Faculty Bibliography

PURPOSE/OBJECTIVE:This study evaluated the extent of prostate displacement during SBRT on an MRI Linac using comprehensive motion management (CMM) and identified variables associated with intrafraction motion (IFM). METHODS:212 patients with clinically localized prostate cancer were treated with 5-fraction SBRT on a 1.5 T MR-Linac where IFM was continuously tracked and gated by CMM. Pre-beam positional shifts were identified from MRI registration prior to beam delivery. Intrafraction positional variability during beam delivery was evaluated, and multivariable analysis identified variables associated with IFM. RESULTS:In 614 fractions (62.7%), a > 1.5 mm pre-beam positional shift led to an adapt-to-position (ATP) plan correction. Mean anterior-posterior and superior-inferior pre-beam shifts were 2.2 mm and 2.1 mm, respectively. For 962 evaluable fractions, the median beam-on-time was 13.7 min with a mean duty cycle of 95.8%. Sustained > 3 mm displacement was observed in 520 fractions (54.1%) with a median cumulative duration of 24 s; >5 mm displacement was observed in 209 fractions (21.7%) with a median duration of 12.4 s. The ATS + ATP workflow was associated with reduced odds of sustained > 3 mm motion (p = 0.035), while older age was associated with increased odds (p = 0.011). CONCLUSIONS:Significant prostate shifts can occur immediately prior to and during radiation beam delivery, frequently exceeding applied margins and potentially leading to tumor underdosage. Continuous motion tracking and gating during prostate SBRT is an important tool in reducing IFM and enhance treatment delivery accuracy.

PURPOSE/UNASSIGNED:This study aims to evaluate the feasibility of delivering escalated doses to the dominant intraprostatic lesion (DIL) as noted on T2-weighted magnetic resonance imaging and diffusion-weighted imaging while maintaining dose to the surrounding normal tissue structures within dose-volume constraints. METHODS AND MATERIALS/UNASSIGNED:A total of 50 consecutive patients were treated with prostate stereotactic body radiation therapy (SBRT), via a simultaneous integrated boost to the DIL, on a 1.5-Tesla magnetic resonance imaging linear accelerator platform. These patients were treated with SBRT to 40 Gy in 5 fractions every other day, and the DIL was simultaneously boosted to 45 Gy in 5 fractions. No patient was treated with androgen deprivation therapy. The normal tissue structures and the prostate and DIL were recontoured, and a postfraction plan was generated to retrospectively generate the doses delivered to the prostate and the DIL boost target for each of these 250 therapy sessions. RESULTS/UNASSIGNED:< .05). Despite excellent target coverage, the rectum, urethra, and bladder dose constraints were generally maintained. At 6 months from completion of therapy, the median prostate-specific antigen result was 1.1 ng/mL (range, 0-5.6 ng/mL) compared to the median pre-SBRT prostate-specific antigen of 6.8 ng/mL (range, 3.45-31 ng/mL). No patient developed late grade 3 or higher urinary or rectal toxicities at median follow-up of 10.8 months (range, 6.4-15.7 months). CONCLUSIONS/UNASSIGNED:With real-time adaptive planning on a magnetic resonance imaging linear accelerator, dose escalation was achieved in most cases with the intended doses without significantly compromising the dose-volume constraints of the surrounding normal tissue structures. These dosimetric findings were associated with an excellent tolerance profile at 12 months and a low incidence of urinary or rectal toxicity.

PURPOSE/OBJECTIVE:To evaluate the degree and rate of bladder filling during magnetic resonance imaging-guided linear accelerator (MRL) prostate stereotactic body radiotherapy (SBRT), and to determine the association of degree of bladder filling with intra-fractional prostatic motion requiring positional shifts during therapy. The impact of bladder filling on post-treatment target and normal tissue dosimetry was also evaluated. METHODS:Sixty-two consecutive prostate SBRT patients treated on the MRL with an empty bladder and a five-fraction regimen were evaluated. Bladder filling patterns during each treatment session and the frequency of required shifts to address intra-fractional prostate motion were studied. During each fraction, three MR image acquisitions were obtained: an immediate baseline T2-weighted sequence, a verification sequence after the plan was generated prior to treatment delivery, and a sequence post-treatment. Bladder filling rates were evaluated at these time points for each fraction and across the five treatment fractions. Multivariate analysis identified variables associated with increased bladder filling rates and the likelihood of positional target adjustments of the prostate during real-time adaptive planning. Post-treatment MR structures were used to recalculate plans for analysis of intra-fractional dosimetric variations in target and normal tissue doses. RESULTS:The median baseline bladder volume at fraction 1 was 88 cc (range 35-245), increasing to 138 cc (range 55-340) at verification MR and 156 cc (range 69-475) post-treatment. Bladder volume increases from baseline to verification MR and from verification MR to post-treatment MR were consistent across the cohort. Multivariate analysis identified the use of alpha receptor blockers during treatment (beta - 17.36 mL; 95 % CI - 32.97, -1.74; p = 0.030) and lower baseline bladder volume (beta 11.62 mL; 95 % CI 4.20, 19.05; p = 0.002) as significant factors in limiting both absolute bladder volume and the rate of bladder filling during adaptive SBRT fractions. Conversely, the need for a positional target shift at verification MR was associated with larger bladder volume (OR 1.20; 95 % CI 0.98, 1.46; p = 0.075) and high International Prostate Symptom Score (OR 5.42; 95 % CI 1.34, 21.89; p = 0.018). Post-treatment dosimetric analysis revealed no notable compromises to prostate target coverage (D95Gy median -0.19 Gy, IQR 0.49) or normal tissue constraints. CONCLUSIONS:This analysis of bladder filling dynamics in patients undergoing prostate SBRT with real-time adaptive planning demonstrated predictable bladder filling patterns using an empty bladder regimen. Dose-volume constraints were consistently achieved for both target volumes and normal tissues. The finding that alpha receptor blockers reduced the rate of bladder filling during treatment fractions may have implications for improving treatment consistency and patient comfort in real-time adaptive planning workflows.

