Feasibility and clinical usefulness of modelling glioblastoma migration in adjuvant radiotherapy.

Glioblastoma (GBM) is one of the most common primary brain tumours in adults, with a dismal prognosis despite aggressive multimodality treatment by a combination of surgery and adjuvant radiochemotherapy. A detailed knowledge of the spreading of glioma cells in the brain might allow for more targeted escalated radiotherapy, aiming to reduce locoregional relapse. Recent years have seen the development of a large variety of mathematical modelling approaches to predict glioma migration. The aim of this study is hence to evaluate the clinical applicability of a detailed micro- and meso-scale mathematical model in radiotherapy. First and foremost, a clinical workflow is established, in which the tumour is automatically segmented as input data and then followed in time mathematically based on the diffusion tensor imaging data. The influence of several free model parameters is individually evaluated, then the full model is retrospectively validated for a collective of 3 GBM patients treated at our institution by varying the most important model parameters to achieve optimum agreement with the tumour development during follow-up. Agreement of the model predictions with the real tumour growth as defined by manual contouring based on the follow-up MRI images is analyzed using the dice coefficient. The tumour evolution over 103-212 days follow-up could be predicted by the model with a dice coefficient better than 60% for all three patients. In all cases, the final tumour volume was overestimated by the model by a factor between 1.05 and 1.47. To evaluate the quality of the agreement between the model predictions and the ground truth, we must keep in mind that our gold standard relies on a single observer's (CB) manually-delineated tumour contours. We therefore decided to add a short validation of the stability and reliability of these contours by an inter-observer analysis including three other experienced radiation oncologists from our department. In total, a dice coefficient between 63% and 89% is achieved between the four different observers. Compared with this value, the model predictions (62-66%) perform reasonably well, given the fact that these tumour volumes were created based on the pre-operative segmentation and DTI.

Optimizing Adjuvant Stereotactic Radiotherapy of Motor-Eloquent Brain Metastases: Sparing the nTMS-Defined Motor Cortex and the Hippocampus.

Brain metastases can effectively be treated with surgical resection and adjuvant stereotactic radiotherapy (SRT). Navigated transcranial magnetic stimulation (nTMS) has been used to non-invasively map the motor cortex prior to surgery of motor eloquent brain lesions. To date, few studies have reported the integration of such motor maps into radiotherapy planning. The hippocampus has been identified as an additional critical structure of radiation-induced deficits. The aim of this study is to assess the feasibility of selective dose reduction to both the nTMS-based motor cortex and the hippocampi in SRT of motor-eloquent brain metastases. Patients with motor-eloquent brain metastases undergoing surgical resection and adjuvant SRT between 07/2014 and 12/2018 were retrospectively analyzed. The radiotherapy treatment plans were retrieved from the treatment planning system ("original" plan). For each case, two intensity-modulated treatment plans were created: the "motor" plan aimed to reduce the dose to the motor cortex, the "motor & hipp" plan additionally reduce the dose to the hippocampus. The optimized plans were compared with the "original" plan regarding plan quality, planning target volume (PTV) coverage, and sparing of organs at risk (OAR). 69 plans were analyzed, all of which were clinically acceptable with no significant differences for PTV coverage. All OAR were protected according to standard protocols. Sparing of the nTMS motor map was feasible: mean dose 9.66 ± 5.97 Gy (original) to 6.32 ± 3.60 Gy (motor) and 6.49 ± 3.78 Gy (motor & hipp), p<0.001. In the "motor & hipp" plan, dose to the ipsilateral hippocampi could be significantly reduced (max 1.78 ± 1.44 Gy vs 2.49 ± 1.87 Gy in "original", p = 0.003; mean 1.01 ± 0.92 Gy vs. 1.32 ± 1.07 Gy in "original", p = 0.007). The study confirms the results from previous studies that inclusion of nTMS motor information into radiotherapy treatment planning is possible with a relatively straightforward workflow and can achieve reduced doses to the nTMS-defined motor area without compromising PTV coverage. Furthermore, we demonstrate the feasibility of selective dose reduction to the hippocampus at the same time. The clinical significance of these optimized plans yet remains to be determined. However, with no apparent disadvantages these optimized plans call for further and broader exploration.

