Radiosurgical System Research Articles

Experimental and Monte Carlo based dosimetric investigation of a novel 3mm radiosurgery 3MV beam using the microSilicon detector.

The ZAP-X system is a novel gyroscopic radiosurgical system based on a 3MV linear accelerator and collimator cones with a diameter between 4 and 25mm. Advances in imaging modalities to detect small and early-stage pathologies allow for an early and less invasive treatment, where a smaller collimator matching the anatomical target could provide better sparing of surrounding healthy tissue. A novel 3mm collimator cone for the ZAP-X was developed. This study aims to investigate the usability of a commercial diode detector (microSilicon) for the dosimetric characterization of this small collimator cone; and to investigate the underlying small field perturbation effects. Profile measurements in five depths as well as PDD and output ratio measurements were performed with a microSilicon detector and radiochromic EBT3 films. In addition, comprehensive Monte Carlo simulations were performed to validate the measurement observations and to quantify the perturbation effects of the microSilicon detector in these extremely small field conditions. It is shown that the microSilicon detector enables an accurate dosimetric characterization of the 3mm beam. The profile parameters, such as the FWHM and 20%-80% penumbra width, agree within 0.1 to 0.2mm between film and detector measurements. The output ratios agree within the measurement uncertainty between microSilicon detector and films, whereas the comparisons of the PDD results show good agreement with the Monte Carlo simulations. The analysis of the perturbation factors of the microSilicon detector reveals a small field correction factor of approximately 3% for the 3mm circular beam and a correction factor smaller than 1.5% for field diameters above 3mm. It could be shown that the microSilicon detector is well-suitable for the characterization of the new 3mm circular beam of the ZAP-X system.

Accurate machine-specific reference and small-field dosimetry for a self-shielded neuro-radiosurgical system.

The newly available ZAP-X stereotactic radiosurgical system is designed for the treatment of intracranial lesions, with several unique features that include a self-shielding, gyroscopic gantry, wheel collimation, non-orthogonal kV imaging, short source-axis distance, and low-energy megavoltage beam. Systematic characterization of its radiation as well as other properties is imperative to ensure its safe and effective clinical application. To accurately determine the radiation output of the ZAP-X with a special focus on the smaller diameter cones and an aim to provide useful recommendations on quantification of small field dosimetry. Six different types of detectors were used to measure relative output factors at field sizes ranging from 4to 25mm, including the PTW microSilicon and microdiamond diodes, Exradin W2 plastic scintillator, Exradin A16 and A1SL ionization chambers, and the alanine dosimeter. The 25mm cone served as the reference field size. Absolute dose was determined with both TG-51-based dosimetry using a calibrated PTW Semiflex ion chamber and measurements using alanine dosimeters. The average radiation output factors (maximum deviation from the average) measured with the microDiamond, microSilicon, and W2 detectors were: for the 4mm cone, 0.741 (1.0%); for the 5mm cone: 0.817 (1.0%); for the 7.5mm cone: 0.908 (1.0%); for the 10mm cone: 0.946 (0.4%); for the 12.5mm cone: 0.964 (0.2%); for the 15mm cone: 0.976 (0.1%); for the 20mm cone: 0.990 (0.1%). For field sizes larger than 10mm, the A1SL and A16 micro-chambers also yielded consistent output factors within 1.5% of those obtained using the microSilicon, microdiamond, and W2 detectors. The absolute dose measurement obtained with alanine was within 1.2%, consistent with combined uncertainties, compared to the PTW Semiflex chamber for the 25mm reference cone. For field sizes less than 10mm, the microSilicon diode, microDiamond detector, and W2 scintillator are suitable devices for accurate small field dosimetry of the ZAP-X system. For larger fields, the A1SL and A16 micro-chambers can also be used. Furthermore, alanine dosimetry can be an accurate verification of reference and absolute dose typically measured with ion chambers. Use of multiple suitable detectors and uncertainty analyses were recommended for reliable determination of small field radiation outputs.

