Dynamic Conformal Arc Technique Research Articles

Improving the Efficiency of Single-Isocenter Multiple Metastases Stereotactic Radiosurgery Treatment

PurposeMultiple brain metastases can be treated efficiently with Stereotactic Radio Surgery (SRS) using a Single Isocenter Dynamic Conformal Arc (SIDCA) technique. Currently plans are manually optimized, which may lead to unnecessary table angles and arcs being used. This study aimed to evaluate an automatic 4π-optimization SIDCA algorithm for treatment efficiency and plan quality.Methods and MaterialsAutomatic 4π-optimized SIDCA plans were created and compared to the manually optimized clinical plans for 54 patients who received single-fraction SRS to 2-10 metastases. The number of table angles and arcs were compared with a paired t-test using a Bonferroni-corrected significance level of p<0.05/4=0.0125. The reduction in treatment time was estimated from the difference in the number of table angles and arcs. Plan quality was assessed through the volume-averaged inverse Paddick Conformity Index (CI) and Gradient Index (GI), and the volume of normal brain surrounding each metastasis receiving 12 Gy (local V12 Gy). For a five-patient subset the automatic plans were manually adjusted further. CI and GI were assessed for non-inferiority using a one-sided t-test with the non-inferiority limit equal to the 95% inter-observer reproducibility limit from a separate planning study (corrected significance level p<0.05/(4−1)=0.017).ResultsThe automatic plans significantly improved treatment efficiency with a mean reduction in the number of table angles and arcs of −0.5±0.1 and −1.3±0.2 respectively (± standard error, both p<0.001). Estimated treatment time saving was −2.7±0.5minutes, 14% of the total. The volume-averaged CI and GI were non-inferior to the clinical plans (both p<0.001), although there was a small systematic shift in CI of 0.07±0.01. The resulting difference in local V12 Gy, 0.25±0.04cm3, was not clinically significant. Minor manual adjustment of the automatic plans removed these slight differences whilst preserving the improved treatment efficiency.ConclusionsAutomatic 4π-optimization can generate SIDCA SRS plans with improved treatment efficiency and non-inferior plan quality.

The Impact of Multileaf Collimator Size on Single Isocenter Dynamic Conformal Arcs-Based Radiosurgery for Brain Metastases.

To compare the plan quality of stereotactic radiosurgery (SRS) between 2.5-mm and 5-mm multileaf collimator (MLC) and investigate the factors' influence onthe differences by MLC size. Seventy-six treatment plans including 145 targets calculated with a single isocenter multiple noncoplanar dynamic conformal arc (DCA) technique using automatic multiple brain metastases (MBM) treatment planning system. Conformity index (CI), gradient index (GI), lesion underdosage volume factor (LUF), healthy tissue overdose volume factor (HTOF), geometric conformity index (g), and mean dose to normal organs were compared between 2.5-mm and 5-mm MLC. Then the factors that influenced the differences of these parameters were investigated. The impact of target size was also investigated forCI and GI values of individual targets (n=145), and differences between 2.5-mm and 5-mm MLC were analyzed. All parameters except for LUF were significantly better in plans with 2.5 mm MLC. Target size was a significant factor for difference inHTOF, and distance between targets was a significant factorfor difference inbrain dose and GI. Among 145 metastases, the average inverse CI was 1.35 and 1.47 with 2.5-mm and 5-mm MLC, respectively (p<0.001). The average GI was 3.21 and 3.53, respectively (p<0.001). For individual targets, target size was a significant factor in CI and GI both with 2.5-mm and 5-mm MLC (p-value: <0.001, each). CI and GI were significantly better with 2.5-mm than 5-mm MLC. CI was almost >0.67 except for ≤5mm targets with 5-mm MLC. Also, GI was almost smaller than 3.0 for >10 mm targets both with 2.5-mm and 5-mm MLC. MBM with 5-mm MLC was almost fine. However, it may be better to use a conservative margin for larger metastases. It may also be better to avoid SRS with5-mm MLC for patients with ≤5 mm target size.

RADT-21. LINEAR ACCELERATOR BASED RADIOSURGERY WITH DYNAMIC CONFORMAL ARCS FOR PATIENTS WITH MULTIPLE BRAIN METASTASES: A PROSPECTIVE COHORT STUDY

