A taskforce for national improvement of innovation implementation in radiotherapy.

BACKGROUND AND PURPOSE: Previous research among Dutch radiotherapy centres (RTCs) showed that 69% of innovations was simultaneously implemented in 7/19 centres, with a success rate of 51%. However, no structure to share lessons learned about the implementation process existed. Therefore, a national Taskforce Implementation (TTI) was raised to stimulate efficient implementation of innovations. The aim of the current study was to develop and pilot-evaluate a website for facilitating mutual learning on implementation issues. MATERIAL AND METHODS: First, we made an inventory in all Dutch RTCs on their 10 most valuable innovations between 2019 and 2022. In-depth interviews, structured according to the Consolidated Framework for Implementation Research, were performed on the four most mentioned topics. A website was built, and pilot evaluated 1 year after the launch, using a qualitative survey amongst the TTI members. RESULTS: In 13/18 centres, 19 interviews were conducted on 1) automation, 2) patient participation, 3) adaptive radiotherapy 4) surface guided radiotherapy and tracking. Most innovations (13/16) were implemented with a delay, with many comparable challenges: e.g. shortage of personnel (7/16) and prioritization of projects (9/16). The website allows users to upload and search for projects, including implementation experiences. After 1 year, 14 projects were uploaded. The qualitative evaluation was largely positive with room for improvement, i.e.75 % would recommend the website to others. CONCLUSION: This study showed that RTCs experience comparable challenges when implementing innovations, thereby underlining the need for a platform to share implementation-lessons learned. The first concept of this platform was evaluated positively.

Toward planning target volume margin reduction for the prostate using intrafraction motion correction with online kV imaging and automatic detection of implanted gold seeds.

PURPOSE: The imaging application Auto Beam Hold (ABH) allows for the online analysis of 2-dimensional kV images acquired during treatment. ABH can automatically detect fiducial markers and initiate a beam interrupt. In this study, we investigate the practical use and results of this intrafraction monitoring tool for patients with prostate cancer who have implanted gold seeds treated with a RapidArc technique. METHODS AND MATERIALS: A total of 105 patients were included. For setup, the seeds were lined up using 2 orthogonal 2-dimensional kV images. After the setup procedure, ABH was applied at an interval of 3 seconds. The software requires a maximum-allowed deviation to be defined for each seed, which is referred to as a deviation limit (DL). Online, the ABH application evaluates the position of the seeds and indicates for each seed whether or not it exceeds the DL. Patients were divided in 3 groups. For the first group ABH was used with the DL at 6 mm, which corresponds to the planning target volume (PTV) margin. For the second group, the DL was set at 5 mm with an unchanged PTV margin of 6 mm. For the third group, the PTV margin was reduced to 5 mm with a DL of 5 mm. Offline, we performed an analysis of the number of beam stops and resulting re-setups. RESULTS: ABH initiated a beam interrupt 223 times (13%) during a total of 1736 sessions. By decreasing the DL from 6 mm to 5 mm, the amount of workload for re-setups increased from 6% (group 1) to 14% (groups 2 and 3). Re-setup, 3-dimensional shifts larger than the PTV margin were found in 44%, 35%, and 45% for groups 1,2, and 3, respectively. CONCLUSIONS: Intrafraction imaging of prostate position during treatment using automatic detection of implanted gold seeds was successfully implemented. PTV margins were safely reduced from 6mm to 5mm without a substantial increase in workload.

Analysis of components of variance determining probability of setup errors in CBCT-guided stereotactic radiotherapy of lung tumors.

PURPOSE: Online tumor matching for SABR lung setup requires margins for inaccuracies due to intra-fraction variability of breathing-averaged tumor position (BATP) and CBCT image guidance. We studied intra-fraction variability during SABR delivery using VMAT, corrected these for measurement inaccuracies, and quantified the CBCT image-guidance uncertainties. MATERIALS AND METHODS: For 193 fractions in 38 patients positioned without immobilization devices, CBCT scans were acquired before and after 2 arcs of a RapidArc treatment. A hidden marker test was performed to determine the accuracy of the CBCT system and an inter-observer test was performed to measure registration accuracy. Intra-fraction variability was calculated after correction for these components of variance, and the prediction interval for setup inaccuracies was determined. RESULTS: Correction for measurement inaccuracies reduced the intra-fraction variability of the BATP from 1.9 to 1.6 mm in AP, from 1.7 to 1.4 mm in SI and from 1.5 to 1.1 mm in LR direction (1 SD). Intra-fraction variability in bony anatomy after correction was ≤ 1 mm (1 SD). The 95% prediction interval to account for CBCT image-guidance uncertainties and intra-fraction variability was determined, and was found to be within our institutional PTV margins of 5 mm. CONCLUSIONS: Our findings show that it is essential to account for measurement and system inaccuracies when obtaining data for validating PTV margins from online CBCT image guidance.

