Gabriele Kragl

Dosimetric characteristics of 6 and 10 MV unflattened photon beams

PURPOSE To determine dosimetric properties of unflattened megavoltage photon beams. MATERIALS AND METHODS Dosimetric data including depth dose, profiles, output factors and phantom scatter factors from three different beam qualities provided by Elekta Precise linacs, operated with and without flattening filter were examined. Additional measurements of leaf transmission, leakage radiation and surface dose were performed. In flattening filter free (FFF) mode a 6-mm thick copper filter was placed into the beam to stabilize it. RESULTS Depths of dose maxima for flattened and unflattened beams did not deviate by more than 2mm and penumbral widths agreed within 1mm. In FFF mode the collimator exchange effect was found to be on average 0.3% for rectangular fields. Between maximum and minimum field size head scatter factors of unflattened beams showed on average 40% and 56% less variation for 6 and 10MV beams than conventional beams. Phantom scatter factors for FFF beams differed up to 4% from the published reference data. For field sizes smaller than 15cm, surface doses relative to the dose at d(max) increased for unflattened beams with maximum differences of 7% at 6MV and 25% at 10MV for a 5x5cm(2) field. For a 30x30cm(2) field, relative surface dose decreased by about 10% for FFF beams. Leaf transmission on the central axis was 0.3% and 0.4% lower for unflattened 6 and 10MV beams, respectively. Leakage radiation was reduced by 52% for 6MV and by 65% for 10MV unflattened beams. CONCLUSIONS The results of the study were independently confirmed at two radiotherapy centres. Phantom scatter reference data need to be reconsidered for medical accelerators operated without a flattening filter.

Flattening filter free beams in SBRT and IMRT: Dosimetric assessment of peripheral doses

PURPOSE Recently, there has been a growing interest in operating medical linear accelerators without a flattening filter. Due to reduced scatter, leaf transmission and radiation head leakage a reduction of out-of-field dose is expected for flattening filter free beams. The aim of the present study was to determine the impact of unflattened beams on peripheral dose for advanced treatment techniques with a large number of MUs. MATERIAL AND METHODS An Elekta Precise linac was modified to provide 6 and 10 MV photon beams without a flattening filter. Basic beam data were collected and implemented into the TPS Oncentra Masterplan (Nucletron). Leakage radiation, which predominantly contributes to peripheral dose at larger distances from the field edge, was measured using a Farmer type ionisation chamber. SBRT (lung) and IMRT (prostate, head&neck) treatment plans were generated for 6 and 10 MV for both flattened and unflattened beams. All treatment plans were delivered to the relevant anatomic region of an anthropomorphic phantom which was extended by a solid water slab phantom. Dosimetric measurements were performed with TLD-700 rods, radiochromic films and a Farmer type ionisation chamber. The detectors were placed within the slab phantom and positioned along the isocentric longitudinal axis. RESULTS Using unflattened beams results in a reduction of treatment head leakage by 52% for 6 and 65% for 10 MV. Thus, peripheral doses were in general smaller for treatment plans calculated with unflattened beams. At about 20 cm distance from the field edge the dose was on average reduced by 23 and 31% for the 6 and 10 MV SBRT plans. For the IMRT plans (10 MV) the average reduction was 16% for the prostate and 18% for the head&neck case, respectively. For all examined cases, the relative deviation between peripheral doses of flattened and unflattened beams was found to increase with increasing distance from the field. CONCLUSIONS Removing the flattening filter lead to reduced peripheral doses for advanced treatment techniques. The relative difference between peripheral doses of flattened and unflattened beams was more pronounced when the nominal beam energy was increased. Patients may benefit by decreased exposure of normal tissue to scattered dose outside the field.

Photon beam quality variations of a flattening filter free linear accelerator

PURPOSE Recently, there has been an increasing interest in operating conventional linear accelerators without a flattening filter. The aim of this study was to determine beam quality variations as a function of off-axis ray angle for unflattened beams. In addition, a comparison was made with the off-axis energy variation in flattened beams. METHODS Two Elekta Precise linear accelerators were modified in order to enable radiation delivery with and without the flattening filter in the beam line. At the Medical University Vienna (Vienna, Austria), half value layer (HVL) measurements were performed for 6 and 10 MV with an in-house developed device that can be easily mounted on the gantry. At St. Lukes Hospital (Dublin, Ireland), measurements were performed at 6 MV in narrow beam geometry with the gantry tilted around 270 degrees with pinhole collimators, an attenuator, and the chamber positioned on the table. All attenuation measurements were performed with ionization chambers and a buildup cap (2 mm brass) or a PMMA mini phantom (diameter 3 cm, measurement depth 2.5 cm). RESULTS For flattened 6 and 10 MV photon beams from the Elekta linac the relative HVL(theta) varies by about 11% for an off-axis ray angle theta = 10 degrees. These results agree within +/- 2% with a previously proposed generic off-axis energy correction. For unflattened beams, the variation was less than 5% in the whole range of off-axis ray angles up to 10 degrees. The difference in relative HVL data was less than 1% for unflattened beams at 6 and 10 MV. CONCLUSIONS Off-axis energy variation is rather small in unflattened beams and less than half the one for flattened beams. Thus, ignoring the effect of off-axis energy variation for dose calculations in unflattened beams can be clinically justified.