Cyberknife treatment for different types of tumor

CyberKnife radiosurgery is a frameless, image guided robotic stereotactic radiosurgical system (IGR-SRS) which uses potent, ablative radiation dose with limited treatment volume. The main aim of this study is to provide knowledge of this treatment for different types of tumor. To deliver radiation treatment from multiple angles, with real time target tracking through a combined orthogonal radiograph imaging and optic motion tracking system cyberKnife is a new and advanced technique. Radiation is delivered to the tumor within a predetermined range, for this an object recognition computer algorithm and a fudicial marker placed within a tumor. CyberKnife treatment cost USD 6,800 (Indian Rupees approx. 4,89,000). In this review you will get idea about different types of tumor get treated with cyberKnife radiosurgery (SRS). CyberKnife have submillimeter accuracy in metastatic brain tumor. By giving appropriate tracking method for stage I lung cancer and lung oligometastases stereotactic radiotherapy is an efficient treatment. So, at well-defined tumor sites cyberKnife is a new stereotactic body radiation therapy which delivers high dose of radiation in short period of time. For head and neck tumor treatment cyberKnife is most widely used. This treatment is helpful for adrenal gland tumor and prostate tumor, spinal metastases.

Stereotactic body radiation therapy for inoperable localized kidney cancer

Purpose of the study: to evaluate the overall survival and local control in patients with inoperable kidney cancer treated with stereotactic body radiotherapy (SBRT). Material and Methods. From 2011 to 2021, 42 patients with histologically verified kidney cancer underwent SBRT to a total dose ranging from 30 to 45 Gr in 3 fractions using the CyberKnife robotic radiosurgical system. SBRT was performed for inoperable patients, taking into account various factors (age, severity of concomitant pathology, ASA IV physical status level), patients’ refusal of surgical treatment, and tumor size of ≤5.0 cm in diameter. SBRT was not performed for patients with tumor size of ≥5.0 cm in diameter, impaired renal function, generalization of cancer and decompensation of concomitant pathology. The median age of the patients was 67.9 years. 93 % of patients had stage I kidney cancer. The primary tumor was diagnosed in 37 cases, recurrence after previous surgical treatment in 6, including 1 patient with recurrence of cancer of both kidneys. The average tumor volume was 28.1 cm3. Results. The median overall survival (OS) was not reached due to the fact that the majority of patients (n=36) were alive at the time of the study. The 1-, 3- and 5-year OS rates were 94.1 %, 88.7 %, and 72.6 %, respectively. The 1-year local control rate was 93 %. At 3-6 months, 26.2 % of patients showed partial response, 66.8 % had stable disease and 7 % had disease progression (according to the RECIST 1.1 criteria). Renal toxicity occurred in 23.8 % of patients 3 months after radiation therapy; however, kidney function was restored in these patients within a year. Conclusion. SBRT has demonstrated high efficacy with minimal toxicity in the treatment of inoperable patients with kidney cancer.

Intraoperative radiotherapy with a photonic radiosurgical system in the treatment of locally advanced rectal cancer

Aim of the study: to evaluate the results of using intraoperative radiotherapy (IO RT) in patients with locally advanced rectal cancer (RC).Material and Methods. A total of 172 patients with histologically confirmed stage II –III (pT3–4 or pN+) RC were included in this retrospective analysis; of those, 92 (53,5 %) were treated with IO RT alone and 80 (46,5 %) received both neoadjuvant EBRT and IO RT. The median follow-up was 25 months for the IO RT group and 22 months for the EBRT/IO RT group (p=0,52).Results. The incidence of toxicity was low in both groups with an overall complication rate of 5,4 %. There were no statistically significant differences between both groups in 4-year overall survival rates (59,1 % in the IO RT group versus 67,4 % in the EBRT / IO RT group, p=0,66), progression-free survival (53,6 versus 55,1 %, p=0,51) and local progression free survival (59,4 versus 65,4 %, p=0,70).Conclusion. IO RT for locally advanced RC is a safe method that ensures adequate local control and can be considered as an effective treatment method both in an isolated version and in integration into a multimodal treatment algorithm in a specialized oncological clinics.

Bothersome Hematospermia Following Stereotactic Body Radiation Therapy for Prostate Cancer.