Abstract PURPOSE Linear accelerator (LINAC)-based stereotactic radiosurgery (SRS) is gaining popularity as an alternative due to its high delivery efficiency and improved accessibility. This study focuses on the radiosurgery protocol of a single institution, utilizing single-isocenter multi-target SRS treatment with an inverse-optimized dynamic conformal arc technique. Materials &amp; METHODS A prospective cohort study was conducted from August 2020. The study protocol was approved by the Research Ethics Committee. Patients with brain tumors referred for SRS were treated based on the indications. Prescription doses were determined by target size and radiation oncologists. The protocol included a 1-2 mm expansion of tumors for the planning target volume (PTV), with a requirement of at least 85% PTV coverage. At least three non-coplanar arcs are needed. Competing risk survival analysis was used to assess the cumulative incidence of failure. RESULTS A total of 57 patients with 190 brain tumors underwent SRS in this study. 12 patients (21.1%) received at least 2 radiation courses for recurrent lesions. Tumor sizes ranged from 0.45 to 8.0 cm (median 1.41 cm). The maximal dose relation ranged between 71.5% and 91.4% (median 81.6). The conformal index ranged from 1.08 to 2.19 (median 1.19), and the dose gradient index ranged from 2.19 to 6.95 (median 3.59). The 1-year cumulative incidence of local failure and intracranial distant failure rates were 10.8% and 35.6%, respectively. CONCLUSION LINAC-based SRS using dynamic conformal arcs achieves optimal tumor coverage and a high dose gradient with ease. It demonstrates a low rate of local failure, effectively protects normal brain tissue, and allows for the postponement of whole brain radiotherapy, enabling patients to maintain a normal quality of life. In conclusion, LINAC-based SRS with dynamic conformal arcs may be a beneficial treatment option for brain metastases, even with a high lesion count, particularly in institutions equipped with LINAC.

Full Dose SBRT in Combination With Mediastinal Chemoradiation for Locally Advanced, Non-Small Cell Lung Cancer: A Practical Guide for Planning, Dosimetric Results From a Phase 2 Study, and a Treatment Planning Guide for the Phase 3 NRG Oncology LU-008 Trial

Stereotactic body radiation therapy (SBRT) has been utilized with high effectiveness in early stage NSCLC but has not been studied extensively in locally advanced NSCLC. We conducted a phase II study delivering SBRT to the primary tumor followed by conventionally fractionated chemoradiation to the involved lymph nodes for patients with node positive locally advanced NSCLC. This manuscript serves as both a guide to planning techniques utilized on this trial and the subsequent phase III study, NRG Oncology LU-008, and to report patient dosimetry and toxicity results. We initiated a phase II multi-center single arm study evaluating SBRT to the primary tumor (50-54 Gy in 3-5 fractions) followed by conventionally fractionated chemoradiation to 60 Gy in 2 Gy fractions with doublet chemotherapy to the involved lymph nodes for patients with stage III or unresectable stage II NSCLC. Patients eligible for adjuvant immunotherapy received up to 12 months of durvalumab. We report a detailed guide for the entire treatment process from CT simulation through treatment planning and delivery. The dosimetric outcomes from the 60 patients who completed therapy on study are reported both for target coverage and normal structure doses. We also report correlation between radiation related toxicities and dosimetric parameters. 60 patients were enrolled between 2017-2022. Planning techniques utilized were primarily VMAT for both SBRT to the primary tumor and conventionally fractionated radiation to the involved nodes, with a minority of cases utilizing dynamic conformal arcs technique or static DMLC IMRT. Grade 2 or higher pneumonitis was associated with lung dose V5Gy >70 % and grade 2 or higher pulmonary toxicity was associated with lung dose V10Gy>50%. Only 3 patients (5%) experienced grade 3 or higher pneumonitis. Grade 2 or higher esophagitis was associated with esophageal doses, including mean dose >20Gy, V60Gy >7%, and D1cc >55Gy. Only one patient (1.7%) experienced grade 3 esophagitis. SBRT to the primary tumor followed by conventionally fractionated chemoradiation to the involved lymph nodes is feasible with planning techniques as described. Radiation related toxicity on this phase II study was low. This manuscript serves as a guideline for the recently activated NRG Oncology LU-008 phase III trial evaluating this experimental regimen.

The Optimal Choice of Technique for Stereotactic Radiosurgery—A LINAC-Based Dosimetric Study between DCA, DCA-SSO, DCA-SSO-VDR, and VMAT

Abstract Introduction Advanced radiation therapy delivery techniques require greater understanding of various planning sequences and methods. The aim of this study is to determine a class solution that finds the best possible technique to deliver for stereotactic radiosurgery between dynamic conformal arc (DCA) techniques using various options such as DCA, DCA + SSO (segment shape optimization), and DCA + SSO + VDR (variable dose rate) using noncoplanar beam arrangement and volumetric modulated arc therapy (VMAT) using coplanar beams. Materials and Methods In this dosimetric study, 11 brain cases were retrospectively planned for various techniques and analyzed for the Paddick conformity index (CI), Radiation Therapy Oncology Group homogeneity index (HI), Paddick gradient index (GI), treatment time in terms of monitor units (MU) and normal brain dose (V12Gy). The paired t-test was performed to know the statistical significance between the techniques. Results In terms of CI, GI, and control of the normal brain dose, the VMAT plan was superior to other techniques. But, HI was found to be better with DCA. Above all, VMAT delivered higher MU than any other technique. The p-values between DCA + SSO and DCA, DCA + SSO + VDR and DCA + SSO, and VMAT and DCA + SSO + VDR are as follows: CI: 0.0004, 0.015, and 0.03; GI: 0.03, 0.33, and 0.29; HI: 0.008, 0.04, and 0.06; V12 Gy of normal brain: 0.1, 0.01, 0.38. VMAT requires approximately 41 ± 17% more MU than DCA + SSO + VDR. Conclusion VMAT using coplanar beams is preferable among all the techniques, considering the dosimetric parameters studied. If VMAT is not available in the facility, DCA + SSO + VDR technique using non coplanar beams can be used to deliver SRS treatment.