Automatic Radiotherapy Planning for Glioblastoma Radiotherapy With Sparing of the Hippocampus and nTMS-Defined Motor Cortex.

BACKGROUND: Navigated transcranial magnetic stimulation (nTMS) of the motor cortex has been successfully implemented into radiotherapy planning by a number of studies. Furthermore, the hippocampus has been identified as a radiation-sensitive structure meriting particular sparing in radiotherapy. This study assesses the joint protection of these two eloquent brain regions for the treatment of glioblastoma (GBM), with particular emphasis on the use of automatic planning. PATIENTS AND METHODS: Patients with motor-eloquent brain glioblastoma who underwent surgical resection after nTMS mapping of the motor cortex and adjuvant radiotherapy were retrospectively evaluated. The radiotherapy treatment plans were retrieved, and the nTMS-defined motor cortex and hippocampus contours were added. Four additional treatment plans were created for each patient: two manual plans aimed to reduce the dose to the motor cortex and hippocampus by manual inverse planning. The second pair of re-optimized plans was created by the Auto-Planning algorithm. The optimized plans were compared with the "Original" plan regarding plan quality, planning target volume (PTV) coverage, and sparing of organs at risk (OAR). RESULTS: A total of 50 plans were analyzed. All plans were clinically acceptable with no differences in the PTV coverage and plan quality metrics. The OARs were preserved in all plans; however, overall the sparing was significantly improved by Auto-Planning. Motor cortex protection was feasible and significant, amounting to a reduction in the mean dose by >6 Gy. The dose to the motor cortex outside the PTV was reduced by >12 Gy (mean dose) and >5 Gy (maximum dose). The hippocampi were significantly improved (reduction in mean dose: ipsilateral >6 Gy, contralateral >4.6 Gy; reduction in maximum dose: ipsilateral >5 Gy, contralateral >5 Gy). While the dose reduction using Auto-Planning was generally better than by manual optimization, the radiated total monitor units were significantly increased. CONCLUSION: Considerable dose sparing of the nTMS-motor cortex and hippocampus could be achieved with no disadvantages in plan quality. Auto-Planning could further contribute to better protection of OAR. Whether the improved dosimetric protection of functional areas can translate into improved quality of life and motor or cognitive performance of the patients can only be decided by future studies.

Monte-Carlo simulation of the Siemens Artiste linear accelerator flat 6 MV and flattening-filter-free 7 MV beam line.

The aim of our work is to provide the up-to-now missing information on the Siemens Artiste FFF 7 MV beam line using a Monte-Carlo model fit to the realistic dosimetric measurements at the linear accelerator in clinical use at our department. The main Siemens Artiste 6MV and FFF 7MV beams were simulated using the Geant4 toolkit. The simulations were compared with the measurements with an ionization chamber in a water phantom to verify the validation of simulation and tuning the primary electron parameters. Hereafter, other parameters such as surface dose, spectrum, electron contamination, symmetry, flatness/unflatness, slope, and characteristic off-axis changes were discussed for both Flat and FFF mode. The mean electron energy for the FFF beam was 8.8 MeV and 7.5 MeV for Flat 6 MV, the spread energy and spot size of the selected Gaussian distribution source were 0.4 MeV and 1mm, respectively. The dose rate of the FFF beam was 2.8 (2.96) times higher than for the flattened beam for a field size of 10×10 (20×20) cm2. The electron contamination has significant contribution to the surface dose especially for the flattened beam. The penumbra, surface dose and the mean energy of photons decrease by removing the flattening filter. Finally, the results show that off-axis changes have no strong effect on the mean energy of FFF beams, while this effect was more considerable for the flattened beam.

Imaging dose and secondary cancer risk in image-guided radiotherapy of pediatric patients.