Isotoxic radiosurgery planning for brain metastases.

PURPOSE/OBJECTIVE(S): Radionecrosis (RN) has previously been correlated with radiosurgery (RS) dose, lesion volume, and the volume of the brain receiving specific doses, i.e. V10-14Gy. A knowledge-based individualized estimation of the optimum RS dose has been derived based on lesional volume and brain toxicity parameters. METHODS AND MATERIALS: A prediction model for brain toxicity parameters and estimation of the optimum RS dose was derived using 30 historical linac-based dynamic conformal arc RS plans for single brain metastases (BM) (0.2-20.3cc) with risk-adapted dose prescription ranging from 15 to 24Gy. Derivation of the model followed a three-step process: (1) Derivation of formulas for the prediction of brain toxicity parameters V10-18Gy; (2) Establishing the relationship of the coefficients used for the prediction of V12Gy with prescription dose; (3) Derivation of the optimum prescription dose for a given maximum V12Gy as a function of a given lesion volume. Model validation was performed on 65 new patients with 138 lesions (44 with multiple BM) treated with non-coplanar volumetric modulated stereotactic arc treatment (VMAT). RESULTS: A linear dependence with the PTV size was found for all investigated brain toxicity parameters (V10-18Gy). Individualized RS prescription doses can be calculated for any given PTV size based on a linear relationship between V12Gy and PTV size, according to the formula PD=[V12Gy+0.96+(1.44×PTV)]/[0.12+(0.12×PTV)]. A very good correlation (R(2)=0.991) was found between the predicted V12Gy and the resulting V12Gy in 65 new patients with 138 lesions treated with non-coplanar VMAT technique in our clinic. CONCLUSIONS: A simple formula is proposed for estimation of the optimal individual RS dose for any given lesion volume for patients with (multiple) BM. This formula is based on calculation of the brain toxicity parameter, V12Gy, for the normal brain minus PTV.

Effect of Dosimetric Outliers on the Performance of a Commercial Knowledge-Based Planning Solution.

PURPOSE: RapidPlan, a commercial knowledge-based planning solution, uses a model library containing the geometry and associated dosimetry of existing plans. This model predicts achievable dosimetry for prospective patients that can be used to guide plan optimization. However, it is unknown how suboptimal model plans (outliers) influence the predictions or resulting plans. We investigated the effect of, first, removing outliers from the model (cleaning it) and subsequently adding deliberate dosimetric outliers. METHODS AND MATERIALS: Clinical plans from 70 head and neck cancer patients comprised the uncleaned (UC) ModelUC, from which outliers were cleaned (C) to create ModelC. The last 5 to 40 patients of ModelC were replanned with no attempt to spare the salivary glands. These substantial dosimetric outliers were reintroduced to the model in increments of 5, creating Model5 to Model40 (Model5-40). These models were used to create plans for a 10-patient evaluation group. Plans from ModelUC and ModelC, and ModelC and Model5-40 were compared on the basis of boost (B) and elective (E) target volume homogeneity indexes (HIB/HIE) and mean doses to oral cavity, composite salivary glands (compsal) and swallowing (compswal) structures. RESULTS: On average, outlier removal (ModelC vs ModelUC) had minimal effects on HIB/HIE (0%-0.4%) and sparing of organs at risk (mean dose difference to oral cavity and compsal/compswal were ≤0.4 Gy). Model5-10 marginally improved compsal sparing, whereas adding a larger number of outliers (Model20-40) led to deteriorations in compsal up to 3.9 Gy, on average. These increases are modest compared to the 14.9 Gy dose increases in the added outlier plans, due to the placement of optimization objectives below the inferior boundary of the dose-volume histogram-predicted range. CONCLUSIONS: Overall, dosimetric outlier removal from or addition of 5 to 10 outliers to a 70-patient model had marginal effects on resulting plan quality. Although the addition of >20 outliers deteriorated plan quality, the effect was modest. In this study, RapidPlan demonstrated robustness for moderate proportions of salivary gland dosimetric outliers.