Radiosurgery for Five to Fifteen Brain Metastases: A Single Centre Experience and a Review of the Literature.

PurposeStereotactic radiosurgery (SRS) is now mainstream for patients with 1-4 brain metastases however the management of patients with 5 or more brain metastases remains controversial. Our aim was to evaluate the clinical outcomes of patients with 5 or more brain metastases and to compare with published series as a benchmarking exercise.MethodsPatients with 5 or more brain metastases treated with a single isocentre dynamic conformal arc technique on a radiosurgery linac were identified from the institutional database. Endpoints were local control, distant brain failure, leptomeningeal disease and overall survival. Dosimetric data were extracted from the radiosurgery plans. Series reporting outcomes following SRS for multiple brain metastases were identified by a literature search.Results36 patients, of whom 35 could be evaluated, received SRS for 5 or more brain metastases between February 2015 and October 2021. 25 patients had 5-9 brain metastases (group 1) and 10 patients had 10-15 brain metastases (group 2). The mean number of brain metastases in group 1 was 6.3 (5-9) and 12.3 (10-15) in group 2. The median cumulative irradiated volume was 4.6 cm3 (1.25-11.01) in group 1 and 7.2 cm3 (2.6-11.1) in group 2. Median follow-up was 12 months. At last follow-up, local control rates per BM were 100% and 99.8% as compared with a median of 87% at 1 year in published series. Distant brain failure was 36% and 50% at a median interval of 5.2 months and 7.4 months after SRS in groups 1 and 2 respectively and brain metastasis velocity at 1 year was similar in both groups (9.7 and 11). 8/25 patients received further SRS and 7/35 patients received whole brain radiotherapy. Median overall survival was 10 months in group 1 and 15.7 months in group 2, which compares well with the 7.5 months derived from the literature. There was one neurological death in group 2, leptomeningeal disease was rare (2/35) and there were no cases of radionecrosis.ConclusionWith careful patient selection, overall survival following SRS for multiple brain metastases is determined by the course of the extracranial disease. SRS is an efficacious and safe modality that can achieve intracranial disease control and should be offered to patients with 5 or more brain metastases and a constellation of good prognostic factors.

Comparison of the dosimetric planning efficiency of dynamic conformal arc (DCA) and volumetric modulated arc therapy (VMAT) techniques for stereotactic body radiotherapy (SBRT) of lung cancer using internal target volume (ITV)

Comparison of the dosimetric planning efficiency of dynamic conformal arc (DCA) and volumetric modulated arc therapy (VMAT) techniques for stereotactic body radiotherapy (SBRT) of lung cancer using internal target volume (ITV)

Feasibility and potential advantages using VMAT in SRS metastasis treatments.

Utilization of stereotactic radiosurgery (SRS) for brain metastases (BM) has become the technique of choice as opposed to whole brain radiation therapy (WBRT). The aim of this work is to evaluate the feasibility and potential benefits in terms of normal tissue (NT) and dose escalation of volumetric modulated arc therapy (VMAT) in SRS metastasis treatment. A VMAT optimization procedure has therefore been developed for internal dose scaling which minimizes planner dependence. Five patient-plans incorporating treatment with frame-based SRS with dynamic conformal arc technique (DA) were re-planned for VMAT. The lesions selected were between 4-6 cm3. The same geometry used in the DA plans was maintained for the VMAT cases. A VMAT planning procedure was performed attempting to scale the dose in inner auxiliary volumes, and to explore the potential for dose scaling with this technique. Comparison of dose-volume histogram (DVH) parameters were obtained. VMAT allows a superior NT sparing plus conformity and dose scaling using the auxiliary volumes. The VMAT results were significantly superior in NT sparing, improving both the V10 and V12 values in all cases, with a 2-3 cm3 saving. In addition, VMAT improves the dose coverage D95 by about 0.5 Gy. The objective of dose escalation was achieved with VMAT with an increment of the Dmean and the Dmedian of about 2 Gy. This work shows a benefit of VMAT in SRS treatment with significant NT sparing. A VMAT optimization procedure, based on auxiliary inner volumes, has been developed, enabling internal dose escalation.