A Monte Carlo study of a flattening filter-free linear accelerator verified with measurements

A Monte Carlo model of an Elekta Precise linear accelerator has been built and verified by measured data for a 6 and 10 MV photon beam running with and without a flattening filter in the beam line. In this study the flattening filter was replaced with a 6 mm thick copper plate, provided by the linac vendor, in order to stabilize the beam. Several studies have shown that removal of the filter improves some properties of the photon beam, which could be beneficial for radiotherapy treatments. The investigated characteristics of this new beam included output, spectra, mean energy, half value layer and the origin of scattered photons. The results showed an increased dose output per initial electron at the central axis of 1.76 and 2.66 for the 6 and 10 MV beams, respectively. The number of scattered photons from the accelerator head was reduced by (31.7 ± 0.03)% (1 SD) for the 6 MV beam and (47.6 ± 0.02)% for the 10 MV beam. The photon energy spectrum of the unflattened beam was softer compared to a conventional beam and did not vary significantly with the off-axis distance, even for the largest field size (0-20 cm off-axis).

Dosimetric Considerations to Determine the Optimal Technique for Localized Prostate Cancer Among External Photon, Proton, or Carbon-Ion Therapy and High-Dose-Rate or Low-Dose-Rate Brachytherapy

PURPOSE To assess the dosimetric differences among volumetric modulated arc therapy (VMAT), scanned proton therapy (intensity-modulated proton therapy, IMPT), scanned carbon-ion therapy (intensity-modulated carbon-ion therapy, IMIT), and low-dose-rate (LDR) and high-dose-rate (HDR) brachytherapy (BT) treatment of localized prostate cancer. METHODS AND MATERIALS Ten patients were considered for this planning study. For external beam radiation therapy (EBRT), planning target volume was created by adding a margin of 5 mm (lateral/anterior-posterior) and 8 mm (superior-inferior) to the clinical target volume. Bladder wall (BW), rectal wall (RW), femoral heads, urethra, and pelvic tissue were considered as organs at risk. For VMAT and IMPT, 78 Gy(relative biological effectiveness, RBE)/2 Gy were prescribed. The IMIT was based on 66 Gy(RBE)/20 fractions. The clinical target volume planning aims for HDR-BT ((192)Ir) and LDR-BT ((125)I) were D(90%) ≥34 Gy in 8.5 Gy per fraction and D(90%) ≥145 Gy. Both physical and RBE-weighted dose distributions for protons and carbon-ions were converted to dose distributions based on 2-Gy(IsoE) fractions. From these dose distributions various dose and dose-volume parameters were extracted. RESULTS Rectal wall exposure 30-70 Gy(IsoE) was reduced for IMIT, LDR-BT, and HDR-BT when compared with VMAT and IMPT. The high-dose region of the BW dose-volume histogram above 50 Gy(IsoE) of IMPT resembled the VMAT shape, whereas all other techniques showed a significantly lower high-dose region. For all 3 EBRT techniques similar urethra D(mean) around 74 Gy(IsoE) were obtained. The LDR-BT results were approximately 30 Gy(IsoE) higher, HDR-BT 10 Gy(IsoE) lower. Normal tissue and femoral head sparing was best with BT. CONCLUSION Despite the different EBRT prescription and fractionation schemes, the high-dose regions of BW and RW expressed in Gy(IsoE) were on the same order of magnitude. Brachytherapy techniques were clearly superior in terms of BW, RW, and normal tissue sparing, with lowest values for HDR-BT.