Frameless high dose rate stereotactic lung radiotherapy: intrafraction tumor position and delivery time.

Intrafraction change in tumor position (Δ) was evaluated for stereotactic lung radiotherapy delivered with flattening filter free volumetric modulated arc therapy. In 140 fractions from 32 patients mean Δ (±SD) was -0.7±1.4 mm (vertical), -0.7±1.3 mm (longitudinal) and +0.2±1.2 mm (lateral) with mean vector 2.1±1.2 mm. Mean delivery time was 4.4±3.4 min (mean beam-on 1.9±0.4 min).

Dosimetric impact of intrafraction motion during RapidArc stereotactic vertebral radiation therapy using flattened and flattening filter-free beams.

PURPOSE: To study the dosimetric impact of relatively short-duration intrafraction shifts during a single fraction of RapidArc delivery for vertebral stereotactic body radiation therapy (SBRT) using flattened (FF) and flattening filter-free (FFF) beams. METHODS AND MATERIALS: The RapidArc plans, each with 2 to 3 arcs, were generated for 9 patients using 6-MV FF and 10-MV FFF beams with maximum dose rates of 1000 and 2400 MU/min, respectively. A total of 1272 plans were created to estimate the dosimetric consequences in target and spinal cord volumes caused by intrafraction shifts during one of the arcs. Shifts of 1, 2, and 3 mm for periods of 5, 10, and 30 seconds, and 5 mm for 5 and 10 seconds, were modelled during a part of the arc associated with high doses and steep dose gradients. RESULTS: For FFF plans, shifts of 2 mm over 10 seconds and 30 seconds could increase spinal cord Dmax by up to 6.5% and 13%, respectively. Dosimetric deviations in FFF plans were approximately 2-fold greater than in FF plans. Reduction in target coverage was <1% for 83% and 96% of the FFF and FF plans, respectively. CONCLUSION: Even short-duration intrafraction shifts can cause significant dosimetric deviations during vertebral SBRT delivery, especially when using very high dose rate FFF beams and when the shift occurs in that part of the arc delivering high doses and steep gradients. The impact is greatest on the spinal cord and its planning-at-risk volume. Accurate and stable patient positioning is therefore required for vertebral SBRT.

Flattening filter free vs flattened beams for breast irradiation.

PURPOSE: Flattening filter free (FFF) beams offer the potential for a higher dose rate, shorter treatment time, and lower peripheral dose. To investigate their role in large-field treatments, this study compared flattened and FFF beams for breast irradiation. METHODS AND MATERIALS: Ten left breast clinical plans comprising 2 tangential beams and a medially located 3-field simultaneous integrated boost (SIB) were replanned. Full intensity modulated radiotherapy (IMRT), hybrid IMRT, electronic tissue compensator (ETC), and multiple static field treatment plans were created for the elective breast volume using flattened and FFF beams, in combination with a 3-field IMRT SIB. Plan quality was assessed and delivery times were measured for all plans for 1 patient. Out-of-field doses were measured using an ionization chamber for an IMRT plan optimized on a corner of simple cubic phantom for both flattened and FFF beams. RESULTS: For each technique, mean target volume metrics (planning target volume coverage, homogeneity, conformity) were typically within 3% for flattened and FFF beams. Larger mean differences in boost conformity favoring flattened hybrid (7.2%) and full IMRT (5.5%) plans may have reflected limitations in plan normalization. Calculated heart and ipsilateral lung doses were comparable; however, both flattened and FFF low-dose phantom measurements were substantially higher than calculated values, rendering the comparison of low dose in the contralateral breast uncertain. Beam delivery times were on average 31% less for FFF. CONCLUSIONS: In general, target volume metrics for flattened and FFF plans were comparable. The planning system did not seem to allow for accurate peripheral dose evaluation. FFF was associated with a potentially shorter treatment time. All 4 IMRT techniques allowed FFF beams to generate acceptable plans for breast IMRT.