Acuros-Based Planning with Density Override for Lung SBRT by a Dynamic Conformal Arc Technique: Comparative Evaluation with AAA-Based Planning in Four-Dimensional Dose

The purpose of this study was to evaluate a planning strategy based on Acuros with density override in comparison with AAA without and with the override. Ten lung-tumor patients were selected with each PTV size around 2 - 4 cm and were imaged using slow scan, followed by four-dimensional (4D) imaging limited to the target. On each phase-specific image, gross tumor volume (GTV) was contoured. Summed over all phases, an integrated GTV (iGTV) was generated and copied to the slow scan. A treatment plan was created using a dynamic-conformal-arc technique with AAA to prescribe 60 Gy to 95% of PTV (iGTV + 0.5 cm). Each AAA-based plan was regenerated by overriding the density of the setup margin of PTV by GTV density (modeling tumor-position uncertainty). It was also regenerated with Acuros and the override. The three plans were validated in 4D dose to PTV, after similarly overriding PTV density (phase-specific), accurately calculating with Acuros, and summing the phase-specific plans through organ/dose registration. The Acuros-based plan with the override, the AAA-based plan, and the AAA-based plan with the override provided 4D PTV doses of 63.9, 67.9, and 62 Gy at D95%, respectively, averaged over all patients. The override with Acuros and AAA produced lesser 4D doses, closer to the associated 3D doses, respectively, than that without the override, with better conformity and inhomogeneity. With the override in common, Acuros provided a greater dose to PTV than that by AAA. The Acuros with the override, which was more accurate than the AAA without the override, is clinically recommended.

Is Monte Carlo-Based Dose Calculation Algorithm Required for All Lung Tumors Treated with Stereotactic Body Radiotherapy?

The primary objective of this study is to compare the accuracy of a Boltzmann equation solvers algorithm against a Monte Carlo based dose calculation algorithm for island and peripheral lung tumors treated with Stereotactic Body Radiotherapy (SBRT). Ten lung SBRT patients considered for this retrospective study, five patients with an island lung tumor and five patients with a chest wall or lateral tumor . For each patient, a simulation CT scan and a 10-phase 4DCT scans were acquired. The initial delivered plans were planned with dynamic conformal arc technique using a planning system that utilizes the x-ray voxel Monte Carlo (XVMC) algorithm. The patients’ treatment plans were transferred to a treatment planning system utilizes the Acuros AXB algorithm. These plans were retrospectively re-calculated keeping the initial arc angles and multi leaf collimator (MLC) segments. Validation plans were generated by projecting the plans into a 4D detector/cylindrical diode detector array quality assurance (QA) phantom. Quality assurance plans were delivered using a 6X SRS beam. Measured dose distributions were compared with calculated dose distributions using 3% 3mm gamma analysis. Dose-volumetric data including the maximum dose, minimum dose, D95% for internal tumor volume (ITV) and planning tumor volume (PTV) were compared for the two algorithms using the Wilcoxon Rank Sum test. All analyses were performed using data and decision management software with two-tailed p-values reported and a p-value of <0.05 considered significant. The table below summarizes the results for dose-volumetric data for all 10 patients. For the island tumors the ITV minimum dose and the PTV minimum dose showed a trend for a difference between the AXB and XVMC algorithms (p=0.06, p=0.06, respectively). There was no difference observed between the ITV max, or PTV max . For the peripheral tumors there was no difference observed for any of the dose volumetric data. Passing rate for both algorithms is above 90% with (3mm, 3%) gamma analysis. The XVMC requires 16% more monitor unit than the AXB algorithm. The dosimetric performance of the AXB is comparable to the XVMC algorithm. This indicates that a Monte Carlo based dose calculation algorithm is not the only algorithm that can be used for lung tumors treated with SBRT. The AXB algorithm can be used clinically even for island type lung tumor where high heterogeneity is present. Slight differences in minimum doses were noted for island tumors; however, these differences are usually clinically acceptable.Abstract 3828; Table 1CriteriaXVMCAXBITV Max dose (%)131.2±5.6130.5±7.1ITV Min dose (%)106.7 ±6.2108.2 ±9.9PTV Max dose (%)131.2 ±6.2130.6 ±6.7PTV Min dose (%)83.1±5.386.0±3.9D95%101.4±1.9100.6±1.7Gamma (3%, 3mm)95.0 ± 3.093.7 ± 2.0 Open table in a new tab

RADI-41. CLINICAL OUTCOMES FOLLOWING RAPID DELIVERY SINGLE ISOCENTER RADIOSURGERY FOR FIVE OR MORE BRAIN METASTASES: VOLUMETRIC MODULATED ARC THERAPY VS. DYNAMIC CONFORMAL ARC