BackgroundHematospermia following prostate radiation therapy is a benign and often self-limiting side effect. However, it may be bothersome to some men and their partners with a negative impact on sexual quality of life (QOL). This study sought to evaluate the incidence, duration, and resolution of hematospermia in patients following stereotactic body radiation therapy (SBRT) for prostate cancer.Methods227 patients treated with SBRT from 2013 to 2019 at Georgetown University Hospital for localized prostate carcinoma with a minimum follow up of two years were included in this retrospective review of data that was prospectively collected. Patients who were greater than 70 years old and/or received hormonal therapy were excluded. Hematospermia was defined as bright red blood in the ejaculate. Time points for data collection included initial consultation, pre-treatment, 1-, 3-, 6-, 9-, 12-, 18-, 24-month. All patients were treated with the CyberKnife Radiosurgical System (Accuray). Data on hematospermia including duration, resolution and recurrence was collected. Utilization of 5-alpha reductase inhibitors was documented at each visit.Results227 patients (45 low-, 177 intermediate-, and 5 high-risk according to the D’Amico classification) at a median age of 65 years (range 47-70) received SBRT for their localized prostate cancer. The 2-year cumulative incidence of hematospermia was 5.6%(14 patients). For these patients, all but one patient (93%) saw resolution of their hematospermia by two years post-SBRT. The median time for hematospermia was 9 months post-treatment. Of the 14 patients who reported hematospermia, 70% were managed with 5-alpha reductase inhibitors. Hematospermia was transient in most patients with 70% of the men reporting resolution by the next follow-up visit.ConclusionThe incidence of bothersome hematospermia following SBRT was low. Hematospermia, as noted by other studies, often self-resolves. 5-alpha reductase inhibitors may lead to quicker resolution of bothersome hematospermia.

The Zap-X Radiosurgical System in the Treatment of Intracranial Tumors: A Technical Case Report.

BACKGROUND AND IMPORTANCEThe Zap-X system (Zap Surgical Systems Inc, San Carlos, California) is a radically new surgical robot designed for brain and head and neck radiosurgery. It represents the first new dedicated brain stereotactic radiosurgery platform in almost half a century optimizing the goals of safety, speed, and accuracy. The Zap-X system was used in a required Chinese National Medical Products Administration clinical study. In early January 2020, 2 patients were treated with the Zap-X robot prior to a national COVID-19 lockdown. Both were closely followed via clinical exam and magnetic resonance imaging (MRI) imaging. Prospectively collected data were used to generate this report.CLINICAL PRESENTATIONTwo female patients, each harboring either a trigeminal schwannoma or petroclival meningioma, were treated with the Zap-X robot. Respective tumor volumes were 2.60 and 4.02 cm3. A radiation dose of 13 Gy was prescribed to the 50% isodose line. At 8 mo of follow-up, preoperative symptoms were either resolved or stable and MRI imaging demonstrated a 31% and 56% reduction in lesion volume, respectively. In both patients, symptoms improved, and tumor volumes decreased, whereas no major complication was observed.CONCLUSIONGiven only 2 patients and short-term follow-up, any conclusions about the safety and efficacy of the Zap-X radiosurgery robot are preliminary. However, in the absence of any other published outcomes to date, this small case series may be of interest to many radiosurgical specialists.

Impact of Age on Patient-Reported Outcomes Following Stereotactic Body Radiation Therapy for Prostate Cancer.

PurposeStereotactic body radiation therapy (SBRT) delivers large radiation doses to the prostate while minimizing exposure to adjacent normal tissues. Large fraction sizes may increase the risks of functional decrements. Elderly men may be at an increased risk of these toxicities due to poor baseline function and hence limited reserve. This study describes patient-reported outcomes following SBRT for clinically localized prostate cancer in the elderly.MethodsBetween 2007 and 2017, 179 hormone-naive elderly patients (≥ 70 years old) and 210 patients under 70 years old with clinically localized prostate cancer were treated with 35-36.25 Gy SBRT in five fractions utilizing the CyberKnife Radiosurgical System (Accuray Inc.). Quality of life (QOL) was assessed using the Expanded Prostate Index Composite-Short Form (EPIC-26) questionnaire at baseline and at 1, 3, 6, 12, 18, 24, 30, and 36 months following the completion of treatment. EPIC scores range from 0 to 100, with lower values representing worsening symptoms.ResultsEPIC scores in the elderly cohort mirrored those in the younger cohort. EPIC urinary obstructive/irritative scores declined at one month post-SBRT (mean change from baseline ≥70: -7.9; <70: -11.1) before returning to baseline at three months post-SBRT (mean change from baseline ≥70: -0.4; <70: -1.4). The EPIC urinary incontinence scores declined slowly over the three years following treatment without recovery (mean change from baseline ≥70: -6.6; <70: -4.8). EPIC Bowel scores transiently declined at one month post-SBRT (mean change from baseline ≥70: -8.5; <70: -9.1) and then experienced a second more protracted decline over the next three years without recovery (mean change from baseline ≥70: -4.5; <70: -1.8). Hormonal EPIC scores were not impacted by radiation treatment or age. Older men had lower baseline and post-treatment EPIC sexual summary scores at all time points. However, there was no clinically significant difference in the EPIC sexual bother score between younger and older men at baseline and following treatment.ConclusionsIn the first three years following treatment, the impact of SBRT treatment on patient-reported outcomes was minimal. Our findings suggest that SBRT for clinically localized prostate cancer should not be deferred in older men solely due to concerns of increased morbidity. Further studies should be conducted to evaluate the impact of age on outcomes or morbidity following SBRT.