Evaluation of treatment plan quality of IMRT and VMAT with and without flattening filter using Pareto optimal fronts

PURPOSE To investigate the differences in treatment plan quality of IMRT and VMAT with and without flattening filter using Pareto optimal fronts, for two treatment sites of different anatomic complexity. MATERIALS AND METHODS Pareto optimal fronts (POFs) were generated for six prostate and head-and-neck cancer patients by stepwise reduction of the constraint (during the optimization process) of the primary organ-at-risk (OAR). 9-static field IMRT and 360°-single-arc VMAT plans with flattening filter (FF) and without flattening filter (FFF) were compared. The volume receiving 5 Gy or more (V5 Gy) was used to estimate the low dose exposure. Furthermore, the number of monitor units (MUs) and measurements of the delivery time (T) were used to assess the efficiency of the treatment plans. RESULTS A significant increase in MUs was found when using FFF-beams while the treatment plan quality was at least equivalent to the FF-beams. T was decreased by 18% for prostate for IMRT with FFF-beams and by 4% for head-and-neck cases, but increased by 22% and 16% for VMAT. A reduction of up to 5% of V5 Gy was found for IMRT prostate cases with FFF-beams. CONCLUSIONS The evaluation of the POFs showed an at least comparable treatment plan quality of FFF-beams compared to FF-beams for both treatment sites and modalities. For smaller targets the advantageous characteristics of FFF-beams could be better exploited.

Radiation therapy with unflattened photon beams: Dosimetric accuracy of advanced dose calculation algorithms

PURPOSE To compare the dosimetric accuracy of advanced dose calculation algorithms for flattened (FF) and unflattened (FFF) photon beams. MATERIAL AND METHODS We compared the enhanced collapsed cone (eCC) algorithm implemented in OncentraMasterplan and the XVMC (MC) code in Monaco. Test plans were created for 10MV FF and FFF beams. Single beam tests were delivered to radiochromic films positioned within a solid water phantom and evaluated with 1D γ-index analysis. Conformal plans were verified with ion chambers in an anthropomorphic thorax phantom. IMRT plans were applied to the Delta4 system and evaluated with γ-criteria of 3% and 3mm. RESULTS 1D γ-index evaluation revealed significantly lower (p<0.05) average γ(mean)-values of 0.46±0.22 for MC calculated FFF profiles compared to average values of 0.53±0.27 detected for FF beams. Respective values for eCC were 0.42±0.27/0.38±0.26 (FF/FFF). When considering off-axis profiles separately, we found significantly reduced average γ(mean)-values for FFF and both algorithms (MC: 0.55±24 vs. 0.45±0.21, eCC: 0.41±0.24 vs. 0.35±0.22). No significant differences were detected on-axis. Absolute dosimetry in the anthropomorphic phantom revealed superior results for MC based dose calculation, with mean deviations of 0.8±0.8/0.0±1.0% compared to -0.1±1.7/-0.5±0.1.7% (FF/FFF) for the eCC algorithm. IMRT plans showed similar results for both linac modes. CONCLUSIONS The dose calculation accuracy for unflattened beams was found to be at least as high as for flattened beams. The slightly improved dose calculation accuracy observed for off-axis profiles for single FFF beams did not directly translate into better verification results for composite IMRT plans.

Clinical Comparison of Dose Calculation Using the Enhanced Collapsed Cone Algorithm vs. a New Monte Carlo Algorithm

Purpose:Comparison of the dosimetric accuracy of the enhanced collapsed cone (eCC) algorithm with the commercially available Monte Carlo (MC) dose calculation for complex treatment techniques.Material and Methods:A total of 8 intensity-modulated radiotherapy (IMRT) and 2 stereotactic body radiotherapy (SBRT) lung cases were calculated with eCC and MC algorithms with the treatment planning systems (TPS) Oncentra MasterPlan 3.2 (Nucletron) and Monaco 2.01 (Elekta/CMS). Fluence optimization as well as sequencing of IMRT plans was primarily performed using Monaco. Dose prediction errors were calculated using MC as reference. The dose–volume histrogram (DVH) analysis was complemented with 2D and 3D gamma evaluation. Both algorithms were compared to measurements using the Delta4 system (Scandidos).Results:Recalculated with eCC IMRT plans resulted in lower planned target volume (PTV) coverage, as well as in lower organs-at-risk (OAR) doses up to 8%. Small deviations between MC and eCC in PTV dose (1–2%) were detected for IMRT cases, while larger deviations were observed for SBRT (up to 5%). Conformity indices of both calculations were similar; however, the homogeneity of the eCC calculated plans was slightly better. Delta4 measurements confirmed high dosimetric accuracy of both TPS.Conclusion:Mean dose prediction errors < 3% for PTV suggest that both algorithms enable highly accurate dose calculations under clinical conditions. However, users should be aware of slightly underestimated OAR doses using the eCC algorithm.Ziel:Vergleich der Dosisberechnungsgenauigkeit eines erweiterten Collapsed-Cone-(eCC-)Algorithmus mit einem kommerziell verfügbaren Monte-Carlo-(MC-)Algorithmus anhand von komplexen Bestrahlungstechniken.Material und Methodik: 8 IMRT- und 2 SBRT-Pläne wurden jeweils mit dem eCC-Algorithmus des Bestrahlungsplanungssystems Oncentra 3.2 (Nucletron) und mit dem Monte-Carlo-Algorithmus von Monaco 2.01 (Elekta/CMS) berechnet. Die Fluenz-Optimierung und Sequenzierung der IMRT-Pläne wurden mittels Monaco durchgeführt. Zusätzlich zur DVH-Analyse wurden eine 2D und 3D Gamma-Index-Evaluierung durchgeführt. Darüber hinaus erfolgte eine dosimetrische Überprüfung der Bestrahlungspläne mit dem Delta4-System (Scandidos).Ergebnisse:Im Vergleich zum MC-Algorithmus resultierte die Nachberechnung der IMRT-Pläne mit dem eCC in einer geringeren Abdeckung der Zielvolumina sowie in einer um bis zu 8% niedrigeren Dosis für Risikoorgane (OAR). Bei den IMRT-Fällen waren die Abweichungen zwischen den PTV-Dosen der beiden Algorithmen gering (1–2%), wohingegen die größten Abweichungen (bis zu 5%) für SBRT festgestellt wurden. Die ermittelten Konformitätsindizes waren für beide Algorithmen sehr ähnlich; die Homogenität war für eCC-Pläne geringfügig besser. Die Delta4-Messungen bestätigten die hohe Dosisberechnungsgenauigkeit beider Algorithmen.Schlussfolgerung:Die mittlere Unsicherheit der berechneten PTV-Dosen betrug weniger als 3%, d.h. mit beiden Algorithmen kann eine zuverlässige Dosisvorhersage unter klinischen Bedingungen erzielt werden. Bei der Beurteilung von mit eCC berechneten Plänen sollte die geringfügige Unterschätzung der Dosis in OAR berücksichtigt werden.