BACKGROUND: Recent advances in the delivery of stereotactic radiosurgery (SRS) has led to wider availability and use of this technology. The ability to rapidly delivery radiosurgery to multiple targets utilizing single-isocenter techniques represents an additional step forward for these otherwise time-consuming therapies. The current retrospective study measures and compares clinical outcomes in patients with 5 or more brain metastases treated using a single-isocenter RapidArc (RA) volumetric-modulated arc therapy (VMAT) approach, with those treated using a single-isocenter dynamic conformal arc (DCA) approach. METHODS: We queried retrospectively a radiosurgery database registry of 680 patients from 11/2012- 3/2019. We included patients with 5 or more brain metastases who had documented SRS treatment plans utilizing a single-isocenter approach, either VMAT or DCA. Radiosurgery was delivered using a linac-based platform capable of delivering radiotherapy to multiple simultaneous targets. We obtained information on patient gender, age, number of lesions, primary cancer histology. Patients were separated by collimation plan (VMAT vs DCA); overall survival (OS) was estimated using Kaplan-Meier method. RESULTS: We found 49 patients that met our criteria for inclusion. Median age was 57 yrs. with a majority being females (n=28, 57.2%). Number of targets was between 5 and 20; median number of targets was 7. Most common primary site histology was non-small cell lung (51.0%), followed by breast (14.2%). For DCA collimation group, median OS was 20.5 weeks (95% CI: 12.4, 34.5), with a 6 month OS of 36.0% (95% CI: 19.2%, 54.9%). For VMAT collimation group, median OS was 20.0 weeks (95% CI: 10.8, 46.1), with a 6 month OS of 39.1% (95% CI: 11.3%, N/A). CONCLUSIONS: For patients with 5 or more brain metastases treated with rapid delivery single-isocenter radiosurgery, no significant difference in survival outcomes were demonstrated between DCA and VMAT collimation techniques.

Circular collimator arc versus dynamic conformal arc treatment planning for linac-based stereotactic radiosurgery of an intracranial small single lesion: a perspective of lesion asymmetry

BackgroundAlthough circular collimator arcs (CCA) and dynamic conformal arcs (DCA) are commonly used linear accelerator-based treatment planning techniques for intracranial stereotactic radiosurgery (SRS) of a small single lesion, these two techniques have not been rigorously compared in terms of tumor shape. Therefore, this study compared clinical CCA plans with re-planned DCA plans using conformity index (CI) and V12Gy (volume of normal brain tissue receiving 12 Gy or higher) from a perspective of asymmetry (Asym) of planning target volume (PTV).MethodsNinety-five clinical CCA plans delivered for a small single lesion with PTV size < 1.4 cm3 were selected and re-planned using DCA. PTV Asym (%) was defined and calculated from three dimensions of PTV. A pair of the 95 plans was first considered as one group without grouping and then categorized into two groups with respective to either PTV size or PTV Asym, and four groups with respect to PTV size and PTV Asym. For grouping, median values of PTV size and PTV Asym were used. A non-parametric paired test was performed for CI and V12Gy to compare CCA and DCA plans in each group.ResultsMedian values of PTV size and PTV Asym were 0.415 cm3 (range: 0.076 cm3–1.369 cm3) and 6.12% (range: 0.52–25.74%), respectively. DCA plans had a lower average CI value than CCA plans for all groups. CCA plans had a smaller average V12Gy value than DCA plans for lesions with PTV Asym ≤6.12%, while CCA and DCA plans had similar average V12Gy values for lesions with PTV Asym > 6.12%.ConclusionsThe DCA technique is recommended when a lesion has PTV Asym > 6.12% regardless of PTV size. For lesions with PTV Asym ≤6.12%, a technique choice would depend on the preference of CI or V12Gy.

Dosimetric Evaluation of Automated Single-Isocenteric vs. Conventional Multi-Isocenteric SRS Technique Using Dynamic Conformal Arcs for the Treatment of Multiple Brain Metastasis