Self-Shielding for the ZAP-X®: Revised Characterization and Evaluation.

The ZAP-X® is a newly designed, self-contained, and first-of-its-kind self-shielded therapeutic radiation therapy device dedicated to brain and head and neck stereotactic radiosurgery (SRS). By using an S-band linear accelerator (linac) and employing integrated minimal but sufficient shielding, the ZAP-X does not typically require a radiation bunker. At the same time, the self-shielded features of the ZAP-X are designed for more consistency of radiation protection, reducing the risk to radiation workers and others potentially exposed from a poorly designed or constructed radiotherapy vault.This study postulates that a radiosurgical system can be self-shielded, such that it produces radiation exposure levels deemed safe to the public while operating under a full clinical workload. The goal of self-shielding is achieved under all but the most exceptional clinical conditions.This work is intended to serve as guidance for the radiation safety evaluations of future ZAP-X treatment operations, following local or regional applicable regulatory requirements, and utilizing the unique provision of all or most of the required shielding material as an integral part of the device.

Robotic Stereotactic Accelerated Partial-Breast Irradiation for Early-Stage Breast Cancer: 5-Year Results of a Single-Institution Pilot Study

Outcomes following adjuvant accelerated partial breast irradiation (APBI) in select women with early stage breast cancer are comparable to whole breast irradiation. Robotic stereotactic accelerated partial breast irradiation (SAPBI) with fiducial tracking is an attractive treatment option, but limited data are available regarding the feasibility of this approach and 5 year outcomes have not yet been published. We report our institutional experience treating select women with SAPBI. Women with DCIS and early stage breast cancer treated from November 2008 to September 2015 were evaluated. Treatments were delivered with a robotic radiosurgical system. At least 4 gold fiducials were implanted around the lumpectomy cavity prior to the start of treatment for target tracking. The CTV was delineated on contrast enhanced CT scans using surgical clips and the lumpectomy cavity. In 2014, following the publication of the University of Florence phase III trial, the protocol was amended after 13 patients and the CTV was defined as the lumpectomy cavity with a uniform 1 cm expansion confined to the breast tissue. The PTV, prescribed 30 Gy in 5 fractions, was defined as the CTV with a 3-5 mm uniform expansion. AAPM TG-101 skin dose constraints were respected. Clinical examination and mammography were completed at 6-12 month follow up intervals. Breast cosmesis was scored using The Harvard Breast Cosmesis Grading Scale. Twenty women (median age 65 years) were treated over a median 7 days (range 5-13). Fourteen women had DCIS and 50% of the tumors were located in the upper outer quadrant. The median treated PTV was 63 cm3 (range 15-142), the median PTV/breast volume ratio was 8.3% (range 4.1-25.6) and the median prescription isodose line was 83% (range 75-87). Three or 4 fiducials were successfully tracked in 90% of patients. At a median follow up of 60 months (range 33-108), locoregional control, distant control, and overall survival was 100%. Five women with skin Dmax > 33 Gy (skin = CT surface minus 2 mm) developed chronic grade 1 dermatitis (faint hyperpigmentation), one of whom developed focal grade 1 telangiectasia at 52 months. Three women developed grade 1 fibrosis, one of whom was later diagnosed with fat necrosis on mammography. Two additional asymptomatic women were diagnosed with fat necrosis on routine mammography. There were no rib fractures. One woman required stereotactic biopsy to confirm benign tumor bed calcifications at 66 months. Fifteen evaluable patients maintained Good-Excellent cosmesis. Robotic SAPBI with fiducial tracking is a feasible, well tolerated and highly effective technique for the adjuvant treatment of select early-stage breast cancer patients. Cosmetic outcomes may be improved further by decreasing the skin Dmax to < 33 Gy. A confirmatory multi-institutional registry is actively enrolling patients. Given these encouraging results, future research in this patient population will evaluate robotic SAPBI as a definitive treatment.

Clinical Efficacy of CyberKnife Radiosurgery for Adult Brainstem Glioma: 10 Years Experience at Tianjin CyberKnife Center and Review of the Literature.