BACKGROUND: Daily image-guided radiotherapy (IGRT) can contribute to cover extended body volumes with low radiation dose. The effect of additional imaging dose on secondary cancer development is modelled for a collective of children with Morbus Hodgkin. METHODS: Eleven radiotherapy treatment plans from pediatric patients with Hodgkin's lymphoma were retrospectively analyzed, including imaging dose from scenarios using different energies (kV/MV) and planar/cone-beam computed tomography (CBCT) techniques. In addition to assessing the effect of imaging dose on organs at risk, the excess average risk (EAR) for developing a secondary carcinoma of the lung or breast was modelled. RESULTS: Although the variability between the patients is relatively large due to the different target volumes, the additional EAR due to imaging can be consistently determined. For daily 6MV CBCT, the EAR for developing a secondary cancer at age 50 is over 3 cases per 104 PY (patient-years) for the female breast and 0.7-0.8 per 104 PY for the lungs. This can be decreased by using only planar images (< 1 per 104 PY for the breast and 0.1 for the lungs). Similar values are achieved by daily 360° kV CBCT (0.44-0.57 per 104 PY for the breast and 0.08 per 104 PY for the lungs), which is again reduced for daily 200° kV CBCT (0.02 per 104 PY for the lungs and 0.07-0.08 per 104 PY for the breast). These values increase if an older attained age is considered (e.g., for 70 years, by a factor of four for the lungs). CONCLUSIONS: Daily imaging can be performed with an additional secondary cancer risk of less than 1 per 104 PY if kV CBCT is applied. If MV modalities must be chosen, a similar EAR can be achieved with planar images. A further reduction in risk is possible if the imaging geometry allows for sparing of the breast by a partial rotation underneath the patient.

mARC Treatment of Hypopharynx Carcinoma with Flat and Flattening-Filter-Free Beam Energies - A Planning Study.

BACKGROUND: The recently implemented mARC-rotation-technique is capable to deliver high dose rate bursts. For the case of hypopharynx cancer plans we evaluate whether the mARC can achieve an advantage in treatment time in comparison to IMRT. These plans consider two arcs with flat and flattening filter free (FFF) beam energies. MATERIALS AND METHODS: For 8 hypopharynx-cancer patients step-and-shoot-IMRT and mARC plans were created retrospectively using flat and FFF beam energy. The comparison of the plan scenarios considered measures of quality for PTV coverage and sparing of organs at risk. All plans were irradiated on an anthromorphic phantom equipped with thermoluminescent dosimeters to measure scattered dose and treatment times. RESULTS: A visual comparison of the dose distribution did not show a marked preference for either technique or energy. The statistical evaluation yielded significant differences in favor of the mARC technique and the FFF energy. Scattered dose could be decreased markedly by the use of the mARC technique. Treatment times could be reduced up to 3 minutes with the use of mARC in comparison to IMRT. The high dose rate energy results in another time advantage of about 1 minute. CONCLUSIONS: All four plan scenarios yielded equally good quality plans. A combination of the mARC technique with FFF 7 MV high dose rate resulted in a decrease of treatment times from about 9 minutes to 5-6 minutes in comparison to 6 MV IMRT.

mARC prostate treatment planning with Varian Eclipse for flat vs. FFF beams.

PURPOSE: The modulated arc (mARC) technique as an alternative to VMAT is a rotational IMRT irradiation with burst mode delivery. Varian has recently implemented an option for mARC-planning into the Eclipse treatment planning system (TPS) and so far mARC-planning with this TPS has not been evaluated systematically. Therefore, for prostate treatment with Eclipse we compare mARC with IMRT using flat (6MV) and flattening-filter-free (FFF, 7MV) beam energies. METHODS: For ten prostate cancer patients standardized re-contouring and re-planning was performed with a prescription of 76Gy to the complete planning-target-volume (PTV). IMRT and mARC plans (6MV vs. FFF 7MV) were compared pairwise considering indices for plan quality. All plans were delivered on an anthromorphic phantom equipped with thermoluminescent dosimeters to measure out-of-field dose and treatment times. RESULTS: Regarding PTV coverage, there was no marked preference for either technique or energy. The evaluation of organs at risk showed improved bladder sparing of the mARC plans up to about 75Gy; above this dose the IMRT plans achieved significant better sparing. The use of the FFF-beam-energy and mARC-technique resulted in a significant decrease in out-of-field dose. This combination also led to a drastic reduction of treatment time by factor of three in comparison with 6MV IMRT. CONCLUSION: While highly conformal treatment plans could be created by the use of all modalities, the combination of the high dose rate with mARC appears to be the preferable option as it benefits from a marked decrease in treatment time and out-of-field dose.

