ACR-ARS Practice Parameter for Radiation Oncology

BACKGROUND

This practice parameter was revised collaboratively by the American College of Radiology (ACR), and the American Radium Society. This practice parameter provides updated reference literature regarding radiation oncology practice and its key personnel.

METHODS

This practice parameter was developed according to the process described under the heading The Process for Developing ACR Practice Parameters and Technical Standards on the ACR website ( https://www.acr.org/Clinical-Resources/Practice-Parameters-and-Technical-Standards ) by the Committee on Practice Parameters-Radiation Oncology of the ACR Commission on Radiation Oncology in collaboration with the American Radium Society.

RESULTS

This practice parameter provides a comprehensive update to the reference literature regarding radiation oncology practice in general. The overall roles of the radiation oncologist, the Qualified Medical Physicist, and other specialized personnel involved in the delivery of external-beam radiation therapy are discussed. The use of radiation therapy requires detailed attention to equipment, patient and personnel safety, equipment maintenance and quality assurance, and continuing staff education. Because the practice of radiation oncology occurs in a variety of clinical environments, the judgment of a qualified radiation oncologist should be used to apply these practice parameters to individual practices. Radiation oncologists should follow the guiding principle of limiting radiation exposure to patients and personnel while accomplishing therapeutic goals.

CONCLUSION

This practice parameter can be used as an effective tool to guide radiation oncology practice by successfully incorporating the close interaction and coordination among radiation oncologists, medical physicists, dosimetrists, nurses, and radiation therapists.

Selected-Lesion Stereotactic Radiosurgery (SL-SRS) as a Novel Strategy in the Treatment of Patients With Multiple Brain Metastases

INTRODUCTION

With the diminishing use of whole-brain radiotherapy (WBRT), there is increasing debate regarding the maximum number of brain metastases that should be treated with stereotactic radiosurgery (SRS). In patients with >10-15 lesions, some groups are proposing a new approach - selected-lesion SRS (SL-SRS) - where only a subset of intracranial lesions are chosen for irradiation. This study is an initial look into this practice.

METHODS

This is a cross-sectional exploratory survey study. A survey of 19 questions was created by the International Radiosurgery Research Foundation (IRRF) using open-ended and multiple-choice style questions on SL-SRS practices and indications with the goal of qualitatively understanding how SL-SRS is being implemented worldwide. The survey was distributed to physicians in the United States (US) and internationally who are members of the IRRF and who perform SRS frequently. Ten out of 50 IRRF institutions provided responses reflecting the practices of 16 physicians.

RESULTS

SL-SRS is being performed at 8/10 institutions. The most common reasons for using SL-SRS included patients with prior WBRT, patients with progressing systemic disease with central nervous system (CNS)-penetrating or immunotherapies available, specific requests from medical oncology, and cooperative studies using this approach. Lesion size was cited as the most important factor when choosing to irradiate any single lesion. The majority of respondents reported 30 mm and 40 mm as size cutoffs (by largest dimension) for treatment of a lesion in eloquent and non-eloquent locations, respectively. Eloquence of lesion location and attributable symptoms were also considered important. Progression of untreated lesions was the most common reason reported for bringing patients back for additional treatment.

CONCLUSION

The responses to this survey show that SL-SRS is being used, allowing for small/asymptomatic brain metastases to be left safely unirradiated. It is currently used in patients who have >10-15 lesions with prior WBRT, those with progression of extracranial disease but with acceptable systemic treatment options, and those with poor functional status. The incorporation of this new approach into clinical trials should be considered for the safe study of the efficacy of new CNS-penetrating systemic therapies.

NIMG-92. DOTATATE PET/MR-GUIDED POSTOPERATIVE RADIOTHERAPY IN SUBTOTALLY RESECTED WHO-2 MENINGIOMA: EVALUATION OF PROGRESSION-FREE SURVIVAL IN A PROSPECTIVE COHORT

BACKGROUND

Current postoperative management recommendations for meningioma lack Level 1 evidence. NRG-0539 (NCT00895622) treated recurrent WHO-1 and newly diagnosed and completely resected (by postoperative MRI) WHO-2 meningiomas with postoperative fractionated radiotherapy to 54 Gy, but 3 year PFS remained below 60%. MRI, the standard of care (SOC) for meningioma radiotherapy planning, lacks sensitivity for postoperative small volume disease and osseous or parenchymal invasion. More sensitive and specific imaging biomarkers are needed to improve RT guidance and thereby clinical outcomes in meningioma. [68Ga]-DOTATATE is a PET radiotracer targeting somatostatin receptor 2 (SSTR2), a highly sensitive and specific meningioma biomarker. We developed a dedicated DOTATATE brain PET/MRI protocol allowing meningioma differentiation from post-treatment change, using SUV analysis and Patlak modeling. Our prospective observational trial (NCT04081701) has imaged over 90 patients with meningioma. This IRB-approved study evaluated PFS in patients with WHO-2 tumors who did not achieve GTR by [68Ga]-DOTATATE PET/MRI, managed with PET/MR guided RT, hypothesizing that the PFS of patients with STR by PET/MRI managed with PET/MR guided RT would be higher than for comparable patients enrolled on NRG 0539.

METHODS

92 patients with SSTR2-positive brain neoplasms were enrolled between 9/2019 and 5/2022 and imaged according to our previously published protocol. 7 patients met inclusion criteria (WHO-2 meningioma; postoperative PET/MR with residual activity) who underwent PET/MR-guided RT, followed with SOC MRI. Kaplan-Meier survival analysis was performed.

RESULTS

5/7 subjects (71%) were women; the mean age was 50.7 years. MRI follow-up data were available for a mean of 18.7 months (range: 16-24 months). All patients remain progression-free at this time; Kaplan-Meier analysis demonstrated 2-year PFS to be 100%, which is substantially higher than reported PFS in this clinical population using standard-of-care MRI-guided RT.

CONCLUSIONS

[68Ga]-DOTATATE PET/MR-guidance can improve PFS following postoperative RT in subtotally resected WHO-2 meningiomas.

NIMG-91. DOTATATE PET/MR-GUIDED POSTOPERATIVE MANAGEMENT IN WHO-2 MENINGIOMA: ACTIVE SURVEILLANCE BASED ON PET/MR EVIDENCE OF GROSS TOTAL RESECTION

BACKGROUND

MRI, the standard of care (SOC) for meningioma radiotherapy planning, lacks sensitivity for postoperative small volume disease and osseous or parenchymal invasion. BN003 (NCT03180268) randomizes patients with WHO-2 meningiomas and MRI-determined GTR to observation or 60 Gy IMRT to the resection bed. More sensitive and specific imaging biomarkers may improve clinical outcomes in meningioma by limiting unnecessary radiation of normal tissues and improving radiation targeting. [68Ga]-DOTATATE is a PET radiotracer targeting somatostatin receptor 2 (SSTR2), a highly sensitive and specific meningioma biomarker. We developed a dedicated DOTATATE brain PET/MRI protocol allowing meningioma differentiation from post-treatment change, using SUV analysis and Patlak modeling. Our prospective observational trial (NCT04081701) has imaged over 90 patients with meningioma. This IRB-approved study evaluated PFS in patients with WHO-2 tumors and postoperative GTR by [68Ga]-DOTATATE PET/MRI who were managed solely with active surveillance. We hypothesized that the PFS of patients with GTR by PET/MRI managed with active surveillance would be higher than for patients with MRI-determined GTR, using NRG-BN003’s observation arm (randomized trial comparing observation to fractionated radiotherapy) as a reference standard.

