTV margin recipe with an allowed 10% of patients having nonrandom errors outside the margin, the observed percentage was 10.0% ± 0.2%. When using conventional characterization of errors and the van Herk margin recipe, on average 58.0% ± 24.3% of patients had errors outside the margin, while 10% was prescribed. For populations with no time trends, the novel recipe simplifies to the generally applied M=2.5Σ+0.7σ formula proposed by van Herk et al.ConclusionsIn populations with time trends in setup errors, the use of trendline characterizations in Monte Carlo simulations for establishment of residual errors after a setup correction protocol can avoid application of erroneous margins. The novel margin recipe can be used to accurately control the percentage of patients with nonrandom errors outside the margin. In case of daily image guidance of patients with multiple targets with differential motion, the recipe can be used to establish margins for the targets that are not the primary target for the image guidance (e.g., nodal regions). Probabilistic planning might be improved by using trendline characterization for modelling of setup errors. Population analyses of interfraction setup errors need to take into account potential time trends.

69 Analysis and evaluation of a fixed vertical couch position technique in comparison with a placement based on tattoo and Couch Move Assistant on linacs Elekta Synergy S

Introduction. In our department the daily patient setup is based on tattoos and the offsets, eventually registered in the Record and Verify software MOSAIQ (version 2.6), are then applied with the Couch Move Assistant (CMA). The procedure is: for the four first days of the treatment, a Cone Beam Computed Tomography (CBCT) is taken and the correction is applied even for a displacement of 1 mm. After the four fractions, the mean of a systematic error (SE), if detected, will be calculated and applied for the following fractions. For the prostate treatment if the means is greater than 4.5 mm, the mean of the offsets will be added automatically through MOSAIQ for the rest of the next fractions. When there is prostate marker (gold seeds), a CBCT is taken daily and no SE correction is applied. Based on the publication of Petillion [1] Petillion, S. et al. Efficacy and workload analysis of a fixed vertical couch position technique and a fixed-action–level protocol in whole-breast radiotherapy. J Appl Clin Med Phys 16, 279–290 (2015). Google Scholar et al. in 2015 for the whole breast radiotherapy, a fixed vertical couch position (FVCP) setup is expected to be more precise than a setup based on skin marks: because of skin mobility, imprecision of tattooing during simulation, precision of the CMA module. For a fixed couch vertical positioning, the setup would be as followed: patient setup in left-right (LR) and superior-inferior (SI) based on the skin marks. For the anterior-posterior (AP) setup, it would be based on table height measured in the Treatment Planning System (TPS). The aim of this study is to apply the same methodology for prostate treatment.

Determining Potential Benefits of Volumetric Image Matching in Radiation Therapy

Accurate daily setup of head and neck cancer patients receiving radiation therapy is a high priority due to the presence of critical structures near the target volume. It is generally accepted that volumetric image matching is superior to projection image matching in detecting head roll about the axial plane, however it increases treatment time and peripheral dose. The goal of this study is to quantify the difference in patient setup error inherent to projection and volumetric image matching.

RTHP-22. EDEMA PROGRESSION DURING MRI-GUIDED GLIOBLASTOMA RADIOTHERAPY

Abstract PURPOSE Despite the common finding of pseudoprogression or true progression one month after primary chemoradiotherapy for glioblastoma, there are few studies evaluating brain MRI changes that occur during concurrent chemotherapy and radiotherapy (RT). With the first generation combination MRI-RT device, daily predominantly T2-weighted MRIs are obtained of glioblastoma during RT. We quantified how many patients had significant MRI detectable volumetric changes through the six week course of primary chemoradiotherapy. This is of particular importance since glioblastoma RT is only planned at the beginning of therapy and not commonly re-planned for changes during therapy. METHODS We retrospectively reviewed the daily set-up imaging of 8 patients at our institution who received RT for glioblastoma using the Cobalt-60 MRI-RT system. Patients received standard chemoradiation at 60 Gy in 30 fractions with temozolomide per EORTC22981/26981. We contoured the abnormality on the initial ViewRay setup scan and the set-up scan for fraction 30. After rigid fusion of the contours of the initial setup MRI and fraction 30 MRI, the volumes were compared. RESULTS Of the 8 patients, 3 patients (37.5%) demonstrated edema expansion greater than 5 mm. The maximum distances of T2-weighted abnormality volume growth for these patients were 1.0 cm, 1.5 cm, and 4.1 cm. These findings were correlated with the post-treatment diagnostic MRIs at 3–4 weeks which demonstrated similar FLAIR abnormalities and expansion in T1 with gadolinium contrast volumes within these areas of the radiotherapy fields (pseudoprogression vs. true progression). CONCLUSION Review of MRIs obtained by daily MRI-RT for glioblastoma indicates that 3 of 8 patients had over 5 mm of change in T2-weighted dimensions from beginning to end of radiotherapy. Groups using limited CTV margins for treatment planning should be aware that MRI volumes could significantly increase during radiotherapy.

