Dynamic Conformal Arc Therapy Research Articles

Interfractional target changes in brain metastases during 13-fraction stereotactic radiotherapy

BackgroundThe risk for radiation necrosis is lower in fractionated stereotactic radiotherapy (SRT) than in conventional radiotherapy, and 13-fraction SRT is our method of choice for the treatment of brain metastases ≥ around 2 cm or patients who are expected to have a good prognosis. As 13-fraction SRT lasts for at least 17 days, adaptive radiotherapy based on contrast-enhanced mid-treatment magnetic resonance imaging (MRI) is often necessary for patients undergoing 13-fraction SRT. In this study, we retrospectively analyzed interfractional target changes in patients with brain metastases treated with 13-fraction SRT.MethodsOur analyses included data from 23 patients and 27 metastatic brain lesions treated with 13-fraction SRT with dynamic conformal arc therapy. The peripheral dose prescribed to the planning target volume (PTV) was 39–44.2 Gy in 13-fractions. The gross tumor volume (GTV) of the initial SRT plan (initial GTV), initial PTV, and modified GTV based on the mid-treatment MRI scan (mid-treatment GTV) were assessed.ResultsThe median initial GTV was 3.8 cm3 and the median time from SRT initiation to the mid-treatment MRI scan was 6 days. Compared to the initial GTV, the mid-treatment GTV increased by more than 20% in five lesions and decreased by more than 20% in five lesions. Interfractional GTV volume changes of more than 20% were not significantly associated with primary disease or the presence of cystic components/necrosis. The mid-treatment GTV did not overlap perfectly with the initial PTV in more than half of the lesions.ConclusionsCompared to the initial GTV, the mid-treatment GTV changed by more than 20% in almost one-third of lesions treated with 13-fraction SRT. As SRT usually generates a steep dose gradient as well as increasing the maximum dose of PTV compared to conventional radiotherapy, assessment of the volume and locational target changes and adaptive radiotherapy should be considered as the number of fractions increases.

Technical Note: Collimator angle optimization for multiple brain metastases in dynamic conformal arc treatment planning

To reduce the exposed area by the multileaf collimator between lesions for single-isocenter dynamic conformal arc (DCA) therapy for stereotactic radiosurgery treatment of multiple brain metastases by optimizing the collimator angle orientation. In particular, this is achieved by the avoidance of collimator angles where multiple lesions are exposed by the same leaf pairs. An algorithm that estimates the quality of an arc by considering the target projections onto the plane perpendicular to the central axis of the arc beam. A penalty proportional to the exposure of healthy tissue between metastases is assigned to each control point and each feasible collimator angle from a discretized set of angles. The algorithm can generate two outputs: the fixed optimal collimator angle over all the control points, or the optimal collimator angle trajectory through all the control points considering the rotation speed of the collimator. The first output is based on explicit enumeration of all collimator angles, and the second one generates the optimal trajectory using dynamic programming to find the globally optimal solution with respect to the objective function cost. The algorithm was validated on eight clinical cases having a different number of cranial metastases: two metastases (n=1), three metastases (n=5), four metastases (n=1), and five metastases (n=1). Plans with optimized fixed collimator angles and plans with optimized dynamic collimator trajectories were compared between each other. When comparing optimal dynamic trajectories to fixed optimal collimator trajectories, the resulting plans demonstrated a total reduction of the exposed area between lesions over the entire beam configuration from 21.7% up to 71.3%; similarly, beam-wise reductions ranging from 5.83% to over 90% have been registered. Collimator angle optimization has the potential to reduce the magnitude of the exposed area between lesions in an efficient way for non-isocentric treatments where multiple lesions are treated simultaneously. Dynamic trajectories are capable of limiting the island blocking problem more than optimal fixed trajectories by exploiting the extra degree of freedom of rotating the multileaf collimator. The algorithm can also lead to time saving during the treatment planning process.

Technical Note: A 3D-printed phantom for radiochromic film evaluation of moving lung tumor SBRT without dose convolution.

A common dosimetric quality assurance (QA) method in stereotactic body radiation therapy (SBRT) of lung tumors is to use lung phantoms with radiochromic film. However, in most phantoms, the film moves with the tumor, leading to the blurring effect. This technical note presents the QA performance of a novel phantom in which the film is fixed; this phantom can be used for both patient-specific QA and end-to-end testing. Lung tumor motion was simulated with the CIRS Model 008A phantom. A lung-equivalent insert that consisted of a fixed radiochromic film around which a 2-cm tumor moved in the inferior/superior direction (i.e., mimicking respiration-induced tumor motion) was generated by 3D printing. Two common SBRT plans [dynamic conformal arc (DCA) and volumetric-modulated arc therapy (VMAT)] were calculated on the average intensity projection (AIP) image set in Varian Eclipse using the dose calculation algorithm Acuros XB. The plans were delivered by a Varian TrueBeam STx accelerator using 6-MV flattening filter-free energy. EBT3 films were used for treatment-dose verification. The measured and planned dose distributions were compared by using the local gamma index at 3% and 2 mm. Mean gamma pass rates of film and planned dose distributions were all ≥95%. DCA and VMAT plans did not differ in gamma pass rates. Planned and measured dose distributions agreed well, as did planned and measured gamma maps. With this new insert, measured and planned dose distributions were very similar, which supports the current view in the field that dose calculations on AIP image sets account sufficiently for tumor motion during treatment. The phantom also performed well despite challenging breathing parameters (large tumor amplitude and slow breathing rate) and the application of a complex treatment technique (VMAT). This phantom could facilitate clinical and end-to-end film-based dosimetric QA for lung SBRT. Twenty-seven TH- Radiation dose measurement devices. Eleven Phantoms for dosimetric measurement.