Multi-isocenter dynamic conformal arc (MIDCA) SRS technique is commonly utilized to treat brain metastasis. Conventionally, individual isocenters are used to treat each lesion separately, which is time consuming and uncomfortable for the patients. An automated single isocenter dynamic conformal arc (SIDCA) technique to treat all lesions simultaneously could save machine time and help patient to tolerate the shorter treatment time. Our objective of this study is to analyze and compare the quality of treatment plans generated using single-isocenter vs multi-isocenter techniques employing non-coplanar dynamic conformal arcs . Five multiple brain metastasis patients were selected for this study, who were treated with MIDCA on a treatment planning system. Number of brain lesions in each patient ranged from 3 to 10 and there were total 27 targets in all 5 patients. Prescribed doses ranged from 15 Gy to 20Gy depending on tumor volume and location. All patient’s contours were transferred to single isocenter automated multimets element software. Three to four non-coplanar arcs were used for each lesion in MIDCA plans whereas 5 non-coplanar arcs were used for each patient to treat all lesions in SIDCA plans. For dosimetric plan quality evaluation, conformity index (CI), volume of 12 Gy (V12) and volume of 5Gy (V5) were collected for all the plans and compared. Average CI for SIDCA was 1.45 (SD 0.8) versus 1.63 (SD 0.2) for MIDCA. Average V12 for SIDCA 24.3cc (SD 23.3cc) versus 29.8 cc (SD 29.1cc) for MIDCA technique. As number of targets increases V5 and V12 dramatically increased for SIDCA technique, for 10 lesions plans V12 values were 65.7cc and 50.5cc for SIDCA and MIDCA respectively. Similarly, V5 values were 373cc and 272cc for SIDCA and MIDCA respectively. As expected Monitor units were significantly lower for SIDCA as compared to MIDCA hence shorter treatment delivery time. Single isocenter plans done using automated multiments element software were better in conformity index however they were inferior in lower dose volumes such as 12 and 5 Gy. This difference was observed larger when number of brain lesions increased. Single isocenter technique is very convenient and time saving for treating multiple brain mets SRS, however, it could compromise lower dose conformity and potentially could deliver higher volumes of lower doses to normal brain tissue.

A Retrospective Review on Stereotactic Radiosurgery Cases Planned with Dynamic Conformal Arcs Versus Volumetric Modulated Arc Therapy

This study aimed to acquire an average value of plan quality metrics by retrospectively analyzing past stereotactic radiosurgery (SRS) treatment plans. The study was also designed to compare the plan quality metrics between single-target Dynamic Conformal Arc (DCA) and multi-target Volumetric Modulated Arc Therapy (VMAT) plans. A total of 253 LINAC-based SRS plans treated in 2017 at our institution were reviewed. A total of 605 lesions were included in these plans as treatment targets. 156 plans were generated with a DCA technique treating single target in each plan, and 97 plans used a VMAT technique to treat multiple targets simultaneously in a single plan. The prescription dose was normalized to 20Gy for all plans for this analysis. Two plan quality metrics, tumor Conformity Index (CI) and normal tissue volume receiving more than 12Gy (V12Gy), were calculated for each target. The results were stratified based on target size and compared between different techniques. For multi-targets VMAT plans, the distance of a target to the nearest other target and the number of targets being treated in the same plan were calculated for each target to evaluate their impact on V12Gy. For the subgroups of target volume < 0.2cc, 0.2-0.5cc, 0.5-1.0cc, 1.0-2.0cc, and 2.0-4.0cc, the mean (±SD) CI were 2.1 (±0.3), 1.9 (±0.2), 1.8 (±0.3), 1.6 (±0.2), and 1.6 (±0.1), respectively, in DCA plans and 1.8 (±0.4), 1.5 (±0.2), 1.4 (±0.2), 1.3 (±0.2), and 1.3 (±0.1), respectively, in VMAT plans. The mean V12Gy were 1.2cc, 1.9cc, 3.5cc, 5.2cc, and 7.4cc, respectively, in DCA plans and 1.3cc, 2.2cc, 3.2cc, 4.7cc, and 7.8cc, respectively, in VMAT plans. The V12Gy for a target in VMAT plan did not show apparent dependence on the distance between targets or the total number of targets in single VMAT plan. The average performance level of CI and V12Gy were established based on previous cases and can be used as benchmark for future SRS planning. The multi-target VMAT plans produced consistently lower CI and kept the normal tissue 12Gy per target comparable to DCA plans.

A hybrid planning strategy for stereotactic body radiation therapy of early stage non-small-cell lung cancer.

Currently dynamic conformal arcs (DCA) and volumetric modulated arc therapy (VMAT) are two popular planning techniques to treat lung stereotactic body radiation therapy (SBRT) patients. Of the two, DCA has advantages in terms of multi‐leaf collimator (MLC) motion, positioning error, and delivery efficiency. However, VMAT is often the choice when critical organ sparing becomes important. We developed a hybrid strategy to incorporate DCA component into VMAT planning, results were compared with DCA and VMAT plans. Four planning techniques were retrospectively simulated for 10 lung SBRT patients: DCA, Hybrid‐DCA (2/3 of the doses from DCA beams), Hybrid‐VMAT (2/3 of the doses from VMAT beams) and VMAT. Plan complexity was accessed by modulation complexity score (MCS). Conformity index (CI) for the planning target volume (PTV), V 20 and V 5 for the lung, V 30 for the chestwall, and maximum dose to all other critical organs were calculated. Plans were compared with regard to these metrics and measured agreement between the planned and delivered doses. DCA technique did not result in acceptable plan quality due to target location for five patients. Hybrid‐DCA produced one unacceptable plan, and Hybrid‐VMAT and VMAT produced no unacceptable plans. The CI improved with increasing VMAT usage, as did the dose sparing to critical structures. Compared to the VMAT technique, a total MU reduction of 14%, 25% and 37% were found for Hybrid‐VMAT, Hybrid‐DCA and DCA techniques for 54 Gy patient group, and 9%, 23% and 34% for 50 Gy patient group, suggesting improvement in delivery efficiency with increasing DCA usage. No significant variations of plan complexity were observed between Hybrid‐DCA and Hybrid‐VMAT (P = 0.46 from Mann–Whitney U‐test), but significant differences were found among DCA, Hybrid and VMAT (P < 0.05). Better agreements between the planned and delivered doses were found with more DCA contributions. By adding DCA components to VMAT planning, hybrid technique offers comparable dosimetry to full VMAT, while increasing delivery efficiency and minimizing MLC complexity.