An analysis of patient positioning during stereotactic lung radiotherapy performed without rigid external immobilization.

BACKGROUND AND PURPOSE: Intra-fraction patient motion is incompletely understood and the optimum amount of support or immobilization during stereotactic body radiotherapy (SBRT) is unclear. Rigid immobilization is often advocated, but motion still occurs. In contrast, we deliver the vast majority of SBRT using simple supporting devices, simultaneously emphasizing comfort, frequent position checks and progressive reduction in treatment times. We report spine stability during lung SBRT. MATERIALS AND METHODS: Patients lie on a thin mattress with arms supported above their head and below-knee support. Stereoscopic spine X-rays before and after fraction delivery identified motion in three translational and three rotational directions. RESULTS: Images from 109 fractions in 30 patients resulted in 327 translational and 327 rotational pre- and post-fraction comparisons. Mean RapidArc® delivery time for variable fraction dose was 4.2 min (SD=1.4). 92% and 97% of translational and rotational differences were ≤1 mm and ≤1° in any direction and 98% of translational differences were ≤1.5mm. Mean vertical, longitudinal and lateral motion was 0mm (SD=0.4), 0mm (0.6) and 0mm (0.6). 84% and 94% of the 109 fractions were delivered with ≤1 and ≤1.5mm translation in all three directions and 93% with ≤1° of rotation. Two patients accounted for 10/17 fractions with >1mm translational motion. CONCLUSIONS: Based on pre and post-fraction X-ray imaging during fast lung SBRT, simple support devices can result in spine stability that is comparable to that reported with rigid external immobilization.

Clinical application of a novel hybrid intensity-modulated radiotherapy technique for stage III lung cancer and dosimetric comparison with four other techniques.

PURPOSE: In large stage III lung tumors, planning delivery of doses exceeding 60 Gy can be challenging and time consuming. Intensity modulated radiation therapy (IMRT) can improve target coverage but may increase volumes receiving low-dose irradiation. We clinically implemented a novel hybrid IMRT (h-IMRT) technique that allowed plans to be produced quickly, and compared these plans with 4 other techniques. METHODS AND MATERIALS: h-IMRT was used to treat 14 consecutive patients with planning target volumes (PTVs) exceeding 500 cm(3) (average, 779 cm(3)) with concurrent chemo-radiation therapy to 66 Gy. h-IMRT plans consisted of 2 components: an anterior-posterior/posterior-anterior/posterior-anterior (AP-PA-PA) oblique, open-field technique delivering an average dose of 58 Gy, plus a 3-field IMRT component optimized to achieve a final homogeneous dose of 66 Gy. Total lung V(20) and contralateral lung V(5) were kept as low as possible but preferably less than 35% and less than 50%, respectively. All plans were retrospectively replanned using a 5- to 9-field 3-dimensional conformal technique, full RapidArc, 6-field full IMRT, and a hybrid RapidArc (h-RapidArc) technique similar to the h-IMRT. RESULTS: The h-IMRT, h-RapidArc, and full RapidArc plans could be generated in less than 2 h, with the first 2 plans achieving the lowest V(5) (36%) and V(20) (30%) values together with the smallest hot spots. Both the 3-dimensional conformal and full IMRT plans occasionally led to unacceptable hot spots outside the PTV. Full RapidArc plans were fast and achieved comparable V(20) values but led to slightly higher V(5) values. CONCLUSIONS: Both h-IMRT and h-RapidArc permitted delivery of 66 Gy to large stage III lung tumors, and both were superior to either full IMRT or RapidArc plans for reducing lung doses. The clinical significance of small increases in V(5) during chemo-radiation therapy delivery are unknown, but the present study suggests that h-IMRT and h-RapidArc are preferable for treatment of large tumors.

Fast arc delivery for stereotactic body radiotherapy of vertebral and lung tumors.