METHODS

92 patients with SSTR2-positive brain neoplasms were enrolled between 9/2019 and 5/2022 and imaged according to our previously published protocol. 8 patients met inclusion criteria (WHO-2 meningioma & postoperative PET/MR GTR) and were followed with SOC MRI. Kaplan-Meier survival analysis was performed.

RESULTS

5/8 subjects (62.5%) were women; the mean age was 67 years. MRI follow-up data were available for a mean of 19.75 months (range: 7-38 months). One subject (12.5%) progressed at 21 months; the remainder remain progression-free. Kaplan-Meier analysis demonstrated 3-year PFS to be 80%, which is substantially higher than the reported 3-year PFS of 69% in the literature.

CONCLUSIONS

[68Ga]-DOTATATE PET can improve the specificity of imaging-based assessment of the extent of resection of WHO-2 meningiomas.

CTNI-40. PHASE I TRIAL OF SULFASALAZINE COMBINED WITH STEREOTACTIC RADIOSURGERY FOR RECURRENT GLIOBLASTOMA: STUDY PROTOCOL FOR NCT04205357

Glioblastoma (GBM) is one of the most aggressive, radioresistant types of cancer with a dismal prognosis. Sulfasalazine (SAS) has shown tumor selective radiosensitizing properties in preclinical studies. The antioxidant glutathione (GSH) produced at high levels in glioma cells normally protects against radiation injury by scavenging reactive oxygen species produced during radiation therapy (RT). SAS blocks cysteine uptake through the xCT-channel, a rate-limiting step for GSH production. We have previously shown slowing of tumor growth and prolonged survival when SAS was combined with stereotactic radiosurgery (SRS) in vivo compared to either treatment alone. Our hypothesis is that SAS potentiates the efficacy of SRS for recurrent GBM with a low risk of adverse events (AE). The primary end-point is to establish the recommended dose for efficacy testing in phase II/III trials. This phase 1 dose-escalation trial utilizes a standard 3 + 3 design with 3-6 patients per cohort. Patients will be treated with oral SAS (1.5, 3.0, 4.5 or 6.0 g/day) 3 days prior to single session SRS. The SAS-dose will be escalated depending on the absence/presence of toxicity in the previous cohort of treated patients. If more than 1 of 3-6 patients ( ≥ 33 %) is experiencing grade 3 or higher toxicity levels, the study will be terminated. The dose below will be the recommended dose. Toxicity is graded using the Common Toxicity Criteria for Adverse Events (CTCAE) v5.0 recorded the first 30 days. Secondary end-points are assessments of 1) intratumoral GSH production (GSH-spectroscopy), 2) late AE utilizing 11C-MET-MRI-PET 3) changes in KPS/quality of life (FACT-Br), 4) need for steroidal treatment, 5) progression free and overall survival. Novel treatment modalities are urgently needed. This trial will establish the recommended dose for SAS repurposed as a radiosensitizer for a future phase 2/3 trial and may ultimately lead to improvement of current GBM treatment.

A multidisciplinary management algorithm for brain metastases

The incidence of brain metastases continues to present a management issue despite the advent of improved systemic control and overall survival. While the management of oligometastatic disease (ie, 1-4 brain metastases) with surgery and radiation has become fairly straightforward in the era of radiosurgery, the management of patients with multiple metastatic brain lesions can be challenging. Here we review the available evidence and provide a multidisciplinary management algorithm for brain metastases that incorporates the latest advances in surgery, radiation therapy, and systemic therapy while taking into account the latest in precision medicine-guided therapies. In particular, we argue that whole-brain radiation therapy can likely be omitted in most patients as up-front therapy.

RTID-01. PHASE III MULTICENTER RCT OF POST-SURGICAL STEREOTACTIC RADIOTHERAPY VERSUS SURGICALLY TARGETED RADIATION THERAPY FOR THE TREATMENT OF LARGE NEWLY DIAGNOSED BRAIN METASTASES – TRIAL IN PROGRESS

BACKGROUND

Resection (R) followed by single- or multi-fraction stereotactic radiosurgery (SRT) of brain metastases (BMs) lowers resection bed recurrence compared to R alone. Nevertheless, for larger BMs, 12-month recurrence rates after R+SRT can exceed 20–30%. Aiming to improve outcomes, a permanently implanted collagen tile brachytherapy device (GammaTile, GT Medical Technologies, Tempe, AZ) utilizing Cs-131 seeds embedded within a bioresorbable collagen tile was developed and is described as Surgically Targeted Radiation Therapy (STaRT) to distinguish it from external beam radiotherapy. STaRT allows rapid, intense localized radiation dose delivery directly to the tumor bed with predictable dosimetry immediately at the time of R, which may confer reduced risk for radiation necrosis compared to other therapies. It is hypothesized that R+ STaRT will increase surgical bed recurrence-free survival (SB-RFS), while reducing impact on functional and neurocognitive status compared to R+SRT.

METHODS

Multicenter, randomized, comparison trial of patients with resectable, previously untreated “index” BMs (≥ 2.5–5cm), and 0–3 other tumors, will be preoperatively randomized 1:1 to undergo R+SRT or R+STaRT to the index lesion; unresected tumors in both groups will receive SRT. Planned sample size is 180 from 14 sites. Enrollment opened 03/31/2021. First subject was enrolled 04/07/2021. Primary endpoint is SB-RFS. Secondary endpoints include overall survival, quality of life, neurocognition, functional status, imaging findings and adverse events. Follow-up will be through 24 months. This will be the first randomized trial comparing R+SRT versus R+STaRT delivered by Cs-131 sources in permanently implanted resorbable collagen tile carriers. Primary and secondary outcome measures captured will elucidate the potential risks and benefits of these two RT delivery methods in the setting of newly diagnosed BMs. We will present trial accrual progress, available data, experience and lessons learned.