Impact of a flattening filter free linear accelerator on structural shielding design

PURPOSE The present study aimed to assess the effects of a flattening filter free medical accelerator on structural shielding demands of a treatment vault of a medical linear accelerator. We tried to answer the question, to what extent the required thickness of the shielding barriers can be reduced if instead of the standard flattened photon beams unflattened ones are used. MATERIAL AND METHODS We chose both an experimental as well as a theoretical approach. On the one hand we measured photon dose rates at protected places outside the treatment room and compared the obtained results for flattened and unflattened beams. On the other hand we complied with international guidelines for adequate treatment vault design and calculated the shielding barriers according to the therein given specifications. Measurements were performed with an Elekta Precise™ linac providing nominal photon energies of 6 and 10 MV. This machine underwent already earlier some modifications in order to be able to operate both with and without a flattening filter. Photon dose rates were measured with a LB133-1 dose rate meter manufactured by Berthold. To calculate the thickness of shielding barriers we referred to the Austrian standard ÖNORM S 5216 and to the US American NCRP Report No. 151. RESULTS We determined a substantial photon dose rate reduction for all measurement points and photon energies. For unflattened 6 MV beams a reduction factor ranging from 1.4 to 1.8 was identified. The corresponding values for unflattened 10 MV beams were 2.1 and 3.2. The performed shielding calculations indicated the same tendency: For all relevant radiation components we found a reduction in shielding thickness when unflattened beams were used. The required thickness of primary barriers was reduced up to 8.0%, the thickness of secondary barriers up to 11.4%, respectively. CONCLUSIONS For an adequate dimensioning of treatment vault shielding barriers it is by no means irrelevant if the accommodated linac operates with or without a flattening filter. The lower consumption of shielding space and material for new treatment vaults housing a FFF machine may reduce building costs, whereas for existing vaults one might benefit in terms of increased weekly workload. Also a more frequent use of monitor unit intense treatment techniques as well as aiming at reduced occupational exposure for staff is conceivable.

The technological basis for adaptive ion beam therapy at MedAustron: Status and outlook

The ratio of patients who need a treatment adaptation due to anatomical variations at least once during the treatment course is significantly higher in light ion beam therapy (LIBT) than in photon therapy. The ballistic behaviour of ion beams makes them more sensitive to changes. Hence, the delivery of LIBT has always been supported by state of art image guidance. On the contrary CBCT technology was adapted for LIBT quite late. Adaptive concepts are being implemented more frequently in photon therapy and also efficient workflows are needed for LIBT. The MedAustron Ion Beam Therapy Centre was designed to allow the clinical implementation of adaptive image-guided concepts. The aim of this paper is to describe the current status and the potential future use of the technology installed at MedAustron. Specifically addressed is the beam delivery system, the patient alignment system, the treatment planning system as well as the Record & Verify system. Finally, an outlook is given on how high quality X-ray imaging, MR image guidance, fast and automated treatment planning as well as in vivo range verification methods could be integrated.