PURPOSE: Flattening filter-free (FFF) beams with higher dose rates and faster delivery are now clinically available. The purpose of this planning study was to compare optimized non-FFF and FFF RapidArc plans for stereotactic body radiotherapy (SBRT) and to validate the accuracy of fast arc delivery. METHODS AND MATERIAL: Ten patients with peripheral lung tumors and 10 with vertebral metastases were planned using RapidArc with a flattened 6-MV photon beam and a 10-MV FFF beam for fraction doses of 7.5-18 Gy. Dosimetry of the target and organs at risk (OAR), number of monitor units (MU), and beam delivery times were assessed. GafChromic EBT2 film measurements of FFF plans were performed to compare calculated and delivered dose distributions. RESULTS: No major dosimetric differences were seen between the two delivery techniques. For lung SBRT plans, conformity indices and OAR doses were similar, although the average MU required were higher with FFF plans. For vertebral SBRT, FFF plans provided comparable PTV coverage, with no significant differences in OAR doses. Average beam delivery times were reduced by a factor of up to 2.5, with all FFF fractions deliverable within 4 min. Measured FFF plans showed high agreement with calculated plans, with more than 99% of the area within the region of interest fulfilling the acceptance criterion. CONCLUSION: The higher dose rate of FFF RapidArc reduces delivery times significantly, without compromising plan quality or accuracy of dose delivery.

Impact of the calculation resolution of AAA for small fields and RapidArc treatment plans.

PURPOSE: To investigate the impact of the calculation resolution of the anisotropic analytical algorithms (AAA) for a variety of small fields in homogeneous and heterogeneous media and for RapidArc plans. METHODS: Dose distributions calculated using AAA version 8.6.15 (AAA8) and 10.0.25 (AAA10) were compared to measurements performed with GafChromic EBT film, using phantoms made of polystyrene or a combination of polystyrene and cork. The accuracy of the algorithms calculated using grid resolutions of 2.5 and 1.0 mm was investigated for different field sizes, and for a limited selection of RapidArc plans (head and neck, small meningioma, and lung). Additional plans were optimized to create excessive multileaf collimator modulation and measured on a homogenous phantom. Gamma evaluation criterion of 3% dose difference and 2- or 1-mm distance to agreement (DTA) were applied to evaluate the accuracy of the algorithms. RESULTS: For fields < or = 3 x 3 cm2, both versions of AAA predicted lower peak doses and broader penumbra widths than the measurements. However, AAA10 and a finer calculation grid improved the agreement. For RapidArc plans with many small multileaf collimator (MLC) segments and relatively high number of monitor units (MU), AAA8 failed to identify small dose peaks within the target. Both versions performed better in polystyrene than in cork. In homogeneous cork layers, AAA8 underestimated the average target dose for a clinical lung plan. This was improved with AAA10 calculated using a 1 mm grid. CONCLUSIONS: AAA10 improves the accuracy of dose calculations, and calculation grid of 1.0 mm is superior to using 2.5 mm, although calculation times increased by factor of 5. A suitable upper MU constraint should be assigned during optimization to avoid plans with high modulation. For plans with a relative high number of monitor units, calculations using 1 mm grid resolution are recommended. For planning target volume (PTV) which contains relatively large area of low density tissue, users should be aware of possible dose underestimation in the low density region and recalculation with AAA10 grid 1.0 mm is recommended.

Quality assurance of 4D-CT scan techniques in multicenter phase III trial of surgery versus stereotactic radiotherapy (radiosurgery or surgery for operable early stage (stage 1A) non-small-cell lung cancer [ROSEL] study).