NCOG-10. RADIATION THERAPY CAN BE SAFELY DEFERRED IN OLIGODENDROGLIOMAS

Deferring multimodal aggressive therapies in young patients to delay treatment-induced toxicity without jeopardizing long-term outcome would be of great benefit to our patients. After IRB approval, we retrospectively reviewed 80 confirmed IDH mutant and 1p19q codeleted oligodendrogliomas treated at one institution between 2005 and 2020. Median follow-up was 5 (range 1-26) years. All patients underwent maximal safe resection, followed by observation with routine imaging (n=28), chemotherapy alone (n=27), or radiation with chemotherapy (n=25) as initial upfront therapy. Median progression free survival was 36 (range 1-203), 54 (range 1-306), and 57 (range 4-281) months, respectively. Median overall survival was not reached, with 85% (67/80) alive, 8 on treatment and 59 stable off therapy. Among 35 patients who died or were followed for 10 years, median PFS was 12, 15, and 10 years for observation (n=9), chemotherapy (n=11) and chemoradiotherapy (n=15), with deaths or KPS below 50 in 3, 6, and 8, respectively. Three deaths in the observation group occurred 12, 12 and 17 years after diagnosis; one at 95 years old and another tumor-unrelated. Among 44 patients eventually receiving radiation, 15 suffered toxicity, including pathologically proven necrosis (n=6), cognitive decline with KPS< 50 (n=5), memory loss with KPS > 50 (n=3), and optic neuropathy (n=1). Myelosuppression from PCV was more pronounced after chemoradiation than in the upfront setting. Temozolomide after PCV chemotherapy was well tolerated. Long term follow-up of oligodendroglioma patients is challenging, but essential in determining late toxicities and treatment efficacy. Long-term results of European and North American multicenter cooperative group trials contradicted earlier publications reporting no benefit from early chemotherapy. Some management practice guidelines established a half-century ago persist (i.e., administer radiation therapy early), despite potentially crippling late effects. Deferring upfront radiation therapy is safe, less toxic, and equally efficacious in codeleted oligodendrogliomas.

NEIM-05. [GA68]DOTATATE PET/MRI-BASED RADIOSURGICAL RESPONSE ASESSMENT IN MENINGIOMA

PURPOSE

Postoperative PET/MRI with [68Ga]-DOTATATE can differentiate residual meningioma from postsurgical change, aid in target delineation, and portend a more favorable dosimetry with decreased PTV and organ-at-risk dose. Our purpose was to demonstrate utility of DOTATATE PET/MR for radiosurgical treatment (RT) response assessment in meningiomas.

METHODS

Patients underwent postoperative radiation treatment planning using DOTATATE PET/MRI as part of our IRB-approved prospective trial. Both DOTATATE PET and gadolinium-enhanced T1 weighted MR imaging were incorporated in RT-planning. All patients underwent follow-up DOTATATE PET/MRI at 6-12 months following completion of radiosurgery. Maximum absolute standardized uptake value (SUV) and SUV ratio (SUVR) of lesion/ superior sagittal sinus SUV were obtained. RANO criteria were applied to determine significance of change in size. Statistical analyses were performed using paired t-tests.

RESULTS

13 patients (15% WHO-I, 54% WHO-II, 23% WHO-III, 8% WHO grade unknown) were followed postoperatively with pre- and post-RT DOTATATE PET/MRI. 29 meningiomas were treated. 46% (6/13) of subjects received SBRT and 54% (7/13) received SRS. Post-RT DOTATATE PET/MRI demonstrated a 46.4% SUV decrease (p-value = 0.0001) and a 60.8% SUVR decrease (p-value < 0.0001). Of 21 measurable lesions, the size product decreased by 21%; while this decrease was statistically significant (p-value = 0.0008), it was below the 25% decrease defined as clinically significant by RANO guidelines. To date, all patients remain stable radiographically without evidence of recurrence (mean follow-up post RT: 14 months; range: 6-24 months).

CONCLUSIONS

DOTATATE PET SUV and SUVR demonstrated marked, significant decrease post radiosurgery. Lesion size decrease was statistically significant but not clinically significant by RANO criteria. DOTATATE PET/MR thus represents a promising approach to aid in response assessment for radiosurgically treated meningiomas. Longer-term follow-up is needed to determine the correlation between the degree of post-RT SUV and/or SUVR decrease and progression-free-survival.

Biologically effective dose correlates with linear tumor volume changes after upfront single-fraction stereotactic radiosurgery for vestibular schwannomas

Vestibular schwannomas (VSs) are benign, slow-growing tumors. Management options include observation, surgery, and radiation. In this retrospective trial, we aimed at evaluating whether biologically effective dose (BED) plays a role in tumor volume changes after single-fraction first intention stereotactic radiosurgery (SRS) for VS. We compiled a single-institution experience (n = 159, Lausanne University Hospital, Switzerland). The indication for SRS was decided after multidisciplinary discussion. Only cases with minimum 3 years follow-up were included. The Koos grading, a reliable method for tumor classification was used. Radiosurgery was performed using Gamma Knife (GK) and a uniform marginal prescription dose of 12 Gy. Mean BED was 66.3 Gy (standard deviation 3.8, range 54.1-73.9). The mean follow-up period was 5.1 years (standard deviation 1.7, range 3-9.2). The primary outcome was changes in 3D volumes after SRS as function of BED and of integral dose received by the VS. Random-effect linear regression model showed that tumor volume significantly and linearly decreased over time with higher BED (p < 0.0001). Changes in tumor volume were also significantly associated with age, sex, number of isocenters, gradient index, and Koos grade. However, the effect of BED on tumor volume change was moderated by time after SRS and Koos grade. Lower integral doses received by the VSs were inversely correlated with BED in relationship with tumor volume changes (p < 0.0001). Six (3.4%) patients needed further intervention. For patients having uniformly received the same marginal dose prescription, higher BED linearly and significantly correlated with tumor volume changes after SRS for VSs. BED could represent a potential new treatment paradigm for patients with benign tumors, such as VSs, for attaining a desired radiobiological effect. This could further increase the efficacy and decrease the toxicity of SRS not only in benign tumors but also in other SRS indications.

NIMG-15. EVALUATING FLUCTUATING ENHANCEMENT IN OLIGODENDROGLIOMAS ON MAGNETIC RESONANCE IMAGING

OBJECTIVE

To identify and characterize patterns of fluctuating contrast enhancement on magnetic resonance imaging (MRI) in patients with oligodendrogliomas.

INTRODUCTION

Gliomas, particularly oligodendrogliomas, can exhibit fluctuating enhancement (FE) on MRI that can make it difficult to differentiate between treatment effect and active tumor.

METHODS

We are conducting a single-center retrospective review of clinical and radiographic data for patients with oligodendrogliomas treated at Weill Cornell Medicine (WCM) from 2/2000-5/2018. We have identified patients with FE on MRI and tracked lesions &gt; 5mm in at least one dimension to the resolution of the lesion or last available MRI. We have recorded time from initial diagnosis to development of FE, time from radiation to development of FE, and time from development to resolution of FE as well as molecular characteristics of each tumor.

RESULTS

A total of 122 patients with oligodendrogliomas were identified. Thus far, fluctuating enhancement has been identified in 11 patients (5 men, 6 women) with 38 total fluctuating lesions. Isocitrate dehydrogenase-1 (IDH-1) mutation was present in 5 tumors, and 1p/19q co-deletion was present in 6. Mean time from initial diagnosis to development of FE was 44.6 months. In patients who developed FE after radiation, mean time from radiation to development of FE was 35.0 months. Twenty-seven lesions resolved, and mean time from onset to resolution of FE was 5.6 months, while mean time from start of radiation to resolution of FE was 41.0 months. Additionally, we will perform perfusion analysis on lesions &gt; 5mm and identify patients who underwent surgical biopsy of FE with pathologic diagnosis.