Benchmarking the mARC performance - treatment time and dosimetric linearity.

PURPOSE: The mARC technique is a hybrid rotational IMRT modality operating in "burst mode". While it is generally assumed that it will be slower than VMAT, the real limits of operation have not been defined so far. We here present the first systematic study of the technical limits on mARC treatment. METHODS: The following scenarios are considered: 18, 30, 36 or 45 arclets per rotation (spacing between 20° and 8°), flat and flattening-filter-free (FFF) energy, arclet width 4° or 2°, from 1 MU/arclet to 1000 MU/plan. All scenarios are irradiated, treatment times are measured and treatment parameters reported. Dose linearity was assessed by point dose measurements of the 18 arclet plans with 1-30 MU per arclet. RESULTS: Minimum treatment times (no MLC movement, few MUs) depend strongly on the number of arclets per rotation (1minute for 18 arclets to 1:50min for 45 arclets), and rise linearly with MU/arclets after a given cut-off value depending on scenario, arclet width and available maximum dose rate. MLC movement adds up to 2minutes of treatment time, but generally less (ca. 45seconds in realistic plans). The rules by which irradiation parameters are selected by the firmware can be partly discovered. The choice of dose rate is most clearly defined. For the flat 6 MV energy, the highest available dose rate (300 MU/min) is always applied. For FFF 7 MV dose rate is reduced for arclets with few MUs, so that an arclet is irradiated in no less than 0.3 s. Only for the case of 1 MU/arclet can this constraint not be met (the technical limit on the dose rate if 500 MU/min for FFF 7 MV). In this case, dosimetric linearity is reduced. In all other instances, deviations from linearity at low MU remain below 2%. CONCLUSIONS: Treatment times of down to 90seconds are technically achievable for treatment with FFF beams using up to 36 arclets per rotation (arclet spacing every 10°) for up to 900 MU/plan, comparable to VMAT treatment times. The values provided here are meant to serve as a reference for the design of mARC plans (choice of arclets spacing etc.) and as minimum times against which the performance of different treatment planning systems can be evaluated.

Set-up errors and planning margins in planar and CBCT image-guided radiotherapy using three different imaging systems: A clinical study for prostate and head-and-neck cancer.

PURPOSE: The purpose of this work is to compare the positioning accuracy achieved by three different imaging techniques and planar vs. CBCT imaging for two common IGRT indications. METHODS: A collective of prostate cancer and head-and-neck cancer patients treated at our institution during the year 2013 was retrospectively analyzed. For all treatment fractions (3078 in total), the kind of acquired set-up image and the performed couch shift before treatment were assessed. The distribution of couch corrections was compared for three different imaging systems available at our institution: the treatment beam line operating at 6 MV, a dedicated imaging beam line of nominally 1 MV, and the kVision system at 70-121 kV. Shifts were analyzed for planar and cone-beam CT images. Based on the set-up corrections, CTV to PTV expansion margins were calculated. RESULTS: The difference in set-up corrections performed for the three energies and both techniques (planar vs. CBCT) was not significant for head-and-neck cancer patients. For prostate cancer all shifts had equal variance. Averages ranged from -0.7 to +0.7 mm. The set-up margins calculated on the basis of the observed shifts are 4.0 mm (AP) and 3.8 mm (SI, LR) for the head-and-neck PTV and 6.6 mm (SI), 6.7 mm (AP) and 7.9 mm (LR) for the prostate cancer patients. CONCLUSIONS: For three different linac-based imaging energies and planar/CBCT imaging, no relevant differences in set-up shifts were observed. The suggested set-up margins for these indications are of the order of 4 mm for head-and-neck and 6-8 mm for prostate treatment.

VMAT to arclet plan conversion in a treatment planning system : Feasibility and dosimetric relationship between VMAT, arclet, and stationary fields.

