Stereotactic Body Radiotherapy (SBRT) for Breast Cancer Spinal Metastases is Associated with Low Rates of Long-Term Local Failure (LF) and Vertebral Compression Fracture (VCF) Independent of Molecular Status

PURPOSE/OBJECTIVE(S)

There is limited outcome data specific to breast cancer spinal metastases following spine SBRT. This study aims to report outcomes specific to breast cancer spinal metastases receiving spine SBRT and determine the implication of biomarker status.

MATERIALS/METHODS

We have been maintaining a prospective database since the inception of the spine SBRT program. A retrospective review identified 168 breast cancer patients with 409 spinal segments treated with spine SBRT between January 2008 and January 2023. Molecular subtypes were grouped based on luminal A, luminal B, basal, and HER2 enriched. Patients were followed with q3-monthly full-spine MRI and a clinical assessment. The primary endpoint was MRI-based local failure (LF), and secondary endpoints were overall survival (OS) and vertebral compression fracture (VCF).

RESULTS

The median follow-up was 33 months (range, 3.3-123 months). Amongst the 168 patients, the majority were ECOG 0 or 1 (95%), neurologically intact (94%), polymetastatic (74%), and either luminal A (71%) or luminal B (8%). A total of 17% of patients were HER2+ve versus 83% HER2-ve. Of 409 treated segments the majority (76%) had no prior radiation or surgery (de novo), were SINS stable (60%), had either no or low-grade epidural disease (86%) and treated with 24-28 Gy in 2 fractions (73%). The LF and OS rates at 1, 3, and 5 years were 5%, 11%, and 14%, respectively, and 91%, 65%, and 45%, respectively, independent of molecular subtype on univariate analyses. The cumulative risk of VCF at 2 and 5 years was 7% and 10%, respectively.

CONCLUSION

We observe, in the largest breast cancer spine cohort to date, excellent long-term local control rates independent of molecular sub-group, and acceptable VCF rates.

Outcomes Specific to Spinal Metastases with Paraspinal Disease Extension Following Spine Stereotactic Body Radiotherapy

PURPOSE/OBJECTIVE(S)

Spinal metastases with paraspinal disease (PD) extension are known to have worse outcomes following stereotactic body radiotherapy (SBRT). Characteristics of the PD itself have not been investigated to determine the impact on outcomes such as local control, which is the purpose of this study.

MATERIALS/METHODS

We retrospectively reviewed those patients who had SBRT for spinal metastases with PD disease, identified from a prospectively maintained database. Spinal metastases previously irradiated or surgical resected were excluded. The extent of PD was classified as involving the rib, neuroforamina, and muscle invasion. The gross tumor volume of PD (GTV_PD) and the clinical target volume of PD (CTV_PD) were segregated from the bony compartments based on the treatment plan contours. The outcomes of interest included the cumulative risk of local failure (LF), re-irradiation rates (ReRT), and overall survival (OS). LF and ReRT were estimated for each treated sites using the competing risk model (death as the competing risk), while OS was evaluated per patient using the Kaplan Meier method.

RESULTS

A total of 86 patients with 96 spinal metastases sites with PD were included. Of the 96 treated sites, 65% (62/96), 29% (28/96) and 6% (6/96) of PD spanned 1, 2, and 3 vertebral levels respectively. The median follow-up was 12.4months (range: 0.6-100months). The 6- and 12-month OS for the cohort was 81% and 51%, respectively. 33/86 (38%) patients had radioresistant cancer (gastrointestinal, renal cell carcinoma, thyroid, sarcoma, or melanoma). Involvement of rib, neuroforamina and muscle invasion were observed in 39% (37/96), 65% (62/96) and 21% (20/96) of the treated sites, respectively. Epidural disease was present in 57% (55/96) of treated sites. The median GTV_PD volume was 7cc (range: 0.3-114cc), and the median CTV_PD volume was 24cc (range: 0.4-248cc). The prescribed doses were 24 Gy/2 fractions (fx) (80%), 28 Gy/2 fx (10%) and 30 Gy/4 fx (10%). There were 84 treated sites with at least one post-treatment MRI available for LF assessment. The crude LF risk was 32% (27/84), and the 6- and 12-month cumulative LF rates were 12% and 28%, respectively. There was a trend towards an increased risk of LF when PD involved the rib (35% vs 24% at 1 year respectively, P = 0.07) and muscle (67% vs 20% at 1 year respectively; P = 0.06), but no difference in LF for neuroforamina involvement (26% vs 34% at 1 year respectively, P = 0.5). There were no differences in LF based on cancer radioresistance (P = 0.6), GTV_PD volume (P = 0.3) or CTV_PD volume (P = 0.4). Of the 96 treated sites, 14% (14/96) were re-irradiated (9 with repeat SBRT and 5 with conventional EBRT) at a median of 15 months (range: 4.7-59 months) post initial SBRT. The cumulative incidence of ReRT at 6- and 12-months were 1.2% and 7.3%, respectively.