CONCLUSIONS

FE has been identified in 11 patients thus far. We are expanding our analysis to identify a larger cohort of patients with FE. Characterizing patterns of FE may aid clinicians in differentiating FE due to treatment effect from active tumor.

RADT-16. HYPOFRACTIONATED RADIOTHERAPY AND STEREOTACTIC RADIOSURGERY FOR VESTIBULAR SCHWANNOMAS: A SYSTEMATIC REVIEW AND META-ANALYSIS

BACKGROUND

Vestibular schwannomas (VS) are tumors of the cerebellopontine angle with significant morbidity, causing hearing loss, tinnitus, and trigeminal and facial nerve compromise. Surgical resection is complex and often results in comorbidities, with radiation therapy being used to achieve tumor control. Both single dose stereotactic radiosurgery (SRS) and, in recent years, hypofractionated stereotactic radiotherapy (hSRT), where a total 18-25 Gy dose is given in approximately 3-5 fractions, have been utilized. It remains unclear, however, how each of these approaches fares compared to the other.

METHODS

Ovid MEDLINE, EMBASE, CINAHL, and Cochrane Reviews were searched for studies either comparing hSRT with SRS or focusing on hSRT alone for the treatment of VS. Primary endpoints included tumor control, serviceable hearing, presence of tinnitus, and facial and trigeminal nerve symptoms. A random effects analysis was employed to compare pre- and post-treatment effects (hSRT alone) or SRS and hSRT outcomes (two-arm studies).

RESULTS

This analysis included 21 studies focusing on hSRT alone and 13 studies comparing SRS and hSRT. Significant heterogeneity was observed. Overall, when hSRT was analyzed alone, crude tumor control was achieved in 93% of 1571 patients at average follow-up of 49.9 months. There was no difference intra-group between pre- and post-treatment odds ratios (OR) of tinnitus, facial, or trigeminal impairment. However, serviceable hearing was diminished following hSRT (OR = 0.60, 95% CI: 0.44, 0.83, p = 0.002). Comparison with SRS showed no difference with respect to tumor control (p = 0.84), serviceable hearing (p = 0.65), trigeminal impairment (p = 0.96), or facial nerve impairment (p = 0.32).

CONCLUSIONS

hSRT achieved excellent tumor control and, with the exception of serviceable hearing, did not result in worse cranial nerve symptomatology. Analyzing two-arm studies showed that hSRT is a treatment modality comparable to SRS with respect to tumor control and cranial nerve comorbidities.

NCOG-12. STEREOTACTIC RADIOSURGERY FOR VESTIBULAR SCHWANNOMAS IN NEUROFIBROMATOSIS TYPE 2 PATIENTS: A SYSTEMATIC REVIEW AND META-ANALYSIS

BACKGROUND

One of the hallmarks of neurofibromatosis type 2 (NF2) is the presence of bilateral vestibular schwannomas (VSs) that result in progressive hearing loss and compression of nearby brainstem structures causing cranial nerve palsies. Treatment of these tumors remains challenging, as both surgical removal and expectant management can result in symptom progression. Stereotactic radiosurgery (SRS) has recently been investigated for the management of NF2-associated VSs; however, the role, promises, and pitfalls of this treatment modality remain unclear.

METHODS

Ovid MEDLINE, EMBASE, Web of Science, and Cochrane Reviews were searched for studies assessing SRS outcome in NF2-associated VSs only. Primary endpoints included tumor control, serviceable hearing, presence of tinnitus, and cranial nerve V and VII symptoms.

RESULTS

A total of 16 studies (589 patients harboring 750 tumors) was included in this analysis. Clinical tumor control was achieved in 88% of cases (95% CI: 80%-95%); salvage surgery was needed in 8% (95% CI: 4%-13%) of cases. Treatment resulted in a worsening of pre-treatment serviceable hearing (OR of 0.26, p &lt; 0.01), in an increase of facial nerve impairment (OR of 1.62, p &lt; 0.01), and showed a trend for increase trigeminal nerve impairment as well (OR of 1.42, p = 0.07). The incidence of vestibular symptoms and hydrocephalus requiring shunting could not be assessed since these measures were not reported consistently.

CONCLUSIONS

The treatment of NF2-associated VS continues to pose a challenge, as current SRS regimens result in impaired hearing and worse cranial nerve comorbidities, despite achieving high tumor control. It remains unclear if these findings have to be intended as treatment complications or, rather, continuous disease progression despite tumor control.

45. DELAY OR FAILURE TO ADMINISTER STEREOTACTIC RADIOSURGERY TO THE CAVITY AFTER SURGERY FOR BRAIN METASTASES. AN INTENTION-TO-TREAT ANALYSIS

BACKGROUND

Data regarding the efficacy of adjuvant stereotactic radiosurgery (SRS) for resected brain metastases (BM) is often limited to patients completing SRS within a specified timeframe. We performed an intention-to-treat analysis to determine local recurrence (LR) for all BM patients referred for SRS.

METHODS

We retrospectively identified resected BM patients referred for SRS between 2012 and 2018. Patients were divided based on delay to SRS into four categories: 1) ≤4 weeks, 2) >4–8 weeks, 3) >8 weeks, and 4) never received. We investigated the relationship between delay to SRS and LR, local recurrence-free survival (LRFS), and overall survival, as well as the predictors of and reason for delays.

RESULTS

In our cohort of 159 patients, median age was 64.0 years, 56.5% patients were female, median tumor diameter was 2.9 cm, and gross total resection was achieved in 83.0%. On intention-to-treat analysis, LR was 22.6%. Delays to SRS correlated with LR: 2.3% with SRS ≤4 weeks postoperatively, 14.5% with SRS at >4–8 weeks (p=0.03), 48.5% with SRS at >8 weeks (p<0.001). No LR difference was observed with SRS delayed by >8 weeks, vs. never completed, 48.5% vs. 50.0% (p=0.91). 53 (33.3%) patients comprised these latter two categories. A similar relationship emerged between delay to SRS and LRFS (p<0.01). Non-small cell lung cancer pathology (p=0.04) and earlier year of treatment (p<0.01) were predictive of delays. Common reasons for delays included logistics, management of systemic disease, complications, or comorbidities.

CONCLUSION

A significant number of patients referred for SRS never receive it, or are treated with a delay >8 weeks, conferring equivalent LR risk. Accordingly, the actual efficacy of adjuvant SRS may need reassessment. Reasons for delays and mechanisms for reducing them are discussed. For patients likely to experience significant delays, other techniques, such as preoperative SRS or intraoperative brachytherapy, may be considered.