Background: Brainstem glioma is a rare brain tumor with poor prognosis and difficulty for surgical resection. We sought to retrospectively analyze and evaluate the clinical efficacy of CyberKnife for brainstem gliomas.Methods: From 2006 to 2015, a total of 21 brainstem gliomas patients who received CyberKnife radiosurgery treatment enrolled in this study and 18 patients with follow up. CyberKnife image-guided radiosurgical system were applied consecutively with the median prescribed total dose of 26 Gy (14–33 Gy) at two to six fractions on days utilizing CyberKnife system, and the median biological equivalent doses of 59.8 Gy (33.6–76.56 Gy). The clinic pathlogical features, survival were analyzed to explore the efficacy of CyberKnife radiosurgery in treatment of brainstem glioma.Results: With median follow-up of 54.5 months, patients with brainstem gliomas had median overall survival of 19 months, five patients still alive. The primary endpoints of the 1- and 2-year overall survival rates were 87.5 and 52.4%, respectively. During the treatment course, six patients were observed to have pseudoprogression with mass effect on MRI. Four patients developed radiation complications. Grade 2 radiation-related toxicity were observed in three patients and one patient with grade 3.Conclusion: The efficacy of brainstem gliomas—treated with CyberKnife is efficacious with mild toxicity.

Frame versus Frameless Leksell Stereotactic Radiosurgery.

For more than 65 years localization of brain targets suitable for stereotactic radiosurgery has been performed after application of an intracranial guiding device to the cranial vault. After imaging and dose planning the same frame is used to secure the target at the focus of the intersection of the ionizing radiation beams that create the radiobiological effect. Non-invasive immobilization systems first proposed for linear accelerator or proton radiation technologies have now been developed for the Leksell Gamma Knife ICON radiosurgical system. The ICON technology adds a cone-beam computed tomography (CBCT) scan to the original Perfexion radiosurgical device in order to define the brain stereotactic space. Marketed since 2015, the ICON can be used for standard radiosurgical procedures, most of which remain frame based, but also coupled with a non-invasive thermoplastic mask for carefully selected patients who undergo standard single-session radiosurgical procedures, as well as multisession procedures using repeat mask fixation. Both at UPMC as well as worldwide, mask immobilization has to date been used for approximately 10% of patients with specific characteristics: relatively simple dose plans, short radiation delivery times, and non-anxious patients, most of whom have metastatic or primary brain cancers. In certain cases, multisession radiosurgery is also performed using the mask. The workflow of frame versus frameless procedures is often altered, and is reliant on high-definition imaging, mostly MRI, done prior to dose planning. Since each CBCT takes 10-12 min to set up and acquire, co-register, and review with the treatment plan, and two CBCT scans are necessary to initiate the treatment plan, this workflow must be added to the beam on time. Although frame-based immobilization remains the predominant method to secure target fixation for problems suitable for single-session radiosurgery, the advent of a mask immobilization technique has proven valuable for a select group of patients. It also provides a non-invasive method to perform multisession or fractionated radiation in patients for whom traditional single-session radiosurgery is not feasible.

The use of the cyberknife stereotactic radiosurgery system for the treatment of intracranial and extracranial residual or recurrent chordoma

Objectives: Chordomas are relatively rare primary malignant bone tumors that arise from remnants of the notochord along the craniospinal axis. They tend to be locally invasive around the clival or sacro-coccygeal regions and critical neurological structures are often involved. Complete surgical excision is rarely possible and post-resection radiation enhances local control of the disease. The CyberKnife is a frameless imageguided LINAC radiosurgical system that allows treatment of both intracranial and extracranial lesion. We review here the methodology of the system and present our experience of treating skull base and spinal chordomas. Methods: Between 1994 and 2008, 22 patients with a histological diagnosis of a chordoma underwent stereotactic radiosurgery (SRS) at Stanford University Hospitals. 17 patients underwent frameless SRS at the Stanford CyberKnife Center. A retrospective and prospective review of these patients was undertaken by obtaining clinical and radiological information from an institutional review board-approved prospective database. Results: There were 11 males and 6 females with an average age of 42 years (range 10-73). 12 lesions were in the clivus, 3 in the cervical spine below C2, 1 lumbar and 1 sacral region. All but one patient had prior surgery. 9 had undergone one resection, 5 had two and 2 had three resections. Three patients had prior radiation or proton beam therapy. The average tumor volume treated was 10.9 cm3 (range 1.9-31.6 cm3), with a mean radiation dose of 29.3 Gy (range 18-50 Gy), and a mean maximum intratumoral dose of 41.0 Gy (range 24.1-67.6 Gy). Patients were followed up for an average of 4 years. Tumor control was achieved in 12 out of the 17 cases (70.6%). 5 of the 12 patients showed tumor size reduction. However, 5 out of the 17 cases showed progression with one developing a new lesion at a lower cervical vertebra despite good local control. None of the patients with tumor control developed new neurological deficits, but 4 of the 5 cases with tumor progression showed worsening neurological deficits. There was no radiation associated complications. Conclusions: Good tumor control rates can be achieved with no significant morbidity with CyberKnife based stereotactic radiosurgery. Cases with poor control tended to have complexmultiple surgical resections. Longer follow-up in larger series is required to adequately compare this technique with other forms of radiation therapy.