ACCURACY OF CONTOURING BY NEURO-ONCOLOGISTS FOR DELIVERY OF FRACTIONATED STEREOTACTIC RADIOTHERAPY (FSRT) AND STEREOTACTIC RADIOSURGERY (SRS) FOR BENIGN INTRACRANIAL CONDITIONS; WHAT DO THE NEURO-RADIOLOGIST AND NEURO-SURGEON ADD?

Variability exists between clinical oncologists when contouring gross tumour volume (GTV) and normal tissue organs at risk (OAR) volumes. This variability is the ‘weakest link’ in the context of the highly conformal and highly accurate treatment delivery used for SRS and fSRT. In 2016-17NHS England commissioned 17 SRS centres. The service specification mandates “treatment protocols will ensure that target definition is performed by either a sub-specialised neuro-surgeon and / or neuro-oncologist (clinical oncologist) with input from a neuro-radiologist before a treatment plan is created.” To evaluate the additional contribution by the neuro radiologist we analysed contouring conformality for 90 patients with benign conditions treated in our centre (September 2014 – February 2018) using fSRT or SRS. GTV margins contoured by the clinical oncologist were copied and amended with the neuro-radiologist and sometimes neurosurgeon. Clinical target volumes (CTV) were added depending on the tumour type and grade, 1 mm margin was added to CTV for the planning target volume (PTV). The 90 patients included 71 meningioma, 10 pituitary adenoma, 6 craniopharyngioma, 3 other. Doses used were: 45–59.4 Gy in 30–33 fractions for fSRT and 14-16Gy for SRS. We used Eclipse TPS (v13.7) for Varian (Palo Alto, CA) Clinac iX with millennium MLC (5 mm) and Exactrac imaging system (Brainlab, Munich DE). All plans were created either using dynamic conformal arc (DCA) or VMAT RapidArc (RA) techniques with 6 MV photons and calculated using AAA (v10) on a 1 mm dose grid. Values for the final treated GTV and PTV (A) were compared with the GTV and PTV that were generated by the Clinical oncologist alone (B) will be compared using the Conformity analysis consisted of Jaccard coefficient, Dice coefficient, Geographical Miss and Discordance index as defined below. Results will be presented.

THE ACCURACY OF BRAIN METASTASES CONTOURING FOR STEREOTACTIC RADIOSURGERY (SRS) BY NEURO-ONCOLOGISTS; WHAT DO THE NEURO-RADIOLOGIST AND NEURO-SURGEON ADD?

Variability exists between clinical oncologists when contouring gross tumour volume (GTV) and normal tissue organs at risk (OAR) volumes. This variability is the ‘weakest link’ in the context of the highly conformal and highly accurate treatment delivery used for SRS. In 2016-17NHS England commissioned 17 SRS centres. The service specification mandates “treatment protocols will ensure that target definition is performed by either a sub-specialised neuro-surgeon and / or neuro-oncologist (clinical oncologist) with input from a neuro-radiologist before a treatment plan is created.”. To evaluate the additional contribution by the neuro radiologist we analysed contouring conformality for all 47 patients treated in our centre between June 2017 and February 2018. GTV margins were contoured first by the clinical oncologist, and then copied and amended with input from the neuroradiologist and sometimes neurosurgeon. 1 mm margin was added to the GTV for the planning target volume (PTV). 47 patients, each with 1–4 metastases/resection cavities, resulted in treatment plans for 67 metastases/cavities. Doses used were: 15-24Gy in 1 fraction, 24Gy in 3 fractions or 25-30Gy in 5 fractions (if close to critical optic structures or brainstem). We used Eclipse TPS (v13.7) for Varian (Palo Alto, CA) Clinac iX with millennium MLC (5 mm) and Exactrac imaging system (Brainlab, Munich DE). All plans were created either using dynamic conformal arc (DCA) or VMAT RapidArc (RA) techniques with 6 MV photons and calculated using AAA (v10) on a 1 mm dose grid. Values for the final treated GTV and PTV (A) were compared with the GTV and PTV that were generated by the Clinical oncologist alone (B) will be compared using the Conformity analysis consisted of Jaccard coefficient, Dice coefficient, Geographical Miss and Discordance index as defined below. Results will be presented.