Simultaneous stereotactic radiosurgery of multiple brain metastases using single-isocenter dynamic conformal arc therapy: a\xa0prospective monocentric registry trial

BackgroundSingle-isocenter dynamic conformal arc (SIDCA) therapy is a technically efficient way of delivering stereotactic radiosurgery (SRS) to multiple metastases simultaneously. This study reports on the safety and feasibility of linear accelerator (LINAC) based SRS with SIDCA for patients with multiple brain metastases.MethodsAll patients who received SRS with this technique between November 2017 and June 2019 within a prospective registry trial were included. The patients were irradiated with a dedicated planning tool for multiple brain metastases using a LINAC with a 5 mm multileaf collimator. Follow-up was performed every 3 months, including clinical and radiological examination with cranial magnetic resonance imaging (MRI). These early data were analyzed using descriptive statistics and the Kaplan–Meier method.ResultsA total of 65 patients with 254 lesions (range 2–12) were included in this analysis. Median beam-on time was 23 min. The median follow-up at the time of analysis was 13 months (95% CI 11.1–14.9). Median overall survival and median intracranial progression-free survival was 15 months (95% CI 7.7–22.3) and 7 months (95% CI 3.9–10.0), respectively. Intracranial and local control after 1 year was 64.6 and 97.5%, respectively. During follow-up, CTCAE grade I adverse effects (AE) were experienced by 29 patients (44.6%; 18 of them therapy related, 27.7%), CTCAE grade II AEs by four patients (6.2%; one of them therapy related, 1.5%), and CTCAE grade III by three patients (4.6%; none of them therapy related). Two lesions (0.8%) in two patients (3.1%) were histopathologically proven to be radiation necrosis.ConclusionSimultaneous SRS using SIDCA seems to be a feasible and safe treatment for patients with multiple brain metastases.

Dosimetric study between a single isocenter dynamic conformal arc therapy technique and Gamma Knife radiosurgery for multiple brain metastases treatment: impact of target volume geometrical characteristics

PurposeTo compare linac-based mono-isocentric radiosurgery with Brainlab Elements Multiple Brain Mets (MBM) SRS and the Gamma Knife using a specific statistical method and to analyze the dosimetric impact of the target volume geometric characteristics. A dose fall-off analysis allowed to evaluate the Gradient Index relevancy for the dose spillage characterization.Material and methodsTreatments were planned on twenty patients with three to nine brain metastases with MBM 2.0 and GammaPlan 11.0. Ninety-five metastases ranging from 0.02 to 9.61 cc were included. Paddick Index (PI), Gradient Index (GI), dose fall-off, volume of healthy brain receiving more than 12 Gy (V12Gy) and DVH were used for the plan comparison according to target volume, major axis diameter and Sphericity Index (SI). The multivariate regression approach allowed to analyze the impact of each geometric characteristic keeping all the others unchanged. A parallel study was led to evaluate the impact of the isodose line (IDL) prescription on the MBM plan quality.ResultsFor mono-isocentric linac-based radiosurgery, the IDL around 70–75% was the best compromise found. For both techniques, the GI and the dose fall-off decreased with the target volume. In comparison, PI was slightly improved with MBM for targets < 1 cc or SI > 0.78. GI was improved with GP for targets < 2.5 cc. The V12Gy was higher with MBM for lesions > 0.4 cc or SI < 0.84 and exceeded 10 cc for targets > 5 cc against 6.5 cc with GP. The presence of OAR close to the PTV had no impact on the dose fall off values. The dose fall-off was higher for volumes < 3.8 cc with GP which had the sharpest dose fall-off in the infero-superior direction up to 30%/mm. The mean beam-on time was 94 min with GP against 13 min with MBM.ConclusionsThe dose fall-off and the V12Gy were more relevant indicators than the GI for the low dose spillage assessment. Both evaluated techniques have comparable plan qualities with a slightly improved selectivity with MBM for smaller lesions but with a healthy tissues sparing slightly favorable to GP at the expense of a considerably longer irradiation time. However, a higher healthy tissue exposure must be considered for large volumes in MBM plans.