CONCLUSION

PD involving adjacent rib and muscle may be associated with worse LF following SBRT. Further expansion of the cohort and dosimetric analyses are ongoing.

Stereotactic Body Radiotherapy for Posterior Element Only Spinal Metastases: Outcomes and Validation of Recommended Clinical Target Volume Delineation Practice

PURPOSE/OBJECTIVE(S)

Spine stereotactic body radiotherapy (SBRT) results in improved local control and pain response compared to conventional external beam radiotherapy. Consensus stipulates MRI-based delineation of the clinical target volume (CTV) is critical and is based on spine segment sector involvement. The applicability of these contouring guidelines to metastases confined to the posterior elements is unknown. The purpose of this study was to determine the patterns of failure, as well as the safety of treating posterior element metastases when the vertebral body was intentionally excluded from the CTV.

MATERIALS/METHODS

A retrospective review of a prospectively maintained database of 605 patients and 1412 spine segments treated with spine SBRT was performed. Only treated segments involving the posterior elements alone were included for the analyses. The primary outcome was local failure, as per SPINO recommendations, and secondary outcomes included patterns of failure, toxicities. Clinical and tumor factors were reported with descriptive statistics. The cumulative risk of local failure was estimated using the Fine-Gray method, accounting for death before local failure as a competing risk.

RESULTS

A total of 24/605 patients and 31/1412 segments within the database were treated to the posterior elements only. Local failure occurred in 11/31 segments. The cumulative rate of local recurrence was 9.7% at 12 months and 30.8% at 24 months. Amongst local failures, the most common histologies were renal cell carcinoma (36.4%) and non-small cell lung cancer (36.4%). At baseline, 4/11 (36.4%) segments with local failure (36.4%) had epidural disease and 8/11 (72.7%) had paraspinal disease. Most local failures were treated in the de novo setting (8/11, 72.7%). 6/11 (54.5%) failed exclusively within treated CTV sectors and 5/11 (45.5%) with both treated and adjacent untreated sectors. Of these five, four had disease progression within the untreated vertebral body. No failures occurred exclusively within the untreated vertebral body. One patient (4.2%) experienced a grade 4 skin toxicity and one patient (4.2%) developed an iatrogenic Grade 1 vertebral compression fracture.

CONCLUSION

Posterior element alone metastases are rare. Our analyses support SBRT consensus contouring guidelines such that the vertebral body can be excluded from CTV in spinal metastases confined to the posterior elements.

Outcomes Following Dose Escalated Radiotherapy for High Grade Meningioma

PURPOSE/OBJECTIVE(S)

Conventionally fractionated radiotherapy (RT) is a defined treatment following surgery for atypical and malignant meningioma. However, the optimal radiotherapeutic approach is not well defined. We present the results of our dose-escalation strategy.

MATERIALS/METHODS

Consecutive patients with a histopathologic grade 2 or 3 meningioma treated with RT were retrospectively reviewed. The primary outcome was progression-free survival (PFS), and secondary outcomes included cause-specific survival (CSS), overall survival (OS), local failure and incidence of radiation necrosis. We specifically compared the dose-escalation cohort, defined as those treated with ≥66 Gy EQD2 (equivalent dose in 2 Gy fractions, a/b = 10), to the standard dose cohort receiving <66 Gy EQD2. We defined adjuvant as RT delivered within 6 months of surgery otherwise the treatment was salvage.