Radionécrose après radiothérapie stéréotaxique d’une métastase cérébrale de 3 cm : la technique peut-elle améliorer les résultats dosimétriques ?

Stereotactic radiotherapy is a major issue in the management of brain metastases. Radionecrosis is a major concern, especially for large lesions. Optimizing dosimetric parameters is essential to allow optimal local control rate while minimizing potential toxicity. We report the case of a 30-mm brain metastases treated with stereotactic radiotherapy after initial whole brain radiotherapy, complicated with symptomatic radionecrosis. A dose of 24Gy in three fractions on the 80% isodose were delivered using a dynamic conformal arc technique (Novalis TX®). We realized a dosimetric comparison with: (i) optimization of initial conformal arc plan, (ii) volumetric modulated arctherapy with coplanar arcs and (iii) volumetric modulated arctherapy with coplanar and non-coplanar arcs. The optimal dose planning in terms of planning target volume coverage (99.2%) and normal brain sparing (V24Gy=0.4cm3, V18Gy=6.5cm3, V10Gy=25.4cm3, V5=83.9cm3) was obtained with volumetric modulated arctherapy with coplanar and non-coplanar arcs. Volumetric modulated arctherapy-based stereotactic irradiation with coplanar and non-coplonar arcs seems an interesting option for the treatment of large brain metastases to optimize dosimetric parameters.

A dosimetric comparison of linac-based stereotactic fractionated radiotherapy techniques for pituitary adenoma and craniopharyngioma

AbstractAimTo compare the dosimetric outcomes of linear accelerator-based stereotactic radiotherapy (SRT) techniques—static conformal field (SCF), static conformal arc (SCA) and dynamic conformal arc (DCA), for treating pituitary adenoma and craniopharyngioma.Materials and methodsComputer image sets of 20 patients with pituitary adenoma or craniopharyngioma and treated with post-operative SRT were selected for this study. For each dataset, three SRT plans, with SCF, SCA and DCA techniques were generated using Brain LAB, iPlan RT V.4.5.3, TPS software. The conformity index (CI), homogeneity index (HI), quality of coverage of the target, dose–volume histograms for the target and organs at risk (OARs) and the time taken to deliver treatment was compared across three sets of plan.ResultsThere were 12 patients with pituitary adenoma and eight with craniopharyngioma. The CI and HI were comparable across three techniques. The quality of coverage was superior in DCA technique. OARs were better spared in SCF and DCA techniques. Time taken to deliver treatment was least in SCF technique.ConclusionsThe linac-based SRT techniques SCF, SCA and DCA are efficient in delivering highly conformal and homogenous dose to the target in pituitary adenoma and craniopharyngioma. Among these three techniques, SCF and DCA had acceptable quality of coverage. The dose received by OARs was least in the SCF technique.

Does intensity modulation increase target dose calculation errors of conventional algorithms for lung SBRT?

PurposeConventional dose algorithms (Type A and Type B) for lung SBRT can display considerable target dose errors compared to Type‐C algorithms. Intensity‐modulated techniques (IMRT/VMAT) are increasingly being utilized for lung SBRT. Therefore, our study aimed to assess whether intensity modulation increased target dose calculation errors by conventional algorithms over conformal techniques.MethodsTwenty lung SBRT patients were parallely planned with both IMRT and dynamic conformal arc (DCA) techniques using a Type‐A algorithm, and another 20 patients were parallely planned with IMRT, VMAT, and DCA using a Type‐B algorithm. All 100 plans were recalculated with Type‐C algorithms using identical beam and monitor unit settings, with the Type‐A/Type‐B algorithm dose errors defined using Type‐C recalculation as the ground truth. Target dose errors for PTV and GTV were calculated for a variety of dosimetric end points. Using Wilcoxon signed‐rank tests (p < 0.05 for statistical significance), target dose errors were compared between corresponding IMRT/VMAT and DCA plans for the two conventional algorithms. The levels of intensity modulation were also evaluated using the ratios of MUs in the IMRT/VMAT plans to those in the corresponding DCA plans. Linear regression was used to study the correlation between intensity modulation and relative dose error magnitudes.ResultsOverall, larger errors were found for the Type‐A algorithm than for the Type‐B algorithm. However, the IMRT/VMAT plans were not found to have statistically larger dose errors from their corresponding DCA plans. Linear regression did not identify a significant correlation between the intensity modulation level and the relative dose error.ConclusionIntensity modulation did not appear to increase target dose calculation errors for lung SBRT plans calculated with conventional algorithms.