43. DELAYS IN ADJUVANT STEREOTACTIC RADIOSURGERY REDUCE LOCAL CONTROL FOR RESECTED BRAIN METASTASES

OBJECTIVE

For resected brain metastases (BM), stereotactic radiosurgery (SRS) is often offered to minimize local recurrence (LR). Although the aim is to deliver SRS within a few weeks of surgery, a variety of socioeconomic, medical, and procedural issues can cause delays. We evaluated the relationship between timing of postoperative SRS and LR.

METHODS

We retrospectively identified a consecutive series of BM patients managed with resection and adjuvant SRS, recommended within two weeks of surgery, at our institution from 2012–2018. We assessed the correlation between time to SRS, as well as other demographic, disease, and treatment variables, and LR, distant recurrence (DR), and overall survival (OS).

RESULTS

133 patients met inclusion criteria. Median age was 64.5 years. Approximately half of patients had a single BM, and median BM size was 2.9 cm. Gross total resection was achieved in 111 (83.6%) patients, and >90% received fractionated SRS. Median time to adjuvant SRS was 37.0 days and LR rate was 16.4%. The factor most predictive of LR was time from surgery to SRS. Median time from surgery to SRS was 34.0 days for patients without LR, versus 61.0 days for those with LR (p<0.01). LR was 2.3% with SRS administered ≤4 weeks postoperatively, compared to 23.6% if delayed >4 weeks (p<0.01). Local recurrence-free survival (LRFS) was also improved for patients who had SRS at ≤4 weeks (p=0.02). Delayed SRS was also predictive of DR (p=0.02), but not OS.

CONCLUSIONS

We demonstrate that the strongest predictor of failure of postoperative SRS for BM is the delay to SRS. A cut-off of 4 weeks is a reliable predictor of increased LR. Every effort should be made to perform SRS within 4 weeks of surgery, and if this cannot be achieved, other RT modalities, such as brachytherapy or preoperative SRS, should be strongly considered.

The Prognostic Value of MRI Subventricular Zone Involvement and Tumor Genetics in Lower Grade Gliomas

BACKGROUND AND PURPOSE

Glioblastomas (GBMs) that involve the subventricular zone (SVZ) have a poor prognosis, possibly due to recruitment of neural stem cells. The purpose of this study was to evaluate whether SVZ involvement by lower grade gliomas (LGG), WHO grade II and III, similarly predicts poorer outcomes. We further assessed whether tumor genetics and cellularity are associated with SVZ involvement and outcomes.

METHODS

Forty-five consecutive LGG patients with preoperative imaging and next generation sequencing were included in this study. Regional SVZ involvement and whole tumor apparent diffusion coefficient (ADC) values, as a measure of cellularity, were assessed on magnetic resonance imaging. Progression was determined by RANO criteria. Kaplan-Meier curves and Cox regression analyses were used to determine the hazard ratios (HR) for progression and survival.

RESULTS

Frontal, parietal, temporal, and overall SVZ involvement and ADC values were not associated with progression or survival (P ≥ .05). However, occipital SVZ involvement, seen in two patients, was associated with a higher risk of tumor progression (HR = 6.6, P = .016) and death (HR = 31.5, P = .015), CDKN2A/B mutations (P = .03), and lower ADC histogram values at the 5th (P = .026) and 10th percentiles (P = .046). Isocitrate dehydrogenase, phosphatase and tensin homolog, epidermal growth factor receptor, and cyclin-dependent kinase 4 mutations were also prognostic (P ≤ .05).

CONCLUSIONS

Unlike in GBM, overall SVZ involvement was not found to strongly predict poor prognosis in LGGs. However, occipital SVZ involvement, though uncommon, was prognostic and found to be associated with CDKN2A/B mutations and tumor hypercellularity. Further investigation into these molecular mechanisms underlying occipital SVZ involvement in larger cohorts is warranted.

A simple score to estimate the likelihood of pseudoprogression vs. recurrence following stereotactic radiosurgery for brain metastases: The Bergen Criteria

Background

A major challenge in the follow-up of patients treated with stereotactic radiosurgery (SRS) for brain metastases (BM) is to distinguish pseudoprogression (PP) from tumor recurrence (TR). The aim of the study was to develop a clinical risk assessment score.

Methods

Follow-up images of 87 of 97 consecutive patients treated with SRS for 348 BM were analyzed. Of these, 100 (28.7%) BM in 48 (53.9%) patients responded with either TR (n = 53, 15%) or PP (n = 47, 14%). Differences between the 2 groups were analyzed and used to develop a risk assessment score (the Bergen Criteria).

Results

Factors associated with a higher incidence of PP vs. TR were as follows: prior radiation with whole brain radiotherapy or SRS (P = .001), target cover ratio ≥98% (P = .048), BM volume ≤2 cm³ (P = .054), and primary lung cancer vs. other cancer types (P = .084). Based on the presence (0) or absence (1) of these 5 characteristics, the Bergen Criteria was established. A total score <2 points was associated with 100% PP, 2 points with 57% PP and 43% TR, 3 points with 57% TR and 43% PP, whereas >3 points were associated with 84% TR and 16% PP, P < .001.

Conclusion

Based on 5 characteristics at the time of SRS the Bergen Criteria could robustly differentiate between PP vs. TR following SRS. The score is user-friendly and provides a useful tool to guide the decision making whether to retreat or observe at appropriate follow-up intervals.

Clinical Considerations in Neurosurgical Radiosurgery in the Time of COVID-19

The COVID-19 pandemic is affecting all aspects of the healthcare ecosystem, including administration of stereotactic radiosurgery (SRS). The clinical and logistical challenges created by the COVID-19 public health crisis are clear, but the solutions to these issues are less readily apparent. The goal of this work is to use our experience at a large, academic medical center as a lens for interpreting the many looming issues specific to radiosurgery and its role in the treatment of brain and spine disorders. While the full impact of the pandemic remains to be seen, the aim of this paper is to provide a structural framework to optimize delivery of neurosurgically oriented radiosurgery with proposed clinical workflow strategies. Innovative solutions to the current pandemic crisis affecting the healthcare ecosystem will be driven by increased interdisciplinary and global dialogue.

MNGI-10. ATYPICAL MENINGIOMA: EARLY OUTCOMES WITH OR WITHOUT POSTOPERATIVE RADIATION

BACKGROUND

Adjuvant radiotherapy (RT) in atypical meningioma, especially for gross-totally resected tumors, remains controversial.

METHODS

We retrospectively identified histologically-confirmed cases of WHO Grade II atypical meningioma at a large academic institution from 2004–2018. Clinicodemographic, surgical, radiation therapy (RT), and histopathologic data were collected, as well as imaging and clinical outcomes, with a median follow-up time of 26 months (IQR 32). Patients were stratified by resection status and whether or not upfront RT was administered. Additionally, subanalyses were performed to compare external beam RT (EBRT) and stereotactic radiosurgery (SRS). Progression was defined by radiology report.