PURPOSE: To determine the accuracy of four-dimensional computed tomography (4D-CT) scanning techniques in institutions participating in a Phase III trial of surgery vs. stereotactic radiotherapy (SBRT) for lung cancer. METHODS AND MATERIALS: All 9 centers performed a 4D-CT scan of a motion phantom (Quasar, Modus Medical Devices) in accordance with their in-house imaging protocol for SBRT. A cylindrical cedar wood insert with plastic spheres of 15 mm (ø15) and 30 mm (ø30) diameter was moved in a cosine-based pattern, with an extended period in the exhale position to mimic the actual breathing motion. A range of motion of R = 15 and R = 25 mm and breathing period of T = 3 and T = 6 s were used. Positional and volumetric imaging accuracy was analyzed using Pinnacle version 8.1× at various breathing phases, including the mid-ventilation phase and maximal intensity projections of the spheres. RESULTS: Imaging using eight CT scanners (Philips, Siemens, GE) and one positron emission tomography-CT scanner (Institution 3, Siemens) was investigated. The imaging protocols varied widely among the institutions. No strong correlation was found between the specific scan protocol parameters and the observed results. Deviations in the maximal intensity projection volumes averaged 1.9% (starting phase of the breathing cycle [ø]15, R = 15), 12.3% (ø15, R = 25), and -0.9% (ø30, R = 15). The end-expiration volume deviations (13.4%, ø15 and 2.5%, ø30), were, on average, smaller than the end-inspiration deviations (20.7%, ø15 and 4.5%, ø30), which, in turn, were smaller than the mid-ventilation deviations (32.6%, ø15 and 8.0%, ø30). A slightly larger variation in the mid-ventilation origin position was observed (mean, -0.2 mm; range, -3.6-4.2) than in the maximal intensity projection origin position (mean, -0.1 mm; range, -2.5-2.5). The range of motion was generally underestimated (mean, -1.5 mm; range, -5.5-1). CONCLUSIONS: Notable differences were seen in the 4D-CT imaging protocols for SBRT among centers. However, the observed deviations in target volumes were generally small. They were slightly larger for the mid-ventilation phases and smallest for the end-expiration phases. Steps to optimize and standardize the 4D-CT scanning protocols for SBRT are desirable.

Dosimetric impact of interplay effect on RapidArc lung stereotactic treatment delivery.

PURPOSE: Volumetric modulated arc therapy (RapidArc; Varian Medical Systems, Palo Alto, CA) allows fast delivery of stereotactic radiotherapy for Stage I lung tumors. We investigated discrepancies between the calculated and delivered dose distributions, as well as the dosimetric impact of leaf interplay with breathing-induced tumor motion. METHODS AND MATERIALS: In 20 consecutive patients with Stage I lung cancer who completed RapidArc delivery, 15 had tumor motion exceeding 5 mm on four-dimensional computed tomography scan. Static and dynamic measurements were performed with Gafchromic EBT film (International Specialty Products Inc., Wayne, NJ) in a Quasar motion phantom (Modus Medical Devices, London, Ontario, Canada). Static measurements were compared with calculated dose distributions, and dynamic measurements were compared with the convolution of static measurements with sinusoidal motion patterns. Besides clinical treatment plans, additional cases were optimized to create excessive multileaf collimator modulation and delivered on the phantom with peak-to-peak motions of up to 25 mm. γ Analysis with a 3% dose difference and 2- or 1-mm distance to agreement was used to evaluate the accuracy of delivery and the dosimetric impact of the interplay effect. RESULTS: In static mode film dosimetry of the two-arc delivery in the phantom showed that, on average, fewer than 3% of measurements had γ greater than 1. Dynamic measurements of clinical plans showed a high degree of agreement with the convolutions: for double-arc plans, 99.5% met the γ criterion. The degree of agreement was 98.5% for the plans with excessive multileaf collimator modulations and 25 mm of motion. CONCLUSIONS: Film dosimetry shows that RapidArc accurately delivers the calculated dose distribution and that interplay between leaves and tumor motion is not significant for single-fraction treatments when RapidArc is delivered with two different arcs.

Stereotactic radiotherapy for peripheral lung tumors: a comparison of volumetric modulated arc therapy with 3 other delivery techniques.

PURPOSE: Volumetric modulated arc therapy (RapidArc) allows for fast delivery of stereotactic body radiotherapy (SBRT) delivery in stage I lung tumors. We compared dose distributions and delivery times between RapidArc and common delivery techniques in small tumors. METHODS: In 18 patients who completed RapidArc SBRT for tumors measuring <70 cm(3), new treatment plans were generated using non-coplanar 3D conformal fields (conf-SBRT) and dynamic conformal arc radiotherapy (DCA). For 9 patients with tumors adjacent to the chest wall, co-planar intensity-modulated radiotherapy (IMRT) plans were also generated. PTV dose coverage, organs at risk (OAR) doses and treatment delivery times were assessed. RESULTS: RapidArc plans achieved a superior conformity index (CI) and lower V(45 Gy) to chest wall (p<0.05) compared to all other techniques. RapidArc led to a small increase in V(5 Gy) to contralateral lung compared to conf-SBRT (4.4±4% versus 1.2±1.8%, p=0.011). For other OAR, RapidArc and conf-SBRT plans were comparable, and both were superior to DCA plans. Delivery of a 7.5 Gy-fraction required 3.9 min (RapidArc), 11.6 min (conf-SBRT), and 12 min (IMRT). CONCLUSIONS: In stage I lung tumors measuring <70 cm(3), RapidArc plans achieved both the highest dose conformity and shortest delivery times.

