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Purpose/objective(s) Management of progressive brain tumors following high dose radiotherapy is controversial. In select cases, surgical resection can provide rapid relief of regional mass effect and edema, and highly focal adjuvant re-irradiation can be administered in the form of brachytherapy. Permanent low-dose rate cesium-131 brachytherapy may offer dosimetric and radiobiologic advantages over other isotopes. The purpose of this study was to examine the safety and efficacy of intracranial cesium-131 brachytherapy after surgical resection in a heavily pre-treated patient cohort. Materials/methods A retrospective chart review was conducted of all patients treated with intraoperative intracranial cesium-131 brachytherapy from an institutional database. Treatment consisted of maximal safe resection followed by implantation of suture-stranded cesium-131 seeds, maintaining approximately 10mm spacing between seeds, with a planned target dose of 80 Gy to 5mm depth. Local failure was defined as recurrence in or contacting the resection cavity after salvage resection and brachytherapy. Regional failure was defined as recurrence within 10mm of the resection cavity. Recurrence was defined as a new lesion or growth of ≥25% in any axial dimension on interval MRI. Toxicity was collected based on CTCAE 4.0. Results A total of 35 patients with a median imaging follow up of 17.0 months (interquartile range [IQR] 9.7-25.9) underwent 41 consecutive cesium-131 implants between 2016 and 2020. 20 patients (59%; N = 22 implants) were treated for recurrent brain metastasis, 12 patients (34%; N = 16 implants) were treated for recurrent atypical (N = 8) or anaplastic meningioma (N = 4), and 3 patients (9%) were treated for recurrent glioma (N = 2) or hemangiopericytoma (N = 1). A majority of lesions had received prior surgery (N = 28, 66.7%, range 0-3 prior surgeries) and 34 patients had received prior high dose radiation. 34 of 41 (82.9%) tumors had received prior radiosurgery, and 12 (29.2%) had undergone prior external beam radiation (range 50-60 Gy). 17 of 41 lesions (41.5%) had radiologic evidence of adverse radiation/treatment effect (ARE) prior to salvage therapy. Median tumor size was 3.0cm (IQR 2.3-3.7). Overall actuarial local/loco-regional control at 1-year was 91.6%/84.7% (89.8%/88.9% for metastases and 100%/82.5% for meningiomas). Crude loco-regional control was 86.3% for metastases, 75% for meningiomas, and 66.7% for other lesions. Symptomatic ARE following treatment was observed in 9.8% (N = 4) resection cavities. The surgical complication rate was 14.6% (N = 6 total, N = 3 [7.3%] grade 3-5) including 1 peri-operative death. Conclusion Cesium-131 brachytherapy in this heavily pre-treated cohort resulted in good local control and an acceptable rate of symptomatic adverse radiation effect and toxicity and may be a reasonable treatment option for this select population. Author disclosure W.C. Chen: None. M. Lafreniere: None. C. Phuong: None. M. Lometti: None. O. Morin: None. B.P. Ziemer: None. H. Vasudevan: Research Grant; Children's Tumor Foundation. Patent/License Fees/Copyright; Genentech, Eli Lilly. S. Hervey-Jumper: None. P.V. Theodosopoulos: None. S. Magill: None. S.E. Fogh: Independent Contractor; Accuray. J.L. Nakamura: None. L. Boreta: None. P.K. Sneed: None. M.W. McDermott: Chair; Miami Baptist. D. Raleigh: None. S.E. Braunstein: Advisory Board; Radiation Oncology Questions, LLC.

This study compares conventional and robotic intensity modulated radiation therapy (IMRT) plans for prostate boost treatments and provides clinical insight into the strengths and weaknesses of each. The potential for dose escalation with robotic IMRT is further investigated using the “critical volume tolerance” method proposed by Roach et al. Three clinically acceptable treatment plans were generated for 10 prostate boost patients: (1) a robotic IMRT plan using fixed cones, (2) a robotic IMRT plan using the Iris variable aperture collimator, and (3) a conventional linac based IMRT (c-IMRT) plan. Target coverage, critical structure doses, homogeneity, conformity, dose fall-off, and treatment time, were compared across plans. The average bladder and rectum V75 was 17.1%, 20.0%, and 21.4%, and 8.5%, 11.9%, and 14.1% for the Iris, fixed, and c-IMRT plans, respectively. On average the conformity index (nCI) was 1.20, 1.30, and 1.46 for the Iris, fixed, and c-IMRT plans. Differences between the Iris and the c-IMRT plans were statistically significant for the bladder V75 (P= .016), rectum V75 (P= .0013), and average nCI (P =.002). Dose to normal tissue in terms of R50 was 4.30, 5.87, and 8.37 for the Iris, fixed and c-IMRT plans, respectively, with statistically significant differences between the Iris and c-IMRT (P = .0013) and the fixed and c-IMRT (P = .001) plans. In general, the robotic IMRT plans generated using the Iris were significantly better compared to c-IMRT plans, and showed average dose gains of up to 34% for a critical rectal volume of 5%.