Comparison of Phase-Gated and Amplitude-Gated Dose Delivery to a Moving Target using Gafchromic EBT3 Film.

Introduction:This study compared phase-gated and amplitude-gated dose deliveries to the moving gross tumor volume (GTV) in lung stereotactic body radiation therapy (SBRT) using Gafchromic External Beam Therapy (EBT3) dosimetry film.Materials and Methods:Eighty treatment plans using two techniques (40 phase gated and 40 amplitude gated) were delivered using dynamic conformal arc therapy (DCAT). The GTV motion, breathing amplitude, and period were taken from 40 lung SBRT patients who performed regular breathing. These parameters were re-simulated using a modified Varian breathing mini phantom. The dosimetric accuracy of the phase- and amplitude-gated treatment plans was analyzed using Gafchromic EBT3 dosimetry film. The treatment delivery efficacy was analyzed for gantry rotation, number of monitor unit (MU), and target position per triggering window. The time required to deliver the phase- and amplitude-gated treatment techniques was also evaluated.Results:The mean dose (range) per fraction was 16.11 ± 0.91 Gy (13.04–17.50 Gy) versus 16.26 ± 0.83 Gy (13.82–17.99 Gy) (P < 0.0001) for phase- and amplitude-gated delivery. The greater difference in the gamma passing rate was 1.2% ±0.4% in the amplitude-gated compared to the phase gated. The gantry rotation per triggering time (tt) was 2° ±1° (1.2°–3°) versus 5° ±1° (3°–6°) (P < 0.0001) and MU per tt was 10 ± 3 MU (6–13 MU) versus 24 ± 7 MU (12–32 MU) (P < 0.0001), for phase- versus amplitude-gated techniques. A 90 beam interruption in the phase-gated technique impacted the treatment delivery efficacy, increasing the treatment delivery time in the phase gated for 1664 ± 202 s 1353–1942 s) compared to 36 interruptions in the amplitude gated 823 ± 79 s (712–926 s) (P < 0.0001).Conclusion:Amplitude-gated DCAT allows for better dosimetric accuracy over phase-gated treatment patients with regular breathing patterns.

Which Is Better for Liver SBRT: Dosimetric Comparison Between DCAT and VMAT for Liver Tumors.

Stereotactic body radiotherapy (SBRT) is currently well-adopted as a curative treatment for primary and metastatic liver tumors. Among SBRT methods, dynamic conformal arc therapy (DCAT) and volumetric-modulated arc therapy (VMAT) are the most preferred methods. In this study, we report a comparison study measuring the dose distribution and delivery efficiency differences between DCAT and VMAT for liver SBRT. All patients who were treated with SBRT for primary or metastatic liver tumors with a curative aim between January 2016 and December 2017 at DIRAMS were enrolled in the study. For all patients, SBRT plans were designed using the Monte Carlo (MC) algorithm in Monaco treatment planning system (version 5.1). The planning goals were set according to the RTOG 0813, RTOG 0915, and RTOG 1112 protocols. A plan comparison was made on the metrics of dose volume histogram, planning and delivery efficiency, monitor unit (MU), and dosimetric indices. PTV coverage was evaluated using the following: Dmean, D95%, D98%, D2%, D50%, Dmax, V95%, heterogeneity index (HI), and conformality index (CI). For DCAT and VMAT, respectively, the Dmean was 5942.8 ± 409.3 cGy and 5890.6 ± 438.8 cGy, D50% was 5968.8 ± 413.1 cGy and 5954.3 ± 405.2 cGy, and CI was 1.05 ± 0.05 and 1.03 ± 0.04. The D98% and V95% were 5580.0 ± 465.3 cGy and 20.4 ± 12.0 mL for DCAT, and 5596.0 ± 478.7 cGy and 20.5 ± 12.0 mL for VMAT, respectively. For normal liver, V40, V30, V20, V17, V5, Dmean, Dmax were evaluated for comparison. The V30, V20, and V10 were significantly higher in DCAT; other parameters of normal livers showed no statistically significant differences. For evaluation of intermediate dose spillage, D2cm(%) and R50% of DCAT and VMAT were 45.8 ± 7.9 and 5.6 ± 0.9 and 45.1 ± 6.7 and 5.5 ± 1.2, respectively. Planning and delivery efficiency were evaluated using MU, Calculation time, and Delivery time. DCAT had shorter Calculation time and Delivery time with smaller MU. MU was smaller in DCAT and the average difference was 300.1 MU. For liver SBRT, DCAT is an effective alternative to VMAT plans that could meet the planning goals proposed by the RTOG SBRT protocol and increases plan and delivery effectiveness, while also ignoring the interplay effect.

A robust VMAT delivery solution for single-fraction lung SABR utilizing FFF beams minimizing dosimetric compromise.