AIM: The aim of this study was to make dynamic rotation treatment with mARC available for the non-dedicated Philips Pinnacle treatment planning system by converting SmartArc plans, offering insight into the relationship between SmartArc, mARC, and stationary field irradiation. METHODS: A scripting solution is presented that can be run in the Pinnacle system. This allows for the conversion of SmartArc plans into mARC format. The dose distribution of the converted mARC plan can be evaluated both in the form of a "real" mARC plan with arclets and-as is generally done in treatment planning systems certified for mARC planning-by approximating the arclets as stationary fields. We present the proof of principle and dosimetric comparisons. RESULTS: The converted plans were irradiated without problems. For the measured 3D dose distributions, on average over 90 % points agreed with the calculated dose distributions (mARC and stationary field plans) within the gamma criteria of 3 % deviation in the local dose, 3-mm distance to agreement, for all dose values above 10 % of the maximum. The agreement between the three calculated dose distributions (SmartArc with both converted plans) was above 87 % (above 92 % when comparing mARC with stationary fields). CONCLUSION: Our solution offers the possibility of mARC planning in Pinnacle. The dose comparisons furthermore prove that the dosimetric differences between SmartArc and mARC, when appropriately translated, are minor.

Image quality and dose distributions of three linac-based imaging modalities.

BACKGROUND AND PURPOSE: Linac-based patient imaging is possible with a variety of techniques using different photon energies. The purpose of this work is to compare three imaging systems operating at 6 MV, flattening free filter (FFF) 1 MV, and 121 kV. PATIENTS AND METHODS: The dose distributions of all pretreatment set-up images (over 1,000) were retrospectively calculated on the planning computed tomography (CT) images for all patients with prostate and head-and-neck cancer treated at our institution in 2013. We analyzed the dose distribution and the dose to organs at risk. RESULTS: For head-and-neck cancer patients, the imaging dose from 6-MV cone beam CT (CBCT) reached maximum values at around 8 cGy. The 1-MV CBCT dose was about 63-79 % of the 6-MV CBCT dose for all organs at risk. Planar imaging reduced the imaging dose from CBCT to 30-40 % for both megavoltage modalities. The dose from the kilovoltage CBCT was 4-10 % of the 6-MV CBCT dose. For prostate cancer patients, the maximum dose from 6-MV CBCT reached 13-15 cGy, and was reduced to 66-73 % for 1 MV. Planar imaging reduces the MV CBCT dose to 10-20 %. The kV CBCT dose is 15-20 % of the 6-MV CBCT dose, slightly higher than the dose from MV axes. The dose distributions differ markedly in response to the different beam profiles and dose-depth characteristics.

mARC vs. IMRT radiotherapy of the prostate with flat and flattening-filter-free beam energies.

BACKGROUND: There as yet exists no systematic planning study investigating the novel mARC rotational radiotherapy technique, which is conceptually different from VMAT. We therefore present a planning study for prostate cancer, comparing mARC with IMRT treatment at the same linear accelerator equipped with flat and flattening-filter-free (FFF) photon energies. METHODS: We retrospectively re-contoured and re-planned treatment plans for 10 consecutive prostate cancer patients. Plans were created for a Siemens Artiste linear accelerator with flat 6 MV and FFF 7 MV photons, using the Prowess Panther treatment planning system. mARC and IMRT plans were compared with each other considering indices for plan quality and dose to organs at risk. All plans were exported to the machine and irradiated while measuring scattered dose by thermoluminescent dosimeters placed on an anthropomorphic phantom. Treatment times were also measured and compared. RESULTS: All plans were found acceptable for treatment. There was no marked preference for either technique or energy from the point of view of target coverage and dose to organs at risk. Scattered dose was significantly decreased by the use of FFF energies. While mARC and IMRT plans were of very similar overall quality, treatment time could be markedly decreased both by the use of mARC and FFF energy. CONCLUSIONS: Highly conformal treatment plans could be created both by the use of flat 6 MV and FFF 7 MV energy, using IMRT or mARC. For all practical purposes, the FFF 7 MV energy and mARC plans are acceptable for treatment, a combination of both allowing a drastic reduction in treatment time from over 5 minutes to about half this value.

Planning study and dose measurements of intracranial stereotactic radiation surgery with a flattening filter-free linac.