Prehabilitation to Improve Positioning Reproducibility in Patients Undergoing Pelvic Radiation Therapy

Reproducible patient positioning is essential for precision in radiation therapy (RT) delivery. However, a retrospective review of pre-treatment imaging demonstrated variability in daily patient set-up. We tested the hypothesis that a structured daily pre-treatment stretching regimen is both feasible and effective for minimizing variability in positioning, as measured by sacral slope angles (SSA). After obtaining IRB approval, we had 8 female patients undergoing pelvic radiotherapy perform a structured daily hip exercise regimen (extension and external rotation) immediately prior to both simulation and daily treatment, throughout the RT course. The control group of 20 patients (17 female and 3 male) had usual care. SSA measurements on daily pre-treatment imaging were compared to SSA measurements from the simulation CT for 5 weeks. The extent of SSA variability between two groups and over time was analyzed using a linear mixed model. The same two readers independently measured SSA of all subjects, comparing SSA on the day of simulation to SSA measured on each day of RT. The average variation in SSA among the intervention subjects was 0.913° (±0.582°), with a range among subjects of 0.57°-1.3°. The average variation for the control subjects was 2.27° (±1.43°), with range among subjects of 1.22° - 5.09°. The difference between the two groups was statistically significant (p=0.002). There was a statistically significant SSA variation between groups at each week of treatment (Week (wk) 1: p = 0.007, wk 2: p = 0.008, wk 3: p = 0.01, wk 4: p = 0.005, wk 5: p = 0.008). There was no significant variation among the intervention subjects between week 1 and later weeks (wk 2: p = 0.88, wk 3: p = 0.74, wk 4: p = 0.97, wk 5: p = 0.40), whereas subjects in the control group demonstrated significant SSA variation between week 1 and the fourth week of therapy (wk 2: p = 0.87, wk 3: p = 0.92, wk 4: p = 0.02, wk 5: p = 0.22). There were no reported adverse effects of the stretching regimen. We demonstrated a significant decrease in the variability of SSA by implementing a simple pre-treatment stretching program, whereas control subjects exhibited an increasing variability of SSA over the course of treatment. We conclude that there is a potential benefit for prehabilitation during pelvic RT. A larger clinical trial is required to determine whether a pre-treatment stretching program would result in clinical benefit.

Surface guided radiotherapy (SGRT) improves breast cancer patient setup accuracy.

PurposeThe purpose of the study was to investigate if surface guided radiotherapy (SGRT) can decrease setup deviations for tangential and locoregional breast cancer patients compared to conventional laser‐based setup (LBS).Materials and MethodsBoth tangential (63 patients) and locoregional (76 patients) breast cancer patients were enrolled in this study. For LBS, the patients were positioned by aligning skin markers to the room lasers. For the surface based setup (SBS), an optical surface scanning system was used for daily setup using both single and three camera systems. To compare the two setup methods, the patient position was evaluated using verification imaging (field images or orthogonal images).ResultsFor both tangential and locoregional treatments, SBS decreased the setup deviation significantly compared to LBS (P < 0.01). For patients receiving tangential treatment, 95% of the treatment sessions were within the clinical tolerance of ≤ 4 mm in any direction (lateral, longitudinal or vertical) using SBS, compared to 84% for LBS. Corresponding values for patients receiving locoregional treatment were 70% and 54% for SBS and LBS, respectively. No significant difference was observed comparing the setup result using a single camera system or a three camera system.ConclusionsConventional laser‐based setup can with advantage be replaced by surface based setup. Daily SGRT improves patient setup without additional imaging dose to breast cancer patients regardless if a single or three camera system was used.