Peripheral lung lesions treated with a single fraction of stereotactic ablative body radiotherapy (SABR) utilizing volumetric modulated arc therapy (VMAT) delivery and flattening filter‐free (FFF) beams represent a potentially high‐risk scenario for clinically significant dose blurring effects due to interplay between the respiratory motion of the lesion and dynamic multi‐leaf collimators (MLCs). The aim of this study was to determine an efficient means of developing low‐modulation VMAT plans in the Eclipse treatment planning system (v15.5, Varian Medical Systems, Palo Alto, USA) in order to minimize this risk, while maintaining dosimetric quality. The study involved 19 patients where an internal target volume (ITV) was contoured to encompass the entire range of tumor motion, and a planning target volume (PTV) created using a 5‐mm isotropic expansion of this contour. Each patient had seven plan variations created, with each rescaled to achieve the clinical planning goal for PTV coverage. All plan variations used the same field arrangement, and consisted of one dynamic conformal arc therapy (DCAT) plan, and six VMAT plans with varying degrees of modulation restriction, achieved through utilizing different combinations of the aperture shape controller (ASC) in the calculation parameters, and monitor unit (MU) objective during optimization. The dosimetric quality was assessed based on RTOG conformity indices (CI100/CI50), as well as adherence to dose–volume metrics used clinically at our institution. Plan complexity was assessed based on the modulation factor (MU/cGy) and the field edge metric. While VMAT plans with the least modulation restriction achieved the best dosimetry, it was found that there was no clinically significant trade‐off in terms of dose to organs at risk and conformity by reducing complexity. Furthermore, it was found that utilizing the ASC and MU objective could reduce plan complexity to near‐DCAT levels with improved dosimetry, which may be sufficiently robust to overcome the interplay effect.

Stereotactic radiotherapy for small and very small tumours (≤1 to ≤3 cc): evaluation of the influence of volumetric-modulated arc therapy in comparison to dynamic conformal arc therapy and 3D conformal radiotherapy as a function of flattened and unflattened beam models

AbstractPurpose:The objective of this article is to evaluate the dosimetric efficacy of volumetric modulated arc therapy (VMAT) in comparison to dynamic conformal arc therapy (DCAT) and 3D conformal radiotherapy (3DCRT) for very small volume (≤1 cc) and small volume (≤3 cc) tumours for flattened (FF) and unflattened (FFF) 6 MV beams.Materials and methods:A total of 21 patients who were treated with single-fraction stereotactic radiosurgery, using either VMAT, DCAT or 3DCRT, were included in this study. The volume categorisation was seven patients each in &lt;1, 1–2 and 2–3 cc volume. The treatment was planned with 6 MV FF and FFF beams using three different techniques: VMAT/Rapid Arc (RA) (RA_FF and RA_FFF), dynamic conformal arc therapy (DCA_FF and DCA_FFF) and 3DCRT (Static_FF and Static_FFF). Plans were evaluated for target coverage (V100%), conformity index, homogeneity index, dose gradient for 50% dose fall-off, total MU and MU/dose ratio [intensity-modulated radiotherapy (IMRT) factor], normal brain receiving &gt;12 Gy dose, dose to the organ at risk (OAR), beam ON time and dose received by 12 cc of the brain.Result:The average target coverage for all plans, all tumour volumes (TVs) and delivery techniques is 96·4 ± 4·5 (range 95·7 ± 6·1–97·5 ± 2·9%). The conformity index averaged over all volume ranges &lt;1, 2, 3 cc&gt; varies between 0·55 ± 0·08 and 0·68 ± 0·04 with minimum and maximum being exhibited by DCA_FFF for 1 cc and Static_FFF/RA_FFF for 3 cc tumours, respectively. Mean IMRT factor averaged over all volume ranges for RA_FF, DCA_FF and Static_FF are 3·5 ± 0·8, 2·0 ± 0·2 and 2·0 ± 0·2, respectively; 50% dose fall-off gradient varies in the range of 0·33–0·42, 0·35–0·40 and 0·38–0·45 for 1, 2 and 3 cc tumours, respectively.Conclusion:This study establishes the equivalence between the FF and FFF beam models and different delivery techniques for stereotactic radiosurgery in small TVs in the range of ≤1 to ≤3 cc. Dose conformity, heterogeneity, dose fall-off characteristics and OAR doses show no or very little variation. FFF could offer only limited time advantage due to excess dose rate over an FF beam.