Self-Shielding Analysis of the Zap-X System.

The Zap-X is a self-contained and first-of-its-kind self-shielded therapeutic radiation device dedicated to brain as well as head and neck stereotactic radiosurgery (SRS). By utilizing an S-band linear accelerator (linac) with a 2.7 megavolt (MV) accelerating potential and incorporating radiation-shielded mechanical structures, the Zap-X does not typically require a radiation bunker, thereby saving SRS facilities considerable cost. At the same time, the self-shielded features of the Zap-X are designed for more consistency of radiation protection, reducing the risk to radiation workers and others potentially exposed from a poorly designed or constructed radiotherapy vault. The hypothesis of the present study is that a radiosurgical system can be self-shielded such that it produces radiation exposure levels deemed safe to the public while operating under a full clinical workload. This study summarizes the Zap-X system shielding and found that the overall system radiation leakage values are reduced by a factor of 50 compared to the occupational radiation limit stipulated by the Nuclear Regulatory Commission (NRC) or agreement states. The goal of self-shielding is achieved under all but the most exceptional conditions for which additional room shielding or a larger restricted area in the vicinity of the Zap-X system would be required.

Treatment Planning for Self-Shielded Radiosurgery.

A five degree of freedom, robotic, radiosurgical system dedicated to the brain is currently under development. In the proposed design, the machine is entirely self-shielded. The main advantage of a self-shielded system is the simplification of the system's installation, which can reduce the cost of radiosurgery. In this way, more patients can benefit from this minimally invasive and highly effective type of procedure. For technical reasons, space inside the shielded region is limited, which leads to constraints on the design. Here, two axes of rotation move a 3-megavolt linear accelerator around the patient’s head at a source axis distance of 400 millimeters (mm), while the integrated patient table is characterized by two additional rotational, and one translational, degrees of freedom. Eight cone collimators of different diameters are available. The system can change the collimator automatically during treatment, using a collimator wheel. Since the linear accelerator can only move with two rotational axes, it is not possible to reposition the beam translationally (as it is in six degrees of freedom robotic radiosurgery). To achieve translational repositioning, it is necessary to move the patient couch. Thus, translational repositioning must be kept to a minimum during treatment. Our goal in this contribution is a preliminary investigation of dose distributions attainable with this type of design. Thus, we do not intend to design and evaluate the treatment planning system itself, but rather to establish that appropriate dose distributions can be achieved with this design under realistic clinical circumstances. Our simulation suggests that dose gradients and conformity for complex target shapes, corresponding to state-of-the-art systems, can be achieved with this construction, although a detailed evaluation of the system itself would be needed in the future.

Prostate specific antigen five years following stereotactic body radiation therapy (SBRT) for low and intermediate risk prostate cancer: An ablative procedure?