PURPOSE: Flattening filter-free (FFF) beams have recently become available for radiation therapy, offering much higher dose rates but complicating treatment owing to the nonflat profile. Stereotactic treatment is one of the most evident scenarios to investigate the use of FFF beams. METHODS AND MATERIALS: We present a planning study of a FFF 7-MV beam for the treatment of brain metastases using multiple noncoplanar arcs. Plan differences as compared with flat 6 MV photon fields are estimated using different measures of quality. Absolute dosimetry and fluence distribution are verified and the out-of-field dose is measured. RESULTS: The FFF 7-MV plans are slightly better than the flat 6-MV plans as evaluated by a number of quality indices, dose to organs at risk, and out-of-field dose, although differences may not be clinically relevant. Verification does not pose any problems. CONCLUSIONS: The FFF 7-MV treatment plans are marginally superior to the flat-beam 6-MV plans in almost all cases, with greatly reduced treatment times (almost 50%).

Comparative planning of flattening-filter-free and flat beam IMRT for hypopharynx cancer as a function of beam and segment number.

Although highly conformal dose distributions can be achieved by IMRT planning, this often requires a large number of segments or beams, resulting in increased treatment times. While flattening-filter-free beams offer a higher dose rate, even more segments may be required to create homogeneous target coverage. Therefore, it is worthwhile to systematically investigate the dependence of plan quality on gantry angles and number of segments for flat vs. FFF beams in IMRT planning. For the practical example of hypopharynx cancer, we present a planning study of flat vs. FFF beams using three different configurations of gantry angles and different segment numbers. The two beams are very similar in physical properties, and are hence well-suited for comparative planning. Starting with a set of plans of equal quality for flat and FFF beams, we assess how far the number of segments can be reduced before the plan quality is markedly compromised, and compare monitor units and treatment times for the resulting plans. As long as a sufficiently large number of segments is permitted, all planning scenarios give good results, independently of gantry angles and flat or FFF beams. For smaller numbers of segments, plan quality decreases both for flat and FFF energies; this effect is stronger for fewer gantry angles and for FFF beams. For low segment numbers, FFF plans are generally worse than the corresponding flat beam plans, but they are less sensitive to a decrease in segment number if many gantry angles are used (18 beams); in this case the quality of flat and FFF plans remains comparable even for few segments.

Commissioning and first clinical application of mARC treatment.

BACKGROUND: The modulated arc (mARC) technique has recently been introduced for Siemens ARTISTE linear accelerators. We present the first experiences with the commissioning of the system and first patient treatments. PATIENTS AND METHODS: Treatment planning and delivery are presented for the Prowess Panther treatment planning system or, alternatively, an in-house code. Dosimetric verification is performed both by point dose measurements and in 3D dose distribution. RESULTS: Depending on the target volume, one or two arcs can be used to create highly conformal plans. Dosimetric verification of the converted mARC plans with step-and-shoot plans shows deviations below 1 % in absolute point dose; in the 3D dose distribution, over 95 % of the points pass the 3D gamma criteria (3 % deviation in local dose and 3 mm distance to agreement for doses > 20 % of the maximum). Patient specific verification of the mARC dose distribution with the calculations has a similar pass rate. Treatment times range between 2 and 5 min for a single arc. CONCLUSIONS: To our knowledge, this is the first report of clinical application of the mARC technique. The mARC offers the possibility to save significant amounts of time, with single-arc treatments of only a few minutes achieving comparable dose distribution to IMRT plans taking up to twice as long.

Dosimetry of an in-line kilovoltage imaging system and implementation in treatment planning.

PURPOSE: To present the beam properties of the Siemens 70-kV and 121-kV linear accelerator-mounted imaging modalities and commissioning of the 121-kV beam in the Philips Pinnacle treatment planning system (TPS); measurements in an Alderson phantom were performed for verification of the model and to estimate the cone-beam CT (CBCT) imaging dose in the head and neck, thorax, and pelvis. METHODS AND MATERIALS: The beam profiles and depth-dose curve were measured in an acrylic phantom using thermoluminescent dosimeters and a soft x-ray ionization chamber. Measurements were imported into the TPS, modeled, and verified by phantom measurements. RESULTS: Modeling of the profiles and the depth-dose curve can be achieved with good quality. Comparison with the measurements in the Alderson phantom is generally good; only very close to bony structures is the dose underestimated by the TPS. For a 200° arc CBCT of the head and neck, a maximum dose of 7 mGy is measured; the thorax and pelvis 360° CBCTs give doses of 4-10 mGy and 7-15 mGy, respectively. CONCLUSIONS: Dosimetric characteristics of the Siemens kVision imaging modalities are presented and modeled in the Pinnacle TPS. Thermoluminescent dosimeter measurements in the Alderson phantom agree well with the calculated TPS dose, validating the model and providing an estimate of the imaging dose for different protocols.