A newly-established SRS service: initial outcome analysis

Keywords: Stereotactic radiotherapy, Stereotactic radiosurgery, Service evaluation, Neuro-oncology, Brain metastases Introduction: Up to 65% of patients with metastatic cancer will develop brain metastases during the course of their disease1,2.The most common sites of originating primary disease are lung, melanoma, renal, breast and colorectal cancer3. Our institution has been commissioned by NHSE to deliver stereotactic radiosurgery (SRS) to benign and metastatic brain tumours since June 2017. A single-institution outcome analysis was carried out to compare local data with published outcomes for SRS to brain metastases. Method and Materials: Each patient received linac-based SRS. Prescription dose ranged from 18-27Gy in 1-3 fractions and was delivered using Rapid Arc™ or dynamic conformal arc (DCA) therapy with ExacTrac™ imaging and immobilisation. Audit was carried out on records of 146 brain metastases patients treated in the first 2 years of operation. Kaplan-Meier curves were created to plot overall survival and progression-free survival (defined as time to progression of treated brain metastases) against a number of variables, e.g. performance status at time of SRS, status of extracranial disease at time of SRS, number of metastases treated, total tumour volume treated. Results: Most common sites of originating primary disease were lung (n=45, 31%), melanoma (n=34, 23%), renal (n=20, 14%), breast (n=17, 12%), lower GI (n=15, 10%). Other primary cancers such as upper GI, gynaecological origin, bladder, germ cell, pancreatic were grouped together for the purpose of this study (n=15, 10%). PTV volume ranged from 0.02cc to 24.3cc with a mean valorigin, bladder, germ cell, pancreatic were grouped together for the purpose of this study (n=15, 10%). PTV volume ranged from 0.02cc to 24.3cc with a mean value of 4.05cc and standard deviation of 5.4cc. 17 patients had died since SRS. Of those, 47% (N=8) were WHO performance status (PS) II, 35% (n=6) were PS I and 17% (n=3) were PS 0 at the time of SRS. Of those who died, mean overall survival for PS 0 patients was 4.6 months, PS I was 6 months and PS II was 5.5 months. Conclusion and Discussion: Results are currently being analysed for statistical significance. Numerical

Dynamic conformal arcs for lung stereotactic body radiation therapy: A comparison with volumetric‐modulated arc therapy

This study constitutes a feasibility assessment of dynamic conformal arc (DCA) therapy as an alternative to volumetric‐modulated arc therapy (VMAT) for stereotactic body radiation therapy (SBRT) of lung cancer. The rationale for DCA is lower geometric complexity and hence reduced risk for interplay errors induced by respiratory motion. Forward planned DCA and inverse planned DCA based on segment‐weight optimization were compared to VMAT for single arc treatments of five lung patients. Analysis of dose‐volume histograms and clinical goal fulfillment revealed that DCA can generate satisfactory and near equivalent dosimetric quality to VMAT, except for complex tumor geometries. Segment‐weight optimized DCA provided spatial dose distributions qualitatively similar to those for VMAT. Our results show that DCA, and particularly segment‐weight optimized DCA, may be an attractive alternative to VMAT for lung SBRT treatments if the patient anatomy is favorable.

Monte Carlo simulation of a 2D dynamic multileaf collimator to improve the plan quality in radiotherapy plan: a proof-of-concept study

The leaf width of a multileaf collimator (MLC) determines the dose conformity to the target volume. The objective of this study was to investigate the feasibility of a two-dimensional dynamic MLC (2DDMLC) to improve the treatment plan quality with a fixed leaf width. The treatment head of the Clinac™ linear accelerator with the Millennium 120™ MLC was modelled with the Geant4 (for GEometry ANd Tracking) tollkit using the Monte Carlo (MC) method. The 2DDMLC produces a beam aperture by moving the MLC bank vertically to the leaf movement. Thus, the effect of the 2DDMLC motion on beam divergence and beam fluence resolution was evaluated by comparing the dose distributions between the conventional MLC motion and the 2DDMLC. Finally, the 2DDMLC was employed for dynamic conformal arc therapy for 13 brain cancer patients. The dose-volumetric parameters, including the dose delivered to 98% of the target volume (D98%), percent volume given 20% of the prescribed dose (V20%), and conformity index (CI) were compared with those of the conventional MLC. For the 6 MV beam of the MC model, the depth dose and lateral dose distribution differed by less than 2% between the simulation and measurement. The 2DDMLC did not significantly influence beam divergence and sharpened the beam. In clinical use, the dose delivered to the target was almost identical between the 2DDMLC and conventional MLC (D98% = 29.74 Gy versus 29.71 Gy, p = 0.18). The CI was improved with the use of the 2DDMLC (CI = 1.49 versus 1.47, p = 0.14). Moreover, irradiation of normal tissue was reduced with the 2DDMLC compared with conventional MLC (V20% = 17.22% versus 17.45%, p < 0.001). The 2DDMLC improved the dose conformity to the target volume and reduced the irradiation of the normal tissue compared with the conventional MLC.

83 Dosimetric study before implementation of stereotactic treatment conditions on a future VersaHD linear accelerator: Impact of table rotations in VMAT and DCA

Introduction. This study was conducted in the context of a forthcoming VersaHD accelerator installation for patients treated with intracranial stereotactic treatment conditions. The future MLC was implemented in our TPS. The aim of this work was to quantify the interest of table rotation for two treatment techniques: Dynamic Conformal Arctherapy (DCA) and Volumetric Modulated Arc Therapy treatment (VMAT).