The purpose of this report was to evaluate the magnitude of the error that would be introduced if only a lateral (LAT) portal image, as opposed to a pair of orthogonal images, was used to verify and correct daily setup errors and organ motion in external beam radiation therapy (EBRT) of prostate cancer. The 3-dimensional (3D) coordinates of gold markers from 12 consecutive prostate patients were reconstructed using a pair of orthogonal images. The data were re-analyzed using only the LAT images. Couch moves from the 2-dimensional (2D)-only data were compared with the complete 3D data set. The 2D-only data provided couch moves that differed on average from the 3D data by 2.3 ± 3.0, 0.0 ± 0.0, and 0.8 ± 1.0 mm in the Lat, AP, and SI directions, respectively. Along AP and SI axes, the LAT image provided positional information similar to the orthogonal pair. The error along the LAT axis may be acceptable provided lateral margins are large enough. A LAT-only setup protocol reduces patient treatment times and increases patient throughput. In most circumstances, with exceptions such as morbidly obese patients, acquisition of only a LAT image for daily targeting of the prostate will provide adequate positional precision.

Purpose: The aim of this study was to measure interfraction prostate bed motion, setup error, and total positioning error in 10 consecutive patients undergoing postprostatectomy radiotherapy. Methods and Materials: Daily image-guided target localization and alignment using electronic portal imaging of gold seed fiducials implanted into the prostate bed under transrectal ultrasound guidance was used in 10 patients undergoing adjuvant or salvage radiotherapy after prostatectomy. Prostate bed motion, setup error, and total positioning error were measured by analysis of gold seed fiducial location on the daily electronic portal images compared with the digitally reconstructed radiographs from the treatment-planning CT. Results: Mean (± standard deviation) prostate bed motion was 0.3 ± 0.9 mm, 0.4 ± 2.4 mm, and −1.1 ± 2.1 mm in the left–right (LR), superior–inferior (SI), and anterior–posterior (AP) axes, respectively. Mean set-up error was 0.1 ± 4.5 mm, 1.1 ± 3.9 mm, and −0.2 ± 5.1 mm in the LR, SI, and AP axes, respectively. Mean total positioning error was 0.2 ± 4.5 mm, 1.2 ± 5.1 mm, and −0.3 ± 4.5 mm in the LR, SI, and AP axes, respectively. Total positioning errors >5 mm occurred in 14.1%, 38.7%, and 28.2% of all fractions in the LR, SI, and AP axes, respectively. There was no significant migration of the gold marker seeds. Conclusions: This study validates the use of daily image-guided target localization and alignment using electronic portal imaging of implanted gold seed fiducials as a valuable method to correct for interfraction target motion and to improve precision in the delivery of postprostatectomy radiotherapy.

A replacement technology for the muon detection system in BaBar using scintillator bars with Wavelength Shifting (WLS) fibers and Avalanche Photo Diodes (APDs). APDs must be used in BaBar due to the high magnetic fields that disrupt the electron multiplying effects of Photo Multiplier Tubes (PMTs). Muons passing through scintillator cause fluors in the scintillator to emit photons, which are captured by WLS fibers and then re-emitted at the ends of the fibers to awaiting APDs. The detector configuration that will produce the largest Light Yield (LY) and produce a clear APD signal that will be larger than background noise is the main goal of the study. Bar dimensions, number of WLS fibers, and placement of fibers are parameters that may be adjusted to produce the largest LY. Additionally, good timing resolution is needed to determine where a muon passed through the detector along the long direction of the bar. A scintillator bar 2cm x 4cm in x-y with four round lmm fibers produced the largest LY out of four different configurations with a timing resolution of 25cm. A Monte Carlo simulation confirmed the relative LY compared to other detectors. This detector was able to produce a 40mV pulse frommore » the APD that was easily discernable above the 2-SmV background noise. Refinement of the fabrication process could produced higher LYs and better timing resolutions, while a redesign of the electronics may increase the signal to noise ratio.« less

