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

Linac primary barrier transmission: Flattening filter free and field size dependence

There is widespread consensus in the literature that flattening filter free (FFF) beams have a lower primary barrier transmission than flattened beams. Measurements presented here, however, show that for energy compensated FFF beams, the barrier transmission can be as much as 70% higher than for flattened beams. The ratio of the FFF barrier transmission to the flattened beam barrier transmission increases with increasing barrier thickness. The use of published FFF TVL data for energy compensated FFF beams could lead to an order of magnitude underestimate of the air kerma rate. There are little data in the literature on the field size dependence of the barrier transmission for flattened beams. Barrier transmission depends on the field size at the barrier, not at isocenter Measurements are presented showing the relative dependence of barrier transmission on the field size, measured at the barrier, for 6 MV and 10 MV beams. An analytical fitting formula is provided for the field size dependence. For field sizes greater than about 150 cm in side length, the field size dependence is minimal. For field sizes less than about 100 cm, the transmission declines rapidly as the field size decreases.

Linear accelerator bunker shielding for stereotactic radiotherapy

Shielding protocols such as NCRP 151, IAEA SRS 47 and IPEM 47 are commonly used for the design of radiotherapy facilities. Some work has been accomplished in updating the basic formula with the advent of IMRT but little consideration has been made for unflattened beams and stereotactic techniques apart from for facilities housing devices like the CyberKnife. The inevitable scenario of a stereotactic-only linear accelerator was considered in this study. The necessary shielding requirements were determined based on stereotactic data from a year's worth treatments from one clinic as well as further measurements of leakage, scatter and use factors. These values were compared to recommendations in the literature. While tenth value layer amounts, workload and barrier widths could be kept at the status quo, major changes could be made to the parameters of leakage, scatter and use factors while still maintaining safety. Some differences could also be seen for the use of IMRT factors. Current shielding protocols were found to inadequately describe methodology for the shielding of a stereotactic-only radiotherapy linac bunker, producing overly-conservative wall thicknesses which is in disagreement with the principles of ICRP.

Comparison of Flattening Filter and Flattening Filter-Free Volumetric Modulated Arc Radiotherapy in Patients with Locally Advanced Nasopharyngeal Carcinoma

BACKGROUND This study aimed to investigate the therapeutic role of flattening filter-free (FFF) mode in volumetric modulated arc therapy (VMAT) compared with flattening filter (FF) mode in patients with locally advanced nasopharyngeal carcinoma (NPC). MATERIAL AND METHODS Ten previously treated patients with NPC underwent treatment re-planning with FFF and FF VMAT. Radiotherapy dose distribution on planning target volume (PTV), organs at risk (OAR), target conformity index (CI), total monitor units (MUs), and therapeutic time were compared. RESULTS Maximum and mean radiotherapy dose in PTV and PGTV (primary lesions of NPC and cervical lymph node metastases) in FFF VMAT planning were significantly increased compared with FF VMAT planning, but PTV and OAR showed no significant differences. The CI value of PTV in FFF VMAT planning was significantly reduced compared with FF planning (P<0.05). No differences were found for the maximum radiotherapy dose in the spinal cord and left and right optic nerve, and the mean radiotherapy dose in the brainstem, left and right parotid gland (P>0.05). The maximum dose in the brainstem in the FFF planning was significantly higher compared with FF planning (P>0.05). The maximum radiotherapy dose in left and right crystalline lens (P<0.05) in FFF planning was significantly reduced compared with FF planning. The total hop count in FFF planning was significantly increased compared with FF planning (P<0.05). CONCLUSIONS Both 6 MV X-ray FFF mode and FF mode in the treatment of patients with NPC showed that FFF VMAT planning provided improved protection for OAR.

Photon dose at the maze entrance door: The comparison of flattening filter and flattening filter free working modes

In recent years, field flattening free accelerators have been introduced in therapy practice. One of the objective of these measurements was to establish if the maze door, designed for accelerators operating with flattening filter can provide adequate shielding in field flattening free mode of operation. Linac installed in this standard one band maze vault is equipped to operate at 6 MV with field flattening filter and in field flattening free mode of operation. Series of measurements of the photon dose at the maze door (with different jaws openings and gantry positions) were performed in both operation modes with and without water canister to mimic standard therapy conditions. In this paper results of photon dose measurements, performed at the maze door of the therapy linear accelerator vault are presented in order to compare photon dose in flattening filter and flattening filter free operation modes. It was obtained that in field flattening free mode of operation, the dose at the maze door is always lower than the dose measured in standard mode of operation with the field flattening filter. In the case when FFF therapy practice should start in some existing therapy vault, no additional shielding measures need to be added at the existing maze door.

Results of a 10-year survey of workload for 10 treatment vaults at a high-throughput comprehensive cancer center