A novel implementation of mARC treatment for non-dedicated planning systems using converted IMRT plans.

BACKGROUND: The modulated arc (mARC) technique has recently been introduced by Siemens as an analogue to VMAT treatment. However, up to now only one certified treatment planning system supports mARC planning. We therefore present a conversion algorithm capable of converting IMRT plans created by any treatment planning system into mARC plans, with the hope of expanding the availability of mARC to a larger range of clinical users and researchers. As additional advantages, our implementation offers improved functionality for planning hybrid arcs and provides an equivalent step-and-shoot plan for each mARC plan, which can be used as a back-up concept in institutions where only one linac is equipped with mARC. METHODS: We present a feasibility study to outline a practical implementation of mARC plan conversion using Philips Pinnacle and Prowess Panther. We present examples for three different kinds of prostate and head-and-neck plans, for 6 MV and flattening-filter-free (FFF) 7 MV photon energies, which are dosimetrically verified. RESULTS: It is generally more difficult to create good quality IMRT plans in Pinnacle using a large number of beams and few segments. We present different ways of optimization as examples. By careful choosing the beam and segment arrangement and inversion objectives, we achieve plan qualities similar to our usual IMRT plans. The conversion of the plans to mARC format yields functional plans, which can be irradiated without incidences. Absolute dosimetric verification of both the step-and-shoot and mARC plans by point dose measurements showed deviations below 5% local dose, mARC plans deviated from step-and-shoot plans by no more than 1%. The agreement between GafChromic film measurements of planar dose before and after mARC conversion is excellent. The comparison of the 3D dose distribution measured by PTW Octavius 729 2D-Array with the step-and-shoot plans and with the TPS is well above the pass criteria of 90% of the points falling within 5% local dose and 3 mm distance to agreement. For all plans, the treatment time was noticeably reduced by conversion to mARC. CONCLUSIONS: We present the feasibility test for converting IMRT step-and-shoot plans from the RTP-output of any treatment planning system (Philips Pinnacle and Prowess Panther, in our case) into mARC plans. The feasibility and dosimetric equivalence is demonstrated for the examples of a prostate and a head-and-neck patient.

Visualisation of respiratory tumour motion and co-moving isodose lines in the context of respiratory gating, IMRT and flattening-filter-free beams.

Respiratory motion during percutaneous radiotherapy can be considered based on respiration-correlated computed tomography (4DCT). However, most treatment planning systems perform the dose calculation based on a single primary CT data set, even though cine mode displays may allow for a visualisation of the complete breathing cycle. This might create the mistaken impression that the dose distribution were independent of tumour motion. We present a movie visualisation technique with the aim to direct attention to the fact that the dose distribution migrates to some degree with the tumour and discuss consequences for gated treatment, IMRT plans and flattening-filter-free beams. This is a feasibility test for a visualisation of tumour and isodose motion. Ten respiratory phases are distinguished on the CT, and the dose distribution from a stationary IMRT plan is calculated on each phase, to be integrated into a movie of tumour and dose motion during breathing. For one example patient out of the sample of five lesions, the plan is compared with a gated treatment plan with respect to tumour coverage and lung sparing. The interplay-effect for small segments in the IMRT plan is estimated. While the high dose rate, together with the cone-shaped beam profile, makes the use of flattening-filter-free beams more problematic for conformal and IMRT treatment, it can be the option of choice if gated treatment is preferred. The different effects of respiratory motion, dose build-up and beam properties (segments and flatness) for gated vs. un-gated treatment can best be considered if planning is performed on the full 4DCT data set, which may be an incentive for future developments of treatment planning systems.