RESULTS

A total of 118 patients with Grade 2 (111/118) or 3 (7/118) meningioma were identified. 54/118 (45.8%) received dose-escalation and 64/118 (54.2%) standard dosing. 34/54 (63.0%) dose-escalated and 45/64 (70.3%) standardly dosed were treated adjuvantly. The median follow-up was 45.4 months (IQR: 24.0-80.0 months) and median OS was 9.7 years. Post-operative residual disease was present in all dose-escalated patients, as compared to 65.6% in the standard dose cohort. PFS at 3-, 4- and 5-years in the dose-escalated vs. standard dose cohort were 78.9%, 72.2% and 64.6% vs. 57.2%, 49.1% and 40.8%, respectively, (p = 0.030). On multivariable (MVA) analysis, dose-escalation (HR: 0.544, 95% CI: 0.303-0.977, p = 0.042) was associated with improved PFS, whereas ≥2 surgeries (HR: 1.989, 95% CI: 1.049-3.773, p = 0.035) and older age (HR: 1.035, 95% CI:1.015-1.056, p<0.001) associated with worse PFS. The cumulative risk of local failure at 3-, 4- and 5-years in the dose-escalated vs. standard dose cohort were 16.9%, 23.8% and 31.8% vs. 39.6%, 45.6% and 53.9%, respectively, favoring dose-escalation (p = 0.016). MVA confirmed dose-escalation as predictive of a lower risk of LF (HR: 0.483, 95% CI: 0.263-0.887, p = 0.019), while ≥2 surgeries prior to RT predicted for greater LF rates (HR:2.145, 95% CI:1.220-3.771, p = 0.008). A trend was observed for prolonged CSS and OS in the dose escalation cohort (p = <0.1). Seven patients (5.9%) developed symptomatic radiation necrosis (RN) with no significant difference between the two cohorts.

CONCLUSION

Dose-escalated radiotherapy for high grade meningioma to at least 66 Gy is associated with significantly improved rates of local control and PFS with an acceptable risk of RN.

Staged Stereotactic Radiosurgery as a Novel Adaptive Approach to Salvage Previously Irradiated Brain Metastases

PURPOSE/OBJECTIVE(S)

We report outcomes specific to a novel 3 fraction (frx) staged stereotactic radiosurgery (St-SRS) regimen designed to salvage metastases previously irradiated and considered to be at high risk of radiation necrosis (RN).

MATERIALS/METHODS

A total of 24 patients with 55 metastases treated with our 3 frx St-SRS approach were reviewed. Prior to each frx, patients were re-simulated and planned with a new MRI to allow for treatment adaption. The primary endpoint was the cumulative incidence of local failure (LF) and secondary endpoints included tumor dynamics and RN rates.

RESULTS

The median follow up was 9.0 months (range: 2.7-40.1 months) and median age was 59-years (range: 32-84). Primary cancers were of breast (44%), lung (33%), melanoma (22%), and gastro-intestinal (1%) origin. Individual metastases treated with St-SRS had initially failed surgery and post-op cavity hypofractionated SRS (HSRS) for 2/55 (4%), SRS alone for 19/55 (34%), whole brain radiation (WBRT) alone for 6/55 (11%), HSRS for 2/55 (4%), and prior SRS and WBRT exposure for 28/55 (51%). 46/55 (84%) were prescribed 8 Gy, 8 Gy, 4 Gy; 8/55 (14%) had 6 Gy, 6 Gy, 4 Gy and 1/55 (2%) had 8 Gy, 8 Gy, 6 Gy. The median number of weeks between frx was 2.6 (range: 1.0-6.8). The median of the mean and maximum target doses were 9.7 Gy (range: 5.4-11.7 Gy) and 12.4 Gy (range, 7.5-16.0 Gy) respectively. The median prescription isodose line was 62% (range: 50-85%). The mean lesion volume and diameter was 3.8cc (range: 0.05-24.8cc) and 1.6cm (range: 0.2-4.4cm), respectively. The mean percent target volume coverage, Paddick Conformality Index and Gradient Index were 100% (range: 97-100%), 0.7 (range: 0.1-0.9), and 3.2 (range: 2.5-6.7), respectively. The mean volume change between staged frxs was -4.2% (range: -69.3 to +63.1%), and based on the first and last St-SRS MRI was -10.8% (range: -86.6% to +68.7%). The crude LF rate was 27%. The median time to LF was 3.4 months (range: 1.2-7.4 months). Amongst those with a LF, 7/15 (46%) were melanoma, 6/15 (40%) HER2 positive breast cancer, 1/15 (7%) gastrointestinal and 1/15 (7%) non-small cell lung carcinoma. 8/15 (53%) had prior WBRT and SRS exposure, 1/16 (7%) surgery and cavity HSRS, 5/15 (33%) SRS alone and 1/15 (7%) WBRT alone. Only asymptomatic RN events were observed in 4/55 (7%).

CONCLUSION

St-SRS is a promising approach to salvage previously irradiated brain metastases, including prior SRS, with a favorable rate of RN. Tumor volume dynamics between stages can be significant warranting adaptive replanning.

