Flattening Filter Free Linear Accelerator Research Articles

Assessment of Photon Beam Skyshine Dose Equivalent Rate of a 4 MeV Radiation Therapy Bunker Using Analytical and Monte Carlo Methods

Purpose: Skyshine radiation dose equivalent dose rate is known as scattered radiation by the room above air to points at the ground level points outside the Linear Accelerator (LINAC) room. Our aim was to estimate skyshine around the LINAC-based radiotherapy by a 4MV LINAC photon beam.
 Materials and Methods: Monte Carlo (MC) MCNP code calculation was conducted to skyshine at the control room, 60Co treatment room, physics, and simulator rooms. National Council on Radiation Protection and Measurements (NCRP) 151 was also used and it reported analytical formulation methods for photon beam calculation. A Flattering Filter (FF) equipped and Flattening Filter-Free (FFF) LINACs photon beams were derived and differences and agreements were discussed.
 Results: The results showed high skyshine for FF equipped relative to FFF LINACs. This effect may be attributed to photon beam hardening by FF in the LINAC head and higher transmission through the ceiling shield and more presence of photons on the roof above the air. NCRP 151 method results were higher than MC simulated photon beam skyshine dose equivalent dose rate and it may be the cause of the inflexible analytical method in contrast to MC simulation. Finally, FFF and FF-equipped LINACs result in skyshine compared and they compared to NCRP 151 report. MC simulation performed reasonably in estimation in different conditions.
 Conclusion: Our results showed that FF-equipped skyshine is higher than FFF LINAC and NCRP 151 is an inflexible method that does not take some effective parameters into account and calculates skyshine higher.

The characteristics of small field beam quality and output factor of 6 MV FFF

Flattening filter free linear accelerator (FFF LINAC) has been installed in Indonesia. To ensure the accuracy of FFF irradiation, we evaluate the characteristic of FFF in regular and small fields. We employed the Monte Carlo (MC) simulation of FFF LINAC 6 MV as a standard reference in this study. Then, we compared the calculation of FFF beam at central and lateral axis to the measurement and TPS at 10 × 10, 1 × 1, 2 × 2, 3 × 3, 4 × 4 cm2 field sizes. Output factor (OF) and beam quality such that TPR20,10 and penumbra of FFF LINAC were evaluated. TPR20,10(S) for 10 × 10 cm2 was 0.01 differ between MC and measurement whereas it varied from 0.588 to 0.703 for small fields. Dose difference of lateral axis was agreed within 3% except for the penumbra region. Output factor of small fields measurement indicated that field size of 1 × 1 cm2 had a large discrepancy to MC according to this works. The TPR20,10(10) of MC, TPS, and measurement was not a significant difference, while the OF was tending to a large deviation in 1 × 1 cm2. The results showed that our FFF LINAC could be used for small field until 2 × 2 cm2.

Volumetric-modulated arc therapy and intensity-modulated radiation therapy treatment planning for prostate cancer with flattened beam and flattening filter free linear accelerators.

This study on patients with localized prostate cancer was set up to investigate valuable differences using flattened beam (FB) and flattening filter free (FFF) mode in the application of intensity‐modulated radiotherapy (IMRT) and volumetric‐modulated arc therapy (VMAT). For ten patients, four different plans were calculated with Oncentra planning system of Elekta, using Synergy machines: IMRT and VMAT, with and without flattening filter. Homogeneity and conformity indexes, dose to the organs at risk, and measurements of peripheral dose and dosimetric plan verification including record of the delivery times were analyzed and statistically evaluated. The indexes for homogeneity and conformity (CTV and PTV) are either advantageous or not significantly different for FFF compared to FB with one minor exception. Regarding the doses to the organs at risk and the measured peripheral dose, equivalent or lower doses were delivered for FFF than with FB. Furthermore, the delivery times were significantly shorter for FFF. VMAT compared to IMRT reveals benefits or at least equivalent values. VMAT‐FFF combines the most advantageous plan quality parameters with the shortest delivery times and reduced peripheral dose and is therefore recommended for the given equipment and cancer localization.

Structural Shielding Design of a 6 MV Flattening Filter Free Linear Accelerator: Indian Scenario.

Detailed structural shielding of primary and secondary barriers for a 6 MV medical linear accelerator (LINAC) operated with flattening filter (FF) and flattening filter free (FFF) modes are calculated. The calculations have been carried out by two methods, one using the approach given in National Council on Radiation Protection (NCRP) Report No. 151 and the other based on the monitor units (MUs) delivered in clinical practice. Radiation survey of the installations was also carried out. NCRP approach suggests that the primary and secondary barrier thicknesses are higher by 24% and 26%. respectively, for a LINAC operated in FF mode to that of a LINAC operated in both FF and FFF modes with an assumption that only 20% of the workload is shared in FFF mode. Primary and secondary barrier thicknesses calculated from MUs delivered on clinical practice method also show the same trend and are higher by 20% and 19%, respectively, for a LINAC operated in FF mode to that of a LINAC operated in both FF and FFF modes. Overall, the barrier thickness for a LINAC operated in FF mode is higher about 20% to that of a LINAC operated in both FF and FFF modes.

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

PurposeThe 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 MethodsWe 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. ResultsWe 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. ConclusionsFor 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.

Photon beam quality variations of a flattening filter free linear accelerator

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. 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. Luke's 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). 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. 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.

SU-FF-T-70: A Treatment Planning Study of Liver Cancer Treatments with a Flattening Filter Free Linear Accelerator

Purpose: Our preliminary studies have shown that the removal of the flattening filter from the beam‐line of a medicallinear accelerator increases the dose rate, reduces head scatter and reduces dose outside the treatment volume. Although, in principle, the filter is unnecessary when a multileaf collimator is present, it remains to be seen whether treatments with unflattened beams are clinically feasible. In this study we investigate the application of flattening filter free photon beams for the treatment of small livercancertumors to determine if these beams offer an advantage for respiratory‐gated radiotherapy. Methods and Materials: This is a treatment planning study. We created plans using Eclipse 8.0 (Varian Medical Systems), which we commissioned with measured (6 MV, 18 MV) and Monte Carlo (10 MV) data for a Clinac 21EX linear accelerator, operated with and without a flattening filter. We selected several conventional 3D conformal treatment plans for flattened beams with field sizes not exceeding 10 cm × 10 cm. These were then compared with treatment plans, including 3D conformal plans and IMRT plans, developed with unflattened beams. Results:Dose distributions for treatment volumes and nearby critical structures were typically equivalent for the intensity‐modulated plans with flattened and unflattened beams. The number of monitor units required to deliver the same dose was much lower in the plans with unflattened beams. For example, at 18 MV the total number of monitor units in the intensity‐modulated plans with unflattened beams was about 1/2 of those in the conformal 3D plans with flattened beams. Conclusion: In this study we show that clinically acceptable treatment plans for livercancer can be obtained with unflattened beams. With respiratory‐gated radiotherapy, the high dose rates achieved with unflattened beams may shorten the overall treatment time dramatically. Conflict of Interest: Research sponsored by Varian Medical Systems.