123 Background: Our previous work on early PSA kinetics following prostate SBRT showed that an initial rapid and then slow PSA decline may result in very low PSA nadirs. This retrospective study sought to evaluate the PSA nadir 5 years following SBRT for low and intermediate risk prostate cancer (PCa). Methods: 65 low and 80 intermediate risk PCa patients were treated definitively with SBRT at Georgetown University Hospital between January 2008 and October 2011. All patients were treated to 35-37.5 Gy in 5 fractions delivered via the CyberKnife Radiosurgical System. Patients who received androgen deprivation therapy were excluded from this study. Pre- and post-treatment PSA and total testosterone levels were obtained during routine follow up visits. Biochemical relapse was defined as a PSA rise &gt; 2 ng/mL above the nadir and analyzed using the Kaplan Meier method. The PSA nadir was defined as the lowest PSA value prior to biochemical relapse or as the lowest value recorded during follow up. Prostate ablation was defined as a PSA nadir &lt; 0.2 ng/mL. Univariate logistic regression analysis was used to evaluate relevant variables on the likelihood of achieving a PSA nadir &lt; 0.2 ng/mL. Results: The median age at the start of SBRT was 72 years. These patients had a median prostate volume of 36 cc with a median 25% of total cores involved. At a median follow up of 5.8 years, 84% and 37% of patients achieved a PSA nadir ≤ 0.5 ng/mL and &lt; 0.2 ng/mL, respectively. Five low and 8 intermediate risk patients experienced a biochemical relapse; those who did not experience a biochemical relapse, achieved a median PSA nadir of 0.2 ng/mL. There was no difference between the 5-year bRFS rate for low (96.6%) and intermediate risk (97.4%) patients and the median time to PSA nadir was 36 months. Initial PSA (p = 0.024) and a lower testosterone at the time of the PSA nadir (p = 0.049) were found to be significant predictors of achieving a PSA nadir &lt; 0.2 ng/mL. Conclusions: SBRT for low and intermediate risk PCa is a convenient treatment option with low PSA nadirs and a high rate of early bRFS. Less than 40% of patients achieved an ablative PSA nadir. Thus, the role of further dose escalation is an area of active investigation.

Sharpening peripheral dose gradient via beam number enhancement from patient head tilt for stereotactic brain radiosurgery44This work was presented in part at the 2015 ISRS Congress in Yokohama Japan.

Sharp dose fall-off is the hallmark of brain radiosurgery for the purpose of delivering high dose radiation to the target while minimizing peripheral dose to regional normal brain tissue. In this study, a technique was developed to enhance the peripheral dose gradient by magnifying the total number of beams focused toward each isocenter through pre-programmed patient head tilting. This technique was tested in clinical settings on a dedicated brain radiosurgical system (GKPFX, Gamma Knife Perfexion, Elekta Oncology) by comparing dosimetry as well as delivery efficiency for 20 radiosurgical cases previously treated with the system. The 3-fold beam number enhancement (BNE) treatment plans were found to produce nearly identical target volume coverage (absolute value < 0.5%, P > 0.2) and dose conformity (BNE CI = 1.41 ± 0.22 versus 1.41 ± 0.11, P > 0.99) as the original treatment plans. The total beam-on time for the 3-fold BNE treatment plans were also found to be comparable (<0.5 min or 2%) with those of the original treatment plans for all the cases. However, BNE treatment plans significantly improved the mean gradient index (BNE GI = 2.94 ± 0.27 versus original GI = 2.98 ± 0.28 P < 0.0001) and low-level isodose volumes, e.g. 20–50% prescribed isodose volumes, by 1.7%–3.9% (P < 0.03). With further 4–5-fold increase in the total number of beams, the absolute gradient index can decrease by as much as −0.5 in absolute value or −20% for a treatment. In conclusion, BNE via patient head tilt has been demonstrated to be a clinically suitable and efficient technique for physically sharpening the peripheral dose gradient for brain radiosurgery.

SU‐F‐T‐648: Sharpening Dose Fall‐Off Via Beam Number Enhancements For Stereotactic Brain Radiosurgery

Purpose:Sharp dose fall‐off is the hallmark of brain radiosurgery to deliver a high dose of radiation to the target while minimizing dose to normal brain tissue. In this study, we developed a technique for the purpose of enhancing the peripheral dose gradient by magnifying the total number of beams focused toward each isocenter via patient head tilt and simultaneous beam intensity modulations.Methods:Computer scripting for the proposed beam number enhancement (BNE) technique was developed. The technique was tested and then implemented on a clinical treatment planning system for a dedicated brain radiosurgical system (GK Perfexion, Elekta Oncology). To study technical feasibility and dosimetric advantages of the technique, we compared treatment planning quality and delivery efficiency for 20 radiosurgical cases previously treated at our institution. These cases included relatively complex treatments such as acoustic schwannoma, meningioma, brain metastasis and mesial temporal lobe epilepsy.Results:The BNE treatment plans were found to produce nearly identical target volume coverage (absolute value &lt; 0.5%, P &gt; 0.2) and dose conformity (BNE CI= 1.41±0.15 versus 1.41±0.20, P&gt;0.9) as the original treatment plans. The total beam‐on time for theBNE treatment plans were comparable (within 1.0 min or 1.8%) with those of the original treatment plans for all the cases. However, BNE treatment plans significantly improved the mean gradient index (BNE GI = 2.9±0.3 versus original GI =3.0±0.3 p&lt;0.0001) and low‐level isodose volumes, e.g. 20‐50% prescribed isodose volumes, by 2.0% to 5.0% (p&lt;0.02). Furthermore, with 4 to 5‐fold increase in the total number of beams, the GI decreased by as much as 20% or 0.5 in absolute values.Conclusion:BNE via head tilt and simultaneous beam intensity modulation is an effective and efficient technique that physically sharpens the peripheral dose gradient for brain radiosurgery.