A simple knowledge-based tool for stereotactic radiosurgery pre-planning.

We studied the dosimetry of single‐isocenter treatment plans generated to treat a solitary intracranial lesion using linac‐based stereotactic radiosurgery (SRS). A common metric for evaluating SRS plan quality is the volume of normal brain tissue irradiated by a dose of at least 12 Gy (V12), which is important because multiple studies have shown a strong correlation between V12 and incidence of radiation necrosis. Unrealistic expectations for values of V12 can lead to wasted planning time. We present a model that estimates V12 without having to construct a full treatment plan. This model was derived by retrospectively analyzing 50 SRS treatment plans, each clinically approved for delivery using circular collimator cone arc therapy (CAT). Each case was re‐planned for delivery via dynamic conformal arc therapy (DCAT), and then scaling arguments were used to extend dosimetric data to account for different prescription dose (PD) values (15, 18, 21, or 24 Gy). We determined a phenomenological expression for the total volume receiving at least 12 Gy (TV12) as a function of both planning target volume (PTV) and PD: TV12/1cc=n∗PD/1Gy+d∗PTV/1cca∗PD/1Gyc, where a,c,n,d are fit parameters, and a separate set of values is determined for each plan type. In addition, we generated a sequence of plots to clarify how the relationship between conformity index (CI) and TV12 depends on plan type (CAT vs DCAT), PTV, and PD. These results can be used to suggest realistic plan parameters and planning goals before the start of treatment planning. In the absence of access to more sophisticated pre‐planning tools, this model can be locally generated and implemented at relatively low cost with respect to time, money, and expertise.

Interfractional Target Volume Changes in Brain Metastases during 13-Fraction Stereotactic Radiotherapy

Advances in systemic therapy have greatly improved the prognosis of patients with brain metastases (BMs); long-term local control and reduced radiation toxicity are important. Fractionated stereotactic radiotherapy (SRT) seems to be associated with a reduced risk of radiation necrosis while providing equivalent local control compared to single-fraction or hypofractionated radiosurgery and is used in our institution for patients with a good prognosis or large BMs. As 13-fraction SRT lasts at least 17 days, we conduct adaptive replanning as necessary, based on contrast-enhanced mid-treatment magnetic resonance imaging (MRI) performed during 13-fraction SRT. We retrospectively analyzed interfractional volume changes and clinical outcomes of BMs treated with 13-fraction SRT. In total, this study included 23 patients with 27 BMs who received 13-fraction SRT with dynamic conformal arc therapy between July 2015 and November 2018. We excluded patients who received postoperative radiotherapy for the surgical cavity, no contrast-enhanced mid-treatment MRI, or single isocenter volumetric-modulated arc therapy for multiple BMs. The median patient age was 70 years and the median Karnofsky Performance Status was 90. There were 15 lung-cancer patients. The planning target volume (PTV) was established by adding a 1-mm margin to the gross tumor volume (GTV), defined as the volume of the contrast-enhancing lesion. The peripheral dose prescribed to the PTV was 39–44.2 Gy in 13 fractions. The GTV of the initial SRT plan (initial GTV) and the modified GTV based on the mid-treatment MRI scan (mid-treatment GTV) were compared. The median initial GTV was 3.8 (range 0.2–26.7) cm3 and the median period from the start of SRT to the mid-treatment MRI scan was 6 (range 3–11) days. In 10 lesions, the mid-treatment GTV had changed by more than 20% (5 had increased and 5 decreased) relative to the initial GTV. Adaptive replanning was conducted for 14 lesions. Of the 10 lesions that had increased or decreased in size by more than 20%, 8 received adaptive replanning. There was no significant relationship between an interfractional GTV volume change of more than 20%, primary disease (lung cancer vs. other cancers), and the presence of necrotic or cystic components. Regarding clinical outcomes, both overall survival rates and local control rates at 6 months were 95.0%, with a median follow-up period of 7.2 months. Two patients experienced grade 3 cerebral edema, two experienced grade 3 seizures, and four experienced grade 1 brain necrosis (CTCAE v5). The mid-treatment GTV changed by more than 20% relative to the initial GTV in almost one-third of lesions receiving 13-fraction SRT. More cases and long-term follow up are required to verify the impact of interfractional volume changes and adaptive replanning on clinical outcomes.

On the need for tuning the dosimetric leaf gap for stereotactic treatment plans in the Eclipse treatment planning system.