The workload for shielding purposes of modern linear accelerators (linacs) consists of primary and scatter radiation which depends on the dose delivered to isocenter (cGy) and leakage radiation which depends on the monitor units (MUs). In this study, we report on the workload for 10 treatment vaults in terms of dose to isocenter (cGy), monitor units delivered (MUs), number of treatment sessions (Txs), as well as, use factors (U) and modulation factors (CI) for different treatment techniques. The survey was performed for the years between 2006 and 2015 and included 16 treatment machines which represent different generations of Varian linear accelerators (6EX, 600C, 2100C, 2100EX, and TrueBeam) operating at different electron and x‐ray energies (6, 9, 12, 16 and 20 MeV electrons and, 6 and 15 MV x‐rays). An institutional review board (IRB) approval was acquired to perform this study. Data regarding patient workload, dose to isocenter, number of monitor units delivered, beam energies, gantry angles, and treatment techniques were exported from an ARIA treatment management system (Varian Medical Systems, Palo Alto, Ca.) into Excel spreadsheets and data analysis was performed in Matlab. The average (± std‐dev) number of treatment sessions, dose to isocenter, and number of monitor units delivered per week per machine in 2006 was 119 ± 39 Txs, (300 ± 116) × 10²cGys, and (78 ± 28) × 10³MUs respectively. In contrast, the workload in 2015 was 112 ± 40 Txs, (337 ± 124) × 10²cGys, and (111 ± 46) × 10³MUs. 60% of the workload (cGy) was delivered using 6 MV and 30% using 15 MV while the remaining 10% was delivered using electron beams. The modulation factors (MU/cGy) for IMRT and VMAT were 5.0 (± 3.4) and 4.6 (± 1.6) respectively. Use factors using 90° gantry angle intervals were equally distributed (~0.25) but varied considerably among different treatment techniques. The workload, in terms of dose to isocenter (cGy) and subsequently monitor units (MUs), has been steadily increasing over the past decade. This increase can be attributed to increased use of high dose hypo‐fractionated regimens (SBRT, SRS) and the increase in use of IMRT and VMAT, which require higher MUs per cGy as compared to more conventional treatment (3DCRT). Meanwhile, the patient workload in terms of treatment sessions per week remained relatively constant. The findings of this report show that variables used for shielding purposes still fall within the recommendation of NCRP Report 151.

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.

Re-irradiating spinal column metastases using IMRT and VMAT with and without flattening filter - a treatment planning study

Background

The aim of this study was to investigate the potential of the flattening filter free (FFF) mode of a linear accelerator for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) for patients with in-field recurrence of vertebral metastases.

Methods

An Elekta Synergy Linac with Agility™ head is used to simulate the treatment of ten patients with locally recurrent spinal column metastases. Four plans were generated for each patient treating the vertebrae sparing the spinal cord: Dual arc VMAT and nine field step and shoot IMRT each with and without flattening filter. Plan quality was assessed considering target coverage and sparing of the spinal cord and normal tissue. All plans were verified by a 2D-ionisation-chamber-array, peripheral doses were measured and compared to calculations. Delivery times were measured and compared. The Wilcoxon test was used for statistical analysis with a significance level of 0.05.

Results

Target coverage, homogeneity index and conformity index were comparable for both flat and flattening filter free beams. The volume of the spinal cord receiving the allowed maximum dose to keep the risk of radiation myelopathy at 0 % was at the same time significantly reduced to below the clinically relevant 1 ccm using FFF mode. In addition the mean dose deposited in the surrounding healthy tissue was significantly reduced in the FFF mode. All four techniques showed equally good gamma scores for plan verification. FFF plans required considerably more MU per fraction dose. Regardless of the large number of MU, out-of-field point dose was significantly lower for FFF plans, with an average reduction of 33 % and mean delivery time was significantly reduced by 22 % using FFF beams. When compared to IMRT FF, VMAT FFF offered even a reduction of 71 % in delivery time and 45 % in peripheral dose.

Conclusions

FFF plans showed a significant improvement in sparing of normal tissue and the spinal cord, keeping target coverage and homogeneity comparable. In addition, delivery times were significantly reduced for FFF treatments, minimizing intrafractional motion as well as strain for the patient. Shortest delivery times were achieved using VMAT FFF. For radiotherapy of spinal column metastases VMAT FFF may therefore be considered the preferable treatment option for the combination of Elekta Synergy Linacs and Oncentra® External Beam v4.5 treatment planning system.

Flattening filter-free accelerators: a report from the AAPM Therapy Emerging Technology Assessment Work Group

This report describes the current state of flattening filter‐free (FFF) radiotherapy beams implemented on conventional linear accelerators, and is aimed primarily at practicing medical physicists. The Therapy Emerging Technology Assessment Work Group of the American Association of Physicists in Medicine (AAPM) formed a writing group to assess FFF technology. The published literature on FFF technology was reviewed, along with technical specifications provided by vendors. Based on this information, supplemented by the clinical experience of the group members, consensus guidelines and recommendations for implementation of FFF technology were developed. Areas in need of further investigation were identified. Removing the flattening filter increases beam intensity, especially near the central axis. Increased intensity reduces treatment time, especially for high‐dose stereotactic radiotherapy/radiosurgery (SRT/SRS). Furthermore, removing the flattening filter reduces out‐of‐field dose and improves beam modeling accuracy. FFF beams are advantageous for small field (e.g., SRS) treatments and are appropriate for intensity‐modulated radiotherapy (IMRT). For conventional 3D radiotherapy of large targets, FFF beams may be disadvantageous compared to flattened beams because of the heterogeneity of FFF beam across the target (unless modulation is employed). For any application, the nonflat beam characteristics and substantially higher dose rates require consideration during the commissioning and quality assurance processes relative to flattened beams, and the appropriate clinical use of the technology needs to be identified. Consideration also needs to be given to these unique characteristics when undertaking facility planning. Several areas still warrant further research and development. Recommendations pertinent to FFF technology, including acceptance testing, commissioning, quality assurance, radiation safety, and facility planning, are presented. Examples of clinical applications are provided. Several of the areas in which future research and development are needed are also indicated.

PACS number: 87.53.‐j, 87.53.Bn, 87.53.Ly, 87.55.‐x, 87.55.N‐, 87.56.bc