Four-dimensional computed tomography prediction of inter- and intrafractional upper gastrointestinal tumor motion during fractionated stereotactic body radiation therapy

PurposeRespiratory-induced tumor motion of upper gastrointestinal (GI) tumors during radiation therapy is often assessed using a single 4-dimensional computed tomography (4D-CT) and presumed to be representative during fractionated treatment regimens. The purpose of this study was to examine the intra- and interfraction correlations of tumor motion between pretreatment 4D-CT and real-time fiducial-based motion tracking in patients treated with fractionated stereotactic body radiation therapy (SBRT) for upper GI malignancies. Methods and materialsFourteen patients with upper GI tumors underwent fractionated SBRT using the CyberKnife radiosurgical system with Synchrony respiratory motion management. Before treatment, each patient underwent a free-breathing 4D-CT scan and fiducial motion was tracked for each phase of the respiratory cycle. Real-time fiducial positions recorded during delivery of each SBRT fraction were extracted from the CyberKnife planning system. Displacements were compared between those predicted by 4D-CT and those recorded by Synchrony during treatment in the left-right (LR), anteroposterior (AP), and superoinferior (SI) directions. ResultsThe 4D-CT scans demonstrated little correlation with real-time mean fiducial displacement as determined by Pearson correlation with coefficients of 0.45, 0.52, and 0.63 in the SI, AP, and LR directions, respectively. Cohort-averaged maximum fiducial displacements based on 4D-CT and real-time tracking were measured to be 3.86 ± 1.40 mm versus 10.73 ± 7.03 mm, 2.29 ± 1.02 mm versus 4.44 ± 3.33 mm, and 1.45 ± 0.49 mm versus 2.67 ± 2.49 mm in the SI, AP, and LR directions, respectively. Mean fiducial displacements were greater than that predicted by the maximum displacements on the corresponding 4D-CT scan in 39%, 22%, and 25% of SBRT fractions in the SI, AP, and LR directions, respectively. ConclusionsComparison of 4D-CT with real-time fiducial tracking demonstrated significant inter- and intrafractional discrepancies, particularly in the SI direction, which could result in compromise of target coverage when planning with a single free-breathing 4D-CT.

Accuracy of Robotic Radiosurgical Liver Treatment Throughout the Respiratory Cycle

To quantify random uncertainties in robotic radiosurgical treatment of liver lesions with real-time respiratory motion management. We conducted a retrospective analysis of 27 liver cancer patients treated with robotic radiosurgery over 118 fractions. The robotic radiosurgical system uses orthogonal x-ray images to determine internal target position and correlates this position with an external surrogate to provide robotic corrections of linear accelerator positioning. Verification and update of this internal-external correlation model was achieved using periodic x-ray images collected throughout treatment. To quantify random uncertainties in targeting, we analyzed logged tracking information and isolated x-ray images collected immediately before beam delivery. For translational correlation errors, we quantified the difference between correlation model-estimated target position and actual position determined by periodic x-ray imaging. To quantify prediction errors, we computed the mean absolute difference between the predicted coordinates and actual modeled position calculated 115 milliseconds later. We estimated overall random uncertainty by quadratically summing correlation, prediction, and end-to-end targeting errors. We also investigated relationships between tracking errors and motion amplitude using linear regression. The 95th percentile absolute correlation errors in each direction were 2.1 mm left-right, 1.8 mm anterior-posterior, 3.3 mm cranio-caudal, and 3.9 mm 3-dimensional radial, whereas 95th percentile absolute radial prediction errors were 0.5 mm. Overall 95th percentile random uncertainty was 4 mm in the radial direction. Prediction errors were strongly correlated with modeled target amplitude (r=0.53-0.66, P<.001), whereas only weak correlations existed for correlation errors. Study results demonstrate that model correlation errors are the primary random source of uncertainty in Cyberknife liver treatment and, unlike prediction errors, are not strongly correlated with target motion amplitude. Aggregate 3-dimensional radial position errors presented here suggest the target will be within 4 mm of the target volume for 95% of the beam delivery.