RESULTS

Of 122 patients, 45 were excluded for lacking adequate records of previous treatment, less than 3 months follow-up, or lacking MR imaging. Of 77 patients analyzed, 57% (44/77) were female; median 59-years-old. 48% (24/50) of gross-total-resections (GTR) received upfront RT – only a single case progressed, at 39 months. Of 26 GTR patients without upfront RT, 8/26 (31%) progressed at median 19.5 months – of these, 2 were lost to follow-up, 5 received salvage RT, and 1 had surgery alone. Adjuvant RT was associated with superior progression free survival (PFS) in GTR (Cox proportional hazard ratio 0.15, likelihood-ratio p=0.025; median PFS not reached). Of 15 subtotal resections (STR) receiving upfront RT, 11 received EBRT and 4 received SRS – 6 progressed (median 37 months), all after EBRT. Upfront SRS demonstrated superior PFS over EBRT following STR (p=0.036). Across the cohort there was one confirmed death, a GTR patient (without RT) who suffered an ischemic stroke at 11 months.

CONCLUSION

This large single-center retrospective analysis indicates adjuvant RT improves PFS in GTR atypical meningiomas, in concordance with prior studies. It is limited by short median follow-up, possibly related to long-term stability in treated patients. In STR tumors, SRS may contribute to improved PFS compared to EBRT.

RTHP-40. EFFECT OF RIND-BASED DOSIMETRIC TECHNIQUES FOR SCALP DOSE REDUCTION IN BRAIN IRRADIATION

OBJECTIVE

Radiation-induced alopecia is an unwanted side effect causing permanent cosmetic distress if hair regrowth does not occur. Rind-based techniques can effectively control dosimetric spread. We evaluated this technique to reduce scalp dose and alopecia while maintaining tumor coverage.

METHODS

Ten consecutive brain tumor plans were retrospectively evaluated. All planning tumor volume (PTV) margins were ≤ 15.0mm from the skin surface. Departmental guidelines for fractionation were followed, with minimum 95% PTV coverage receiving 100% dose. Fractionation variation was accounted for with biologically effective dose calculation (alpha/beta=2). Rind structures encompassed 5mm depth from scalp surface; upper dose limits were customized to minimum values without PTV coverage compromise. Standard comparative plans using identical criteria, without rind structures, were calculated. Scalp dose evaluation was defined for tissue from skin to 5mm depth. Paired T-tests were used for comparative evaluation.

RESULTS

Median age: 58 (range 27–85); 70% female (n=7). Histologies included gliomas (n=7) and meningiomas (n=3). Median PTV distance to skin surface: 13.5mm (range 8.0–15.0). Median PTV minimum and mean dose for rind-based plans: 88.63% (range 73.14–95.2) and 104.39% (range 102.07–107.38) of prescription and 90.90% (range 68.64–98.21) and 103.02% (range 101.91–107.04) for standard plans, respectively. Statistically significant reduction in scalp maximum and mean dose of 19.65% (p=2.72E-06) and 0.48% (p=0.007), respectively, was seen with rind-based planning. Scalp volume receiving 1000cGy-equivalent increased 6.7cc using rind-based techniques, although insignificant (p=0.33). Volume receiving 1500cGy-equivalent was significantly reduced 3.88cc (p=0.03) using rind-based techniques. With median 28.5 day follow-up, of 5 patients treated using rind-based techniques, 40% (n=2) exhibited acute alopecia compared to 100% of those treated with standard plans.

CONCLUSION

Rind-based dosimetric techniques exhibit significant reduction of scalp dose in brain irradiation. 60% of patients treated using this technique experienced no alopecia, versus 0% receiving standard treatment. Further investigation is warranted to better evaluate correlation.

RADI-39. EFFECT OF RIND-BASED DOSIMETRIC TECHNIQUES FOR SCALP DOSE REDUCTION IN BRAIN IRRADIATION

OBJECTIVE: Radiation-induced alopecia is an unwanted side effect causing permanent cosmetic distress if hair regrowth does not occur. Rind-based techniques can effectively control dosimetric spread. We evaluated this technique to reduce scalp dose and alopecia while maintaining tumor coverage. METHODS: Ten consecutive brain tumor plans were retrospectively evaluated. All planning tumor volume (PTV) margins were ≤ 15.0mm from the skin surface. Departmental guidelines for fractionation were followed, with minimum 95% PTV coverage receiving 100% dose. Fractionation variation was accounted for with biologically effective dose calculation (alpha/beta=2). Rind structures encompassed 5mm depth from scalp surface; upper dose limits were customized to minimum values without PTV coverage compromise. Standard comparative plans using identical criteria, without rind structures, were calculated. Scalp dose evaluation was defined for tissue from skin to 5mm depth. Paired T-tests were used for comparative evaluation. RESULTS: Median age: 58 (range 27–85); 70% female (n=7). Histologies included gliomas (n=7) and meningiomas (n=3). Median PTV distance to skin surface: 13.5mm (range 8.0–15.0). Median PTV minimum and mean dose for rind-based plans: 88.63% (range 73.14–95.2) and 104.39% (range 102.07–107.38) of prescription and 90.90% (range 68.64–98.21) and 103.02% (range 101.91–107.04) for standard plans, respectively. Statistically significant reduction in scalp maximum and mean dose of 19.65% (p=2.72E^-06) and 0.48% (p=0.007), respectively, was seen with rind-based planning. Scalp volume receiving 1000cGy-equivalent increased 6.7cc using rind-based techniques, although insignificant (p=0.33). Volume receiving 1500cGy-equivalent was significantly reduced 3.88cc (p=0.03) using rind-based techniques. With median 28.5 day follow-up, of 5 patients treated using rind-based techniques, 40% (n=2) exhibited acute alopecia compared to 100% of those treated with standard plans. CONCLUSION: Rind-based dosimetric techniques exhibit significant reduction of scalp dose in brain irradiation. 60% of patients treated using this technique experienced no alopecia, versus 0% receiving standard treatment. Further investigation is warranted to better evaluate correlation.