PURPOSE/OBJECTIVE:To commission a beam model in ClearCalc (Radformation Inc.) for use as a secondary dose calculation algorithm and to implement its use into an adaptive workflow for an MR-linear accelerator. METHODS:A beam model was developed using commissioning data for an Elekta Unity MR-linear accelerator and entered into ClearCalc. The beam model consisted of absolute dose calculation settings, output factors, percent depth-dose (PDD) curves, mutli-leaf collimator (MLC) transmission and dose leaf gap error, and cryostat corrections. Beam profiles were hard-coded by the manufacturer into the beam model and were compared with Monaco-derived profiles. The beam model was tested by comparing point doses in a homogenous phantom obtained through measurements using an ionization chamber in water, Monaco, and ClearCalc for various field sizes, source-surface distances (SSDs), and point locations. Additional testing including point dose verification for test plans using a heterogeneous phantom and patient plans. Post clinical implementation, performance of ClearCalc was evaluated for the first 41 patients treated, which included 215 adaptive plans. RESULTS:PDDs generated using ClearCalc fell within 1.2% of measurements. Field profile comparison between ClearCalc and Monaco showed an average pass rate of 98% using a 3%/3 mm gamma criteria. Measured cryostat corrections used in the beam model showed a maximum deviation from unity of 1.4%. Point dose and field monitor units (MUs) comparisons in a homogenous phantom (N = 22), heterogeneous phantoms (N = 22), and patient plans (N = 57) all passed with a threshold of 5%/5MU. Clinically, ClearCalc was implemented as a physics check post adaptive planning completed prior to beam delivery. Point dose and field MUs showed good agreement at a 5%/5MU threshold for prostate stereotactic body radiation therapy (SBRT), pelvic lymph nodes, rectum, and prostate and lymph node plans. DISCUSSION/CONCLUSIONS:This work demonstrated commissioning and clinical implementation of ClearCalc into an adaptive planning workflow. No primary or adaptive plan failures were reported with proper beam model testing.

PURPOSE/OBJECTIVE:The IC Profiler (ICP) manufactured by Sun Nuclear Corporation (SNC) is an ionization chamber (IC) array used for linear accelerator dosimetry measurements. Previous work characterized response of the ICP under various conditions, but there is limited work of its implementation into monthly QA measurement procedures. This work quantifies ICP accuracy and variables that affect accuracy for beam output measurements, and demonstrates feasibility of using the ICP for all recommended monthly dosimetry measurements. METHODS:A total of 1985 output measurements on six Varian TrueBeam and Edge linear accelerators were performed using three ICP with quad wedges (QWs) and were compared with conventional IC measurements. The accuracy of the ICP for beam output was characterized as the difference between the ICP and IC. Variables that affect ICP accuracy, including gain settings, calibrations, and template baselining as well as machine or energy-specific bias were investigated. Measurements of profile constancy, energy, dose rate constancy, wedge factors, and gating were performed. RESULTS:The initially observed mean output difference between the ICP and IC was 0.16% (0.61%). When gain settings were optimized, the output difference accuracy improved to -0.02% (0.38%). The output accuracy of the ICP was not dependent on array, dose, temperature and pressure calibrations, or template baselining. Statistically, ICP output accuracy was dependent on machine and beam energy, but clinically, all measurements fell within 0.5% of unity. ICP measurements of energy, dose rate constancy, and wedge factors matched passing results with conventional IC in water measurements. Gating and beam profile constancy measurements demonstrated good stability using the ICP. Finally, monthly dosimetry QA using ICP was completed in an average of 33 min compared to 66 min using the IC. CONCLUSION/CONCLUSIONS:This work demonstrated the feasibility and efficiency of using the ICP, with specific considerations, as a measurement device for dosimetric linear accelerator monthly QA.