The accuracy of frameless stereotactic intracranial radiosurgery.

PURPOSE: To determine the accuracy of frameless stereotactic radiosurgery using the BrainLAB ExacTrac system and robotic couch by measuring the individual contributions such as the accuracy of the imaging and couch correction system, the linkage between this system and the linac isocenter and the possible intrafraction motion of the patient in the frameless mask. MATERIALS AND METHODS: An Alderson head phantom with hidden marker was randomly positioned 31 times. Automated 6D couch shifts were performed according to ExacTrac and the deviation with respect to the linac isocenter was measured using the hidden marker. ExacTrac-based set-up was performed for 46 patients undergoing hypofractionated stereotactic radiotherapy for 135 fractions, followed by verification X-rays. Forty-three of these patients received post-treatment X-ray verification for 79 fractions to determine the intrafraction motion. RESULTS: The hidden target test revealed a systematic error of 1.5 mm in one direction, which was corrected after replacement of the system calibration phantom. The accuracy of the ExacTrac positioning is approximately 0.3 mm in each direction, 1 standard deviation. The intrafraction motion was 0.35±0.21 mm, maximum 1.15 mm. CONCLUSION: Intrafraction motion in the BrainLAB frameless mask is very small. Users are strongly advised to perform an independent verification of the ExacTrac isocenter in order to avoid systematic deviations.

A novel simple approach for incorporation of respiratory motion in stereotactic treatments of lung tumors.

PURPOSE: An internal target volume (ITV) is often used for incorporating tumor motion into radiotherapy planning but it overestimates the margins necessary for breathing motion. We describe a pragmatic approach using maximum- and minimum-intensity projections (MIP and Min-IP) only, for reducing ITVs in stereotactic radiotherapy by using dosimetric margins that compensate for motion-induced dose blurring. PATIENTS AND METHOD: We studied tumor motion characteristics from 26 repeat 4DCT scans derived from 10 patients. These were used to calculate the shift in cranio-caudal direction of the 80% isodose due to dose blurring of the time-averaged dose distribution caused by respiratory motion. The dosimetric margins necessary to compensate for dose blurring were calculated relative to the ITV, which can be determined efficiently using the MIP. Peak-to-peak motion amplitude was determined using the MIP and Min-IP. A programmable respiratory motion phantom was used to investigate imaging artifacts in determining the ITV for realistic motion patterns. Dose profiles were both calculated and measured in lung- and water-equivalent tissue. RESULTS: Using margins for the 80% dose level permitted the use of smaller target volumes relative to the use of ITV-based volumes, with (i) greater reductions seen at the end-inspiration edge than at expiration side due to asymmetric breathing motion patterns and (ii) a linear relationship seen with breathing amplitude. The average reduction of the ITV at a 95% confidence level is given by 0.2×A(pp)-1.3 mm at expiration side, where A(pp) is the peak-to-peak breathing amplitude, and 0.3×A(pp)-2.2 mm at inspiration side. Dosimetric margins did not differ significantly between water-equivalent and lung tissue for 80% isodose. CONCLUSION: A simple margin recipe for breathing motion linear with breathing amplitude can be used to calculate the ITV reductions achievable for stereotactic radiotherapy of lung tumors.

Rapid delivery of stereotactic radiotherapy for peripheral lung tumors using volumetric intensity-modulated arcs.

The delivery of high dose conventional stereotactic body radiotherapy (SBRT) for patients with stage I lung tumors generally takes 30-45min per fraction. The novel volumetric intensity-modulated arc therapy (RA) for planning and delivery enabled much faster treatment for three patients with different fractionation schemes. This reduces the risk of intrafraction motion and is more patient friendly. In addition, in comparison to the conventional plans using 10 static non-coplanar fields, RA plans achieved superior dose conformity around the PTV and reduced chest wall doses.