The dosimetric leaf gap (DLG) and tongue‐and‐groove (T&G) effects are critical aspects in the modeling of multileaf collimators (MLC) in the treatment planning system (TPS). In this study, we investigated the dosimetric impact of limitations associated with the T&G modeling in stereotactic plans and its relationship with the need for tuning the DLG in the Eclipse TPS. Measurements were carried out using Varian TrueBeam STx systems from two different institutions. Test fields presenting MLC patterns with several MLC gap sizes (meanGap) and different amounts of T&G effect (TGi) were first evaluated. Secondly, dynamic conformal arc (DCA) and volumetric modulated arc therapy (VMAT) deliveries of stereotactic cases were analyzed in terms of meanGap and TGi. Two DLG values were used in the TPS: the measured DLG (DLGmeas) and an optimal DLG (DLGopt). Measured and calculated doses were compared according to dose differences and gamma passing rates (GPR) with strict local gamma criteria of 2%/2 mm. The discrepancies were analyzed for DLGmeas and DLGopt, and their relationships with both TGi and meanGap were investigated. DCA arcs involved significantly lower TGi and larger meanGap than VMAT arcs (P < 0.0001). By using DLGmeas in the TPS, the dose discrepancies increased as TGi increased and meanGap decreased for both test fields and clinical plans. Dose discrepancies dramatically increased with the ratio TGi/meanGap. Adjusting the DLG value was then required to achieve acceptable calculations and configuring the TPS with DLGopt led to an excellent agreement with median GPRs (2%/2 mm) > 99% for both institutions. We also showed that DLGopt could be obtained from the results of the test fields. We demonstrated that the need for tuning the DLG is due to the limitations of the T&G modeling in the Eclipse TPS. A set of sweeping gap tests modified to incorporate T&G effects can be used to determine the optimal DLG value.

Single isocenter stereotactic radiosurgery for patients with multiple brain metastases: dosimetric comparison of VMAT and a dedicated DCAT planning tool

BackgroundIn this dosimetric study, a dedicated planning tool for single isocenter stereotactic radiosurgery for multiple brain metastases using dynamic conformal arc therapy (DCAT) was compared to standard volumetric modulated arc therapy (VMAT).MethodsTwenty patients with a total of 66 lesions who were treated with the DCAT tool were included in this study. Single fraction doses of 15–20 Gy were prescribed to each lesion. Patients were re-planned using non-coplanar VMAT. Number of monitor units as well as V4Gy, V5Gy and V8Gy were extracted for every plan. Using a density-based clustering algorithm, V10Gy and V12Gy and the volume receiving half of the prescribed dose were extracted for every lesion. Gradient indices and conformity indices were calculated. The correlation of the target sphericity, a measure of how closely the shape of the target PTV resembles a sphere, to the difference in V10Gy and V12Gy between the two techniques was assessed using Spearman’s correlation coefficient.ResultsThe automated DCAT planning tool performed significantly better in terms of all investigated metrics (p < 0.05), in particular healthy brain sparing (V10Gy: median 3.2 cm3 vs. 4.9 cm3), gradient indices (median 5.99 vs. 7.17) and number of monitor units (median 4569 vs. 5840 MU). Differences in conformity indices were minimal (median 0.75 vs. 0.73) but still significant (p < 0.05). A moderate correlation between PTV sphericity and the difference of V10Gy and V12Gy between the two techniques was found (Spearman’s rho = 0.27 and 0.30 for V10Gy and V12Gy, respectively, p < 0.05).ConclusionsThe dedicated DCAT planning tool performed better than VMAT in terms of healthy brain sparing and treatment efficiency, in particular for nearly spherical lesions. In contrast, VMAT can be superior in cases with irregularly shaped lesions.

197. Stereotactic Body Radiotherapy: Comparison between two IGRT systems

Purpose The aim of this work is to compare the residual setup errors measured with ExacTrac X-ray 6 degree-of-freedom (6D) and kilo-voltage cone-beam computed tomography (CBCT) for patients receiving stereotactic body radiotherapy (SBRT) at Fondazione IOM in collaboration with REM Radioterapia. The first device employs an infrared optical system that performs patients pre-positioning through body markers and isocenter coordinates, and an X-ray imaging system that verifies target position using bone anatomy. CBCT imaging allows positioning verification using not only bone anatomy but also soft tissues. Methods On a TrueBeam-Novalis STx treatment unit, from January to November 2017, 45 patients with extracranial tumours were treated with SBRT for a total of 53 lesions (some patients having more than one lesion) and 152 treatment sessions. Prescribing doses ranged from 24 to 60 Gy in 3 fractions or 20 Gy in a single fraction and planning techniques were dynamic conformal arc therapy or hybrid intensity modulated radiotherapy, with no-coplanar fields. All patients were initially located using personalized immobilization systems and setup corrections were determined and corrected by means of registrations of ExacTrac X-ray images with the corresponding digitally reconstructed radiographs using the ExacTrac 6D fusion. At the end of each treatment session, with the couch at 0°, the residual setup errors were determined by means of registrations of CBCT images with the planning CT using online 3D fusion and for each session, displacements were evaluated to analyse the residual setup errors. Results A modest difference in residual setup errors was found between ExacTrac system and CBCT (figure below). The average of the observed residual displacements for the three space directions, in absolute value, was 1.5 mm in vertical and in lateral directions, 1.2 mm in longitudinal direction. Conclusion This work showed the good agreement on the setup accuracy between ExacTrac X-ray and CBCT for patients treated with SBRT. The two systems present comparable precision, but ExacTrac offers additional benefits such the capability to quantify all rotational errors, fastest automated positioning in 6D even for no-coplanar fields and smaller doses, representing a valid alternative to CBCT with complementary information in image-guided stereotactic radiotherapy.