PURPOSE/OBJECTIVE:Our institution was an early adopter of 5-fraction accelerated partial breast irradiation (ABPI) to treat women with early-stage breast cancer. This study reports long-term oncologic and cosmetic outcomes. METHODS:We included patients receiving APBI 600 cGy × 5 fx delivered every other day or every day between 2010 and 2022. Logistic regression models were used to identify factors associated with development of late toxicities, clinician, and patient-rated cosmesis. Kaplan-Meier methodology was used to calculate overall survival (OS), disease-free survival (DFS), and locoregional recurrence-free survival (LR-RFS). RESULTS:442 patients received APBI either daily (56%) or every other day (44%) in the prone position (92%). At a median follow-up of 48 months (range: 5.96-155 months), 12 (2.7%) patients developed a local recurrence (LR). Out of 258 patients with > 3-month toxicity data available, the most common late grade ≥ 2 adverse event was breast fibrosis (6.2%). On multivariate analysis, daily APBI treatment (vs every other day) did not correlate with an increased risk of any late grade ≥ 2 toxicity though it did correlate with a lower risk of any late grade ≥ 2 fibrosis. Overall, at a median follow-up of 80 months, the rates of good-excellent physician and patient-rated cosmesis were 95% and 85%, respectively, with no difference between patients treated on consecutive vs. every other day. On multivariate analysis, patients who did not receive any adjuvant therapy were at increased risk of developing a LR. Five-year OS, LRFS, and DFS were 97.2%, 97.7%, and 89.5%, respectively. CONCLUSIONS:Five-fraction APBI delivered primarily in the prone position either daily or every other day was effective with low rates of local recurrence, minimal toxicity, and excellent cosmesis at long-term follow-up.

PURPOSE/OBJECTIVE:A 3D stereoscopic camera system developed by .decimal was commissioned and implemented into the clinic to improve the efficiency of clinical electron simulations. Capabilities of the camera allowed simulations to be moved from the treatment vault into any room with a flat surface that could accommodate patient positioning devices, eliminating the need for clinical patient setup timeslots on the treatment machine. This work describes the process used for these simulations and compares the treatment parameters determined by the system to those used in delivery. METHODS:The Decimal3D scanner workflow consisted of: scanning the patient surface; contouring the treatment area; determining gantry, couch, collimator, and source-to-surface distance (SSD) parameters for en face entry of the beam with sufficient clearance at the machine; and ordering custom electron cutouts when needed. Transparencies showing the projection of in-house library cutouts at various clinical SSDs were created to assist in choosing an appropriate library cutout. Data from 73 treatment sites were analyzed to evaluate the accuracy of the scanner-determined beam parameters for each treatment delivery. RESULTS:Clinical electron simulations for 73 treatment sites, predominately keloids, were transitioned out of the LINAC vault using the new workflow. For all patients, gantry, collimator, and couch parameters along with SSD and cone size were determined using the Decimal3D scanner with 57% of simulations using library cutouts. Tolerance tables for patient setup were updated to allow differences of 10, 20 and 5 degrees for gantry, collimator and couch, respectively. Approximately 7% of fractions (N=181 total fractions) were setup outside of the tolerance table based on physician-direction during treatment. This reflects physician preference to adjust the LINAC rather than patient position during treatment setup. No scanner-derived plan was untreatable due to cutout shape inaccuracy or clearance issues. CONCLUSION/CONCLUSIONS:Clinical electron simulations were successfully transitioned out of the LINAC vault using the Decimal3D scanner without loss of setup accuracy as measured through machine parameter determination and electron cutout shape.

BACKGROUND:Long-term follow-up using volumetric measurement could significantly assist in the management of vestibular schwannomas (VS). Manual segmentation of VS from MRI for treatment planning and follow-up assessment is labor-intensive and time-consuming. This study aims to develop a deep learning technique to fully automatically segment VS from MRI. METHODS:This study retrospectively analyzed MRI data of 737 patients who received gamma knife radiosurgery for VS. Treatment planning T1-weighted isotropic MR and manually contoured gross tumor volumes (GTV) were used for model development. A 3D convolutional neural network (CNN) was built on ResNet blocks. Spatial attenuation and deep supervision modules were integrated in each decoder level to enhance the training for the small tumor volume on brain MRI. The model was trained and tested on 587 and 150 patient data, respectively, from this institution (n = 495) and a publicly available dataset (n = 242). The model performance were assessed by the Dice similarity coefficient (DSC), 95% Hausdorff distance (HD95), average symmetric surface (ASSD) and relative absolute volume difference (RAVD) of the model segmentation results against the GTVs. RESULTS:Measured on combined testing data from two institutions, the proposed method achieved mean DSC of 0.91 ± 0.08, ASSD of 0.3 ± 0.4 mm, HD95 of 1.3 ± 1.6 mm, and RAVD of 0.09 ± 0.15. The DSCs were 0.91 ± 0.09 and 0.92 ± 0.06 on 100 testing patients of this institution and 50 of the public data, respectively. CONCLUSIONS:A CNN model was developed for fully automated segmentation of VS on T1-Weighted isotropic MRI. The model achieved good performance compared with physician clinical delineations on a sizeable dataset from two institutions. The proposed method potentially facilitates clinical workflow of radiosurgery for VS patient management.