Molecular Status Predicts for Local Control in Patients with Non-Small Cell Lung Cancer Spinal Metastases Following Spine Stereotactic Body Radiotherapy

PURPOSE/OBJECTIVE(S)

We report outcomes after spine stereotactic body radiotherapy (SBRT) in patients with metastatic non-small cell lung cancer (NSCLC), to determine the significance of programmed death-ligand 1 (PD-L1) status and epidermal growth factor (EGFR) mutation on local failure (LF) rate.

MATERIALS/METHODS

A total of 165 patients and 389 spinal segments were retrospectively reviewed from 2009 to 2021. Baseline patient characteristics, treatment and outcomes were abstracted. Primary endpoint was LF and secondary outcomes included overall survival (OS) and vertebral compression fracture (VCF) rates. OS was estimated using the Kaplan-Meier method. Cumulative LF and VCF rates were calculated using competing risk analysis method. Multivariable analysis (MVA) evaluated factors predictive of LF and VCF.

RESULTS

Median follow-up was 13 months (range, 0.5-95 months). Median OS was 18.4 months (95% CI 11.4-24.6). Median age was 67 years (range, 28.2-89.9). 52% were female, 76% had an adenocarcinoma histology and 61% had a smoking history. 49/165 (29%) had an EGFR mutation. PD-L1 status was analyzed in 109/165 (66%) patients with 16% PD-L1 ≥ 50%, 20% PD-L1 1-49% and 35% PD-L1 <1%. Of 389 segments, 79% were de novo and 21% were previously radiated. At baseline, 35% had a VCF, 27% had epidural disease, 27% had paraspinal extension, and 49% were Spinal Instability in Neoplasia Score (SINS) stable. 239/389 (61%) were treated with either 24 or 28 Gy in 2 SBRT fractions. Within 1 month of SBRT, 39/165 (24%) had a tyrosine kinase inhibitor, 27/165 (16%) immunotherapy (IO) with or without chemotherapy, and 31/165 (19%) chemotherapy alone. LF cumulative incidence at 1- and 2-years was 16.3% (95% CI 12.8-20.3%) and 25.4% (95% CI 20.9%-30%), respectively. EGFR positivity (p<0.0001), PD-L1≥50% (p = 0.013) and treatment with IO within 1 month of SBRT (p = 0.004) predicted for improved local control on MVA. The 1- and 2-year LF rate in EGFR-positive vs. negative patients were 12.9% vs. 16.6% and 17.7% vs. 28.8%, respectively, and in those PD-L1 ≥50% vs PD-L1<50% were 7.8% vs. 19.6% and 7.8% vs. 38.1% respectively. Cumulative incidence of VCF at 1- and 2-years were 6.6% (95% CI 4.4-9.4%) and 8.8% (95% CI 6.1-12.0%). MVA identified prior SBRT to the same treated segment (P<0.0001) and a baseline VCF (p<0.0001) as significant predictors. 18/389 (4.6%) had radiation-induced radiculopathy and no radiation myelopathy events detected.

CONCLUSION

We identify the predictive utility of EGFR mutation and PD-L1 ≥50% status on local control in NSCLC patients with spinal metastases treated with spine SBRT, and a therapeutic benefit with peri-SBRT IO.

Predictors of Tumor Dynamics during a 6-Week Course of Chemoradiotherapy for Glioblastoma

PURPOSE/OBJECTIVE(S)

Our prior imaging studies have shown geometrically meaningful inter-fraction tumor dynamics specific to glioblastoma (GBM). We aim to identify predictors associated with tumor dynamics during a 6-week course of concurrent chemoradiotherapy (CRT) for GBM.

MATERIALS/METHODS

Patients enrolled in a prospective serial magnetic resonance imaging (MRI) study were reviewed. All patients were treated with 54-60 Gy in 30 fractions. The gross tumor volume (GTV) included the surgical cavity and T1c enhanced residual tumor; clinical tumor volume (CTV) included GTV with a 15mm isotropic expansion, respecting anatomical boundaries; planning target volume (PTV) was 4mm expansion. MRIs were obtained at RT planning (F0), fraction 10 (F10), and fraction 20 (F20). Tumor dynamic metrics (relative to F0) assessed included the GTV volume (Vrel), Hausdorff distance (dH) and migration distance (dM). dH is the average distance between two datasets in metric space. dM is the maximum linear displacement of the GTV in any direction. Factors to be determined associated with tumor dynamics included: age, sex, corpus callosum (CC) involvement, extent of surgery (gross total resection (GTR), subtotal resection (STR) or biopsy alone (Bx)), MGMT methylation and IDH mutation status.