2 Dosimetry comparison for brain metastases treatment by stereotactic radiotherapy

Introduction Technical capabilities for brain metastases treatment in stereotactic radiotherapy are in-house Dynamic Conformal Arc Therapy (DCA, with non-coplanar arcs DCAr and without DCA0) and non-coplanar Volumetric-Modulated Arc Therapy (VMAT). This study is to decide on the most efficient method use to get the optimum PTV coverage and dose escalation, OAR sparing, especially healthy brain tissue, and minimum monitor units. Methods On Elekta’s Monaco treatment planning system, eight patients with brain metastases and an average target volume of 9.13 cc [3.58–26.5] were selected. Six of these patients were treated with DCAR / DCA0 and two with VMAT on VERSA HD radiation therapy system. For all these patients, the two other planning techniques were carried out retrospectively. Dosimetric analysis tools HI, CI, CO, OCO [1] as well as PTV coverage, PTV overdose, the 50% isodose line and healthy brain constraints ( V 12 Gy 5.9 cc, V 18 Gy 1 cc) enabled this comparison. The selected indicators were compared using non-parametric Wilcoxon’s rank tests. Results Homogeneity index is significantly higher for DCA0, as well as for DCA R compared to VMAT. The overdose for DCA0 is significantly lower than for the other two techniques. For the eight patients, the 50% isodose line is significantly smaller with DCA R . The healthy brain tissue gain (OCO) is significant using VMAT dosimetric planning rather than DCA0 (p 0.05). VMAT significantly saves healthy brain tissue for 12 Gy isodose line compared to DCA0 (p = 0.031). The same applies for DCA R compared to DCA0 (p = 0.047). Concerning average monitor units, there is 1738 UM for VMAT treatment planning against 1449 (−16%) for DCA R and 1359 (−21.8%) for DCA0. Conclusions Comparison between DCA with coplanar ( DCA r ) and non-coplanar arcs (DCA0) shows up that coplanar arcs were of significant interest for overdose at the center of PTV, for a 50% isodose line narrower and help sparing healthy brain tissue. DCA R and VMAT techniques are comparable on this last point. However, using non-coplanar arcs involves considerable additional processing time and controls. The study will be followed up with other patient data to confirm the use of VMAT treatment planning technique on target volumes from 3.58 to 26.5 cc.

26 Dosimetric comparison of dynamic conformal arc therapy with flattened beams and flattening filter free beams for SBRT lung cancer

Introduction The aim of this study was to compare dosimetric gain using dynamic conformal arc (DCA) with flattened filter (FF) and flattening filter free (FFF) beam for lung stereotactic body radiation therapy (SBRT). Methods and materials In our institution, SBRT lung cancer patients were treated with prescribed dose 60 Gy in 4 or 8 fractions. In Pinnacle 9.10 were planned two partial DCA’s for peripheral tumors and three to four partial DCA’s for central region tumors to help pull dose of the OARs (heart, esophagus, spinal cord and ribs). Treatments were delivered with 6 MV beams and dose rate 600 MU, for TrueBeam STx accelerator. Fifty patients who underwent lung SBRT treatment with DCA with FF were retrospectively analyzed using 6 MV FFF beams with dose rate of 1400 MU. We were observing the planning target volume (PTV) coverage, sparing of organ at risks (OARs) and time of treatment delivery. Results The PTV coverage (D98% > 95%) were similar, 98.3% vs 98% for FF and FFF, with 1887 MU’s vs 1860 MU’s, respectively. Dosimetric gain between FF and FFF for max dose to the heart was 1 Gy vs 0.87 Gy (p 0.0001). Max dose for spinal cord was 8.84 Gy vs 8.38 Gy (p 0.0001). Max dose for esophagus was 9.76 Gy vs 8.94 Gy (p 0.0001). Max dose for ribs was 34.16 Gy vs 31.89 Gy (p 0.0001). Time of treatment delivery was 4.50 min vs 2.43 min (p = 0.0006), for FF and FFF, respectively. Conclusion Treatment delivery with FFF DCA allows better sparing of OARs for SBRT of lung cancer patients. Shorter treatment time with FFF DCA for SBRT of lung cancer patients can reduce potentially discomfort or patient motion.