RADI-26. DOSIMETRIC EVALUATION OF 6 MV VERSUS 10 MV PHOTONS FOR HIPPOCAMPAL AVOIDANCE WHOLE BRAIN RADIOTHERAPY

OBJECTIVE: Whole brain radiotherapy (WBRT) causes neurocognitive decline. Hippocampal avoidance WBRT (HA-WBRT) reduces hippocampal irradiation, potentially mitigating neurocognitive sequelae. We compared hippocampal and brain dosimetry with HA-WBRT with 6 megavoltage (MV) versus 10 MV photon energies. METHODS: Twenty consecutive patients treated with WBRT were retrospectively replanned with HA-WBRT techniques using 6 MV and 10 MV photons. Coplanar volumetric modulated arc therapy was employed, with a prescription dose of 3000 cGy in 10 fractions. Planning was done with Eclipse version 13.6 or 15.6. Nine patients were planned with 2.5 mm multileaf collimator leaves, with the remainder planned with 5 mm leaves. The hippocampi were contoured and a HA structure was generated using a uniform 5 mm expansion. A planning target volume (PTV) was defined as the brain parenchyma minus the HA structure. NRG-CC001 dose constraints were used. For each variable, descriptive statistics were calculated. Comparisons were made using two-tailed Wilcoxon signed rank tests or paired t-tests. RESULTS: The minimum hippocampal dose (D100%) was improved with 6 MV plans, 841 cGy compared to 914 cGy with 10 MV (p< 0.005). The maximum hippocampal dose (D0.03cc) was reduced with 6 MV planning, 1614 cGy versus 1676 cGy for 10 MV (p< 0.0001). With 6 MV photons, a greater number of plans met NRG-CC001 constraints without deviations. 6 MV photons improved PTV coverage by the 95% isodose line, 96.6% compared to 95.9% for 10 MV (p=0.021). 6 MV photon plans decreased the volume of PTV receiving ≥105% of the prescription, 84.2% versus 87.9% for 10 MV (p=0.006). The mean dose, hot spots, and cold spots did not differ by photon energy. PTV dose constraints were always met. CONCLUSION: 6 MV photon HA-WBRT plans are dosimetrically superior to 10 MV, reducing hippocampal radiation dose, without compromise in brain coverage and improved target dose homogeneity.

RADI-15. CLUSTERING AND GROUPING OF BRAIN METS IN RADIOSURGERY TREATMENTS

INTRODUCTION: Radiosurgical treatment of numerous lesions in the brain with ‘single-isocenter’ radiosurgery on a linac often requires using multiple isocenters. With our TPS (Elements, Brainlab) multiple plans need to be generated for each set of lesions, and a sum plan calculated. We investigated how to distribute multiple lesions into two groups for two isocenters to achieve a good summed dose distribution. METHODS: The DICOM RS file is exported and the PTV data is extracted by a MATLAB program that calculates the convex hulls, estimated radii, and the centers of mass for each PTV. Two approaches were tried: (1) Lesions close to each other (closer than a certain limit) are put in different groups and (2) Create clusters by kMeans clustering, which allows close lesions but the groups are distant from each other. MATLAB programs were written for all approaches. Treatment plans were generated for three patients (20, 13, 15 lesions) using each method and compared with the actual treatment plan used to treat the patient based on the intuitive grouping of lesions by the planners. Dose maximums outside the lesions, and volumes in the normal tissue exceeding 75, 50 and 25% of the prescription dose were evaluated. RESULTS AND DISCUSSION: The coverage of all lesions for all plans were 95% of the prescription dose. The first approach allowed lowering the maximum dose between lesions, but with summing dose distributions this advantage disappeared. The maximum dose and the 75, 50 and 25% dose volumes were also all worse than in plans generated by experienced planners and higher normal brain doses are delivered if closely spaced lesions are separated into different isocenters for treatment. However, the clustering approach resulted in the same or better values of these same parameters, i.e. improved dose distributions over the dosimetrist’s intuitively chosen separation.

Stereotactic radiosurgery of the brain: a review of common indications

Over the past half-century since Lars Leksell first utilized radiation to address deep and difficult to treat lesions of the central nervous system (CNS), intracranial stereotactic radiosurgery (SRS) has become an increasingly valued tool in the hands of neurosurgeons and radiation oncologists. Following developments in medical imaging and radiation technology, radiosurgery has evolved from its first application in movement disorders to widespread use for a varied range of malignant and benign conditions. SRS remains a powerful, minimally invasive instrument that offers additional options for intervention to a diverse patient population. In this review, we will touch upon the common indications for SRS, including its use in brain metastases, malignant gliomas, meningiomas, arteriovenous malformations (AVMs), vestibular schwannomas, pituitary adenomas, and functional disorders, as well as consider the future possibilities of combining radiosurgery with immunotherapy.

Hippocampal-sparing and target volume coverage in treating 3 to 10 brain metastases: A comparison of Gamma Knife, single-isocenter VMAT, CyberKnife, and TomoTherapy stereotactic radiosurgery

PURPOSE

Our purpose was to evaluate hippocampal doses and target volume coverage with and without hippocampal sparing when treating multiple brain metastases using various stereotactic radiosurgery (SRS) platforms.

METHODS AND MATERIALS

We selected 10 consecutive patients with 14 separate treatments who had been treated in our department for 3 to 10 brain metastases and added hippocampal avoidance contours. All 14 treatments were planned with GammaPlan for Gamma Knife, Eclipse for single isocenter volumetric modulated arc therapy (VMAT), TomoTherapy Treatment Planning System (TPS) for TomoTherapy, and MultiPlan for CyberKnife. Initial planning was performed with the goal of planning target volume coverage of V100 ≥95% without hippocampal avoidance. If the maximum hippocampal point dose (Dmax) was <6.6 Gy in a single fraction and <40% of the hippocampi received ≤4.5 Gy, no second plan was performed. If either constraint was not met, replanning was performed with these constraints.

RESULTS

There was a median of 6 metastases per plan, with an average total tumor volume of 7.32 mL per plan. The median hippocampal Dmax (in Gy) without sparing averaged 1.65, 9.81, 4.38, and 5.46, respectively (P < .0001). Of 14 plans, 3 Gamma Knife and CyberKnife plans required replanning, whereas 13 VMAT and 8 TomoTherapy plans required replanning. The hippocampal constraints were not achievable in 1 plan on any platform when the tumor was bordering the hippocampus. The mean volume of brain receiving 12 Gy (in mL), which has been associated with symptomatic radionecrosis, was 23.57 with Gamma Knife, 76.77 with VMAT, 40.86 with CyberKnife, and 104.06 with TomoTherapy (P = .01). The overall average conformity indices for all plans ranged from 0.36 to 0.52.

CONCLUSIONS

Even with SRS, the hippocampi can receive a considerable dose; however, if the hippocampi are outlined as organs of risk, sparing these structures is feasible in nearly all situations with all 4 platforms, without detriment to target coverage, and should be considered in all patients undergoing SRS for multiple brain metastases.

SUMMARY

Hippocampi play an important role in memory, and sparing of these structures in whole brain radiation can improve neurocognitive outcomes. The hippocampi are not routinely spared when using stereotactic radiosurgery. We evaluated the incidental dose to the hippocampi when treating multiple brain metastases and sought to examine if hippocampal sparing is feasible without detriment to target coverage. We found that hippocampal sparing is possible without affecting coverage or conformality in most cases across treatment platforms.