RESULTS

A total of 129 patients were reviewed. Median GTV was 20.9cc at F0, 17.6cc at F10 (Vrel 0.85), and 16.1cc at F20 (Vrel 0.78). Patients without CC involvement had more marked GTV volume reduction: Vrel 0.82 vs 1.02 with CC involvement at F10 (P = 0.05), and Vrel 0.77 vs 0.88 with CC involvement at F20 (P = 0.03). Patients with GTR (vs STR vs Bx) had more marked GTV volume reduction across all time points: Vrel 0.78, 0.85 and 1.07 respectively at F10 (P = 0.001), and Vrel 0.69, 0.80, 1.04 respectively at F20 (P = 0.001). The median dH was 8.1mm at F10 and 9.2mm at F20. Patients with CC involvement (vs without CC involvement) had a larger dH: 54% vs 25% had dH>10mm respectively at F10 (P = 0.03), and 73% vs 28% had dH>10mm respectively at F20 (P<0.005). Patients with a GTR had smaller dH at both F10 (P = 0.02) and F20 (P = 0.006). At F20, 20%, 47% and 37% of patients with GTR, STR and Bx had dH>10mm (P = 0.04). The median dM were 4.7mm at F10 and 4.7mm at F20. Patients with CC involvement (vs without CC involvement) had larger dM: 41% vs 12% had dM >10mm respectively at F10 (P = 0.01), and 45% vs 9% had dM >10mm respectively at F20 (P<0.001). Patients with GTR had smaller dM at F10 (P = 0.03) and F20 (P0.002). At F20, 0%, 25% and 19% of patients with GTR, STR and Bx had dM>10mm (P = 0.002). Age, sex, MGMT methylation and IDH mutation status were not associated with Vrel, dH and dM at F10 and F20.

CONCLUSION

We identified CC involvement and extent of surgery to be associated with tumor dynamics at F10 and F20 over the course of CRT for GBM. This offers opportunities to better select patients who may benefit from earlier/ more frequent RT replan/ adaptation to ensure adequate tumor coverage, or to reduce RT toxicities.

Stereotactic Body Radiotherapy (SBRT) for Sacral Metastases: Deviation from Recommended Target Volume Delineation Predicts Higher Risk of Local Failure

PURPOSE/OBJECTIVE(S)

A international consensus recommendation was published to guide target volume delineation specific to sacral stereotactic body radiotherapy (SBRT). We report outcomes after sacrum SBRT, focusing on the impact of contouring deviation on local failure (LF) risk, with an aim to validate this guideline.

MATERIALS/METHODS

All patients who underwent SBRT to any level between S1 and S5 from 2010 to 2021 were identified from a prospectively maintained institutional database. The primary outcome was magnetic resonance-based LF. Secondary outcomes included vertebral compression fracture (VCF) and overall survival (OS). Cumulative LF and VCF rates were calculated per segment using the competing risk analysis method. Kaplan Meier analysis was used to estimate OS per patient. Cox proportional hazards model was used to assess predictive factors of LF, VCF, and OS.