Only a rare concern a few years ago, an increasing number of patients with hip replacements are presenting for prostate radiotherapy.Image artifacts caused by the presence of hip replacements render CTimages useless for prostate delineation and catheter definition. We report on the use of a Mega‐Voltage Cone‐Beam CT (MVCBCT) system to identify the catheters and to complement the regular CT for target definition of High Dose Rate Brachytherapy of prostate cancer patients with hip replacements. Following a routine trans‐rectal ultrasound guided catheter implant and recovery, MVCBCT and regular CT acquisitions were performed within few minutes. Metallic wires were inserted in the catheters during the MVCBCT image acquisition. A series of filters were applied on the MVCBCT image to maximise spatial resolution and image quality. The MVCBCT and CTimages were registered with the Anatomy Modelling module of the planning system using the catheters as landmarks. The fused images were used to delineate the target and organs at risk. Then, the MVCBCT and the volumes of interest were transferred to the brachytherapy planning system where catheters were identified solely from the MVCBCT image. The dose distribution was optimized by following our standard technique using the inverse planning IPSA. Fused MVCBCT‐CT images greatly facilitated target delineation. The registration precision was better than 1.5 mm in the prostate and catheter areas. Catheters were easily visible and accurately defined on MVCBCT. The same dosimetric criteria used for regular HDR prostate planning were and achieved on this patient. HDR treatment based only on CT is not possible for patients with bi‐lateral hip replacements. The use of MVCBCT for planning purpose allows to offer prostate and other pelvic patients with hip prosthesis the most advanced form of HDR brachytherapy. Research supported in part by Siemens and Nucletron.

Purpose: To use a flat panel electronic portal imaging device(EPID) for intensity map verification as part of an intensity modulated radiation therapy(IMRT) QA program. Method and Materials: An a‐Si EPID was used to image each field shape segment of a head and neck IMRTtreatment. The DICOM images produced from each gantry angle were summed together and weighted according to the number of MU measured. Open field portal images were acquired at the end of the treatment, for each gantry angle, and subtracted from each composite image. A GUI was created in the MATLAB environment to read and sort each image according to gantry angle and MU measured, perform the image summations/subtractions, produce and display a final composite image.Results: Subtraction of the open field portal image from the field shape composite image removed the anatomy from the treatment area and produced a true intensity map. Outside of the treatment area the anatomy was visible, thus providing verification of correct radiation delivery location. The composite images were compared to other composite images produced using the same treatment plan delivered with and without a phantom and were found to be in excellent agreement. The images were also compared to the intensity map images created in the treatment planning system. Imaging of each segment of an IMRTtreatment required no additional radiation be delivered to the patient. The open field portal images require additional radiation, however each image needs only to be acquired once during a course of treatment.Conclusion: This work indicates that flat panel EPIDs may be used to image an IMRTtreatment and accurately and efficiently verify the intensity and location of the delivered radiation. Future investigations will use the 2D images to produce a 3D intensity “cloud” image.

Purpose: Current set‐up protocols for prostate patients with gold markers receiving EBRT use orthogonal portal images acquired prior to treatment to position the patient. Times between image acquisition and treatment delivery for different gantry angles range from one to seven minutes, even longer for IMRT. During this time the prostate can shift from its corrected position. Method and Materials: An a‐Si flat panel EPID was used to acquire pre‐treatment portal images for positioning purposes. The panel was also used in fluoroscopic mode, acquisition rate of 3.5 frames/second, during treatment delivery. Movies of treatment delivery, 10–70 frames/movie, were acquired. Intra‐fractional and inter‐/intra‐field (between treatment delivery/during treatment delivery) organ motion was evaluated. Results: To date 83 of 125 days of treatment data have been processed. Initial results indicate that intra‐fractional/intra‐field organ motion is rare, only 4% of days exhibit marker center‐of‐mass moves greater than 2 mm. Intra‐fractional/inter‐field motion greater than 2 mm does however occur for a significant portion of treatment days, 31% of the processed population, appearing to occur during 45–90 second intervals. Qualitative analysis indicates that a large portion of prostate shifts occur due to gas in the colon and/or rectum. Respiration does not seem to be a significant cause of prostate motion. Voluntary global patient motion appears to contribute slightly to organ motion. Conclusion: The prostate is fairly stable for time periods comparable to treatment delivery times, yet is less stable for longer times. A new setup protocol is proposed utilizing a step‐and‐shoot method of treatment. The method of patient positioning would acquire a portal image, adjust the patient based on the image and then treat that single field. This process would be repeated for each gantry angle until treatment is completely delivered, thus reducing the time between portal image acquisition and treatment delivery for a particular gantry angle.