Hypofractionated stereotactic radiosurgery: a new treatment strategy for giant cavernous sinus hemangiomas

OBJECTIVE Cavernous sinus hemangiomas (CSHs) are rare benign vascular tumors that arise from the dural venous sinuses lateral to the sella. Stereotactic radiosurgery (SRS) has emerged as a principal alternative to microresection for small- and medium-sized CSHs. Resection is a reasonable option for large (3-4 cm in diameter) and giant (> 4 cm in diameter) CSHs. However, management of giant CSHs remains a challenge for neurosurgeons because of the high rates of morbidity and even death that stem from uncontrollable and massive hemorrhage during surgery. The authors report here the results of their study on the use of hypofractionated SRS (H-SRS) to treat giant CSH. METHODS Between January 2008 and April 2014, 31 patients with a giant CSH (tumor volume > 40 cm³, > 4 cm in diameter) treated using CyberKnife radiosurgery were enrolled in a cohort study. Clinical status and targeted reduction of tumor volume were evaluated by means of serial MRI. The diagnosis for 27 patients was determined on the basis of typical imaging features. In 4 patients, the diagnosis of CSH was confirmed histopathologically. The median CSH volume was 64.4 cm³ (range 40.9-145.3 cm³). Three or 4 sessions of CyberKnife radiosurgery were used with a prescription dose based on the intent to cover the entire tumor with a higher dose while ensuring dose limitation to the visual pathways and brainstem. The median marginal dose to the tumor was 21 Gy (range 19.5-21 Gy) in 3 fractions for 11 patients and 22 Gy (range 18-22 Gy) in 4 fractions for 20 patients. RESULTS The median duration of follow-up was 30 months (range 6-78 months) for all patients. Follow-up MRI scans revealed a median tumor volume reduction of 88.1% (62.3%-99.4%) at last examination compared with the pretreatment volume. Ten patients developed new or aggravated temporary headache and 5 experienced vomiting during the treatment; these acute symptoms were relieved completely after steroid administration. Among the 30 patients with symptoms observed before treatment, 19 achieved complete symptomatic remission, and 11 had partial remission. One patient reported seizures, which were controlled after antiepileptic drug administration. No radiation-induced neurological deficits or delayed complications were reported during the follow-up period. CONCLUSIONS Hypofractionated SRS was an effective and safe modality for treating giant CSH. Considering the risks involved with microsurgery, it is possible that H-SRS might be able to serve as a definitive primary treatment option for giant CSH.

Radioresistance of Brain Tumors

Radiation therapy (RT) is frequently used as part of the standard of care treatment of the majority of brain tumors. The efficacy of RT is limited by radioresistance and by normal tissue radiation tolerance. This is highlighted in pediatric brain tumors where the use of radiation is limited by the excessive toxicity to the developing brain. For these reasons, radiosensitization of tumor cells would be beneficial. In this review, we focus on radioresistance mechanisms intrinsic to tumor cells. We also evaluate existing approaches to induce radiosensitization and explore future avenues of investigation.

Single versus multiple session stereotactic body radiotherapy for spinal metastasis: the risk-benefit ratio

Spine stereotactic body radiation therapy represents an important advancement in the management of spinal metastases that allows precise delivery of ablative doses of radiation therapy with excellent local control. Although the technique is being increasingly used in clinical practice, the optimal fractionation schedule remains uncertain. In this perspective paper, we review radiobiologic principles that support the use of multiple- versus single-fraction spine stereotactic body radiation therapy schedules and clinical data supporting the multiple-fraction approach. Specifically, we suggest that there may be a local control benefit of fractionation, while helping to limit the risk of toxicities such as vertebral body fracture, pain flare and radiation myelopathy. We conclude with future directions and the need for future study on this important topic.

Duodenal sparing stereotactic body radiation therapy for the treatment of locally advanced pancreatic cancer

254

Background: The proximity of the pancreas to bowel presents a unique challenge for pancreatic cancer SBRT. This study explores the safety and effectiveness of a novel approach optimizing pancreatic tumor coverage and duodenal sparing. Methods: Eighteen patients with locally advanced pancreatic cancer (15 head/3 body) were treated with initial chemotherapy (Gemcitabine or 5-FU/Cisplatin) for 2- 6 months. All underwent endoscopic US-guided fiducial placement in and around the tumor and had a 4D treatment planning CT scan with oral contrast. This CT was used in conjunction with EUS findings, PET, and biphasic CT scans to identify the GrossTumorVolume (GTV) on the expiration phases. The planning target volume (PTV) was created by expanding the GTV by 2 mm. Dose-volume histogram (DVH) endpoints were constructed, keeping V7/15/20 (stomach/duodenum volumes that receive 7 Gy/15 Gy/20 Gy) <40%,/<25%/ <15% ;the dose to 1/3 of the duodenal circumference <20Gy and duodenal point max dose <23 Gy. Additional dose constraints included liver D50<5 Gy, ipsilateral kidney D25<5 Gy, cord Dmax<10Gy. Three 10 Gy fractions, normalized to the 85% isodose were delivered to the PTV on consecutive weekdays using fiducial-based respiratory motion tracking on a dedicated 6 MV linac-integrated stereotactic delivery system . The patients were then offered systemic therapy for 6 months or until tolerance or disease progression. Follow-up occurred at 4 weeks, 12 weeks, and every 3 months. Results: All patients completed SBRT and a median of 5 total cycles of pre-and post-SBRT chemotherapy. Gross target volumes ranged from 14.2-205.7cm3 (median 39.5cm3). With a median follow-up of 10 months, 1 patient developed transient gastroparesis, 1 G 2 abdominal pain;1 G 2 hematologic toxicity. No late toxicity was observed in 15 patients with longer follow-up (median 12 month). All patients were free of local progression at the last follow-up visit (range 3-12 months). Conclusions: Linac-delivered organ-sparing SBRT with chemotherapy in locally advanced pancreatic cancer resulted in excellent local control and also was well tolerated acutely and subacutely. Longer follow-up is warranted. A phase I dose-escalation study is underway.

Radiosurgical dose selection for brain metastasis

Dose selection for brain metastasis radiosurgery is based largely upon clinical data obtained over a half century of radiosurgical treatments for various benign and malignant conditions. It is expected that within the entire radiosurgical process, the step of dose selection will occur within a framework of accurate calibration of dose delivery and accurate and detailed imaging for planning the radiosurgical treatment. Brain metastasis radiosurgery should seek lifelong, uncomplicated control. A low radiosurgery dose that will not control the tumor will not achieve this therapeutic goal, and neither will a radiosurgery dose that controls the tumor but causes symptomatic brain radiation necrosis. The volume of the metastasis being targeted and the volume of normal tissues receiving substantial radiosurgical doses are of paramount importance in dose selection. A high degree of conformality of the high-dose radiosurgical treatment volume to the metastasis has been shown to decrease complications, as does a steep dose gradient between the metastasis and adjacent normal brain tissue. A dose-escalation trial conducted by the Radiation Therapy Oncology Group that differentially dose-escalated radiosurgical doses for tumors of different sizes established that single-fraction doses between 15 and 24 Gy are relatively safe in patients who have received prior fractionated radiation therapy to the brain. Corresponding data do not exist for patients who are treated with primary radiosurgery and no whole brain radiation therapy. A dose-escalation trial for three-fraction radiosurgical treatment of brain metastases is being conducted at Stanford. Knowledge of prior whole brain radiation therapy treatment details, including the dose delivered and the time interval since that treatment was given may affect the choice of radiosurgical dose, as may recent administration of systemic, radiation-potentiating chemotherapy. Physician knowledge and careful judgment, together with careful treatment planning and delivery can minimize the risks associated with brain metastasis radiosurgery.