RESULTS

A total of 215 treated sacral segments in 112 patients were retrospectively reviewed. The median follow-up was 13 months (range, 0.4-116.9). The median age was 64 years (range, 18-86), and 56% were male. Most patients (52%) had treatment to a single segment. The median clinical target volume (CTV) was 129.2 cc (range, 5.8-753.5). Most segments were treated with 30 Gy/4 fractions (51%), 24 Gy/2 fractions (31%), or 30 Gy/5 fractions (10%). Thirty-one percent of segments were of radioresistant histology (gastrointestinal, kidney, melanoma, sarcoma, or thyroid primary), and 51% had extraosseous disease. Sixteen percent of segments were under-contoured per consensus guidelines, with incomplete coverage of the involved sector (71%), omission of the adjacent uninvolved sector (17%), or both (11%) as the causes for deviation. The cumulative incidence of LF was 18.4% (95% CI 13.5-24.0) at 12-months and 23.1% (95% CI 17.6-29.0) at 24-months. On multivariate analysis (MVA), under-contouring (HR 2.4, 95% CI 1.3-4.7, p = 0.008), radioresistant histology (HR 2.4, 95% CI 1,4-4.1, p = 0.001), and extraosseous extension (HR 2.5, 95% CI 1.3-4.7, p = 0.005) were predictors of increased risk of LF. The LF rates at 12/24-months were 15.1%/18.8% for segments contoured per guideline versus 31.4%/40.0% for those under-contoured. The cumulative incidence of VCF was 7.1% (95% CI 4.1-11.1) at 12-months and 12.3% (95% CI 8.2-17.2) at 24-months. On MVA, female gender was the only risk factor for VCF (HR 2.3, 95% CI 1.1-5.2, p = 0.04). The median OS was 29.5 months (95% CI 17.5-59.2). On MVA, primary kidney (HR 4.7, 95% CI 1.7-12.5, p = 0.002) or lung histology (HR 3.4, 95% CI 1.3-8.5, p = 0.010), the presence of liver (HR 2.8, 95% CI 1.2-6.4, p = 0.016) or lung (HR 2.5, 95% CI 1.3-5.1, p = 0.008) metastases, ECOG performance status 2 or 3 (HR 3.3, 95% CI 1.2-8.2, p = 0.013), and the presence of sensory or motor deficit (HR 2.6, 95% CI 1.2-5.4, p = 0.012) were prognostic for worse OS.

CONCLUSION

Sacral SBRT is associated with high rates of efficacy and an acceptable VCF risk. Adherence to target volume delineation consensus guidelines reduces the risk of LF.

Dosimetric Implications of Weekly On-Line MR-Guided Adaptive Radiotherapy (RT) for Glioblastomas (GBM) Growing during RT

PURPOSE/OBJECTIVE(S)

The UNITED trial (NCT04726397) involves once-weekly on-line adaptive RT for patients with GBM on a 1.5T MRI-Linac (MRL). For tumors that continue to enlarge during the course of RT, we hypothesize that the adaptive strategy improves the dosimetric coverage of the target relative to a non-adaptive strategy.

MATERIALS/METHODS

As per the trial protocol, T1+contrast (T1c) and FLAIR MRI sequences were acquired once per week during RT to re-define and adapt the gross tumor volume (GTV) as the contrast enhanced volume, the clinical target volume (CTV) as a 5mm expansion around the GTV plus adjacent FLAIR hyperintense regions considered at-risk by the radiation oncologist, and a 3 mm PTV around the CTV. Nine UNITED patients with tumors that grew throughout RT (GTV and/or CTV) were identified. 5/9 patients were treated with 60 Gy in 30 fractions (6 weekly adaptions) and 4/9 with 40 Gy in 15 fractions (3 weekly adaptions). For the final week's GTV and CTV (GTVfinal and CTVfinal), the dosimetric outcomes of the delivered and adaptive summative plans (Dsum) were compared to dose of the baseline plan (Dbaseline) generated on Fraction 1 of treatment, the latter being indicative of the theoretical situation where no further adaption was taken. We measured the dose to 99% of the GTVfinal and CTVfinal: Dsum99 and Dbaseline99.

RESULTS

For each adaptive fraction, the plan was optimized to achieve an objective of D99 greater than 95% of the prescription (D99>95%) for both the GTV and CTV. The relative increase in GTVfinal and CTVfinal from baseline was on average 119% (range: 98% to 144.7%) and 128% (range: 109% to 176%), respectively. The proportion of CTVfinal that was outside the baseline plan's PTV was on average 11.5% (range: 0% to 32%). The GTVfinal did not extend beyond the baseline PTV for any of the 9 cases. GTVfinal Dsum99 was >95% for all cases while CTVfinal Dsum99 was < 95% in 2 of 9 cases (74%, and 87%). By contrast, the baseline plan, if given for all fractions with no further adaptation, yielded a D99 for CTVfinal of <95% in 5 of 9 cases (28%, 52%, 75%, 84%, and 87%). In general, coverage of the CTVfinal decreased with increasing levels of CTVfinal outside of the baseline PTV. For all 5 cases where CTVfinal D99<95% on the baseline plan, more than 10% of CTVfinal was outside of the baseline PTV. Small margin, weekly adaptive RT on an MRL for GBM maintains coverage of the GTV in the presence of tumor growth while minimizing the degree of normal brain tissue irradiated. Dosimetric impact on non-GTV/CTV brain is outside the scope of the present study.

CONCLUSION

Preliminary results indicate that a once weekly adaptive approach for small margin MR-guided RT improves tumor coverage for progressive tumors compared to a static (baseline) plan without adaptation.