Abstract

Flattening filter‐free (FFF) beams are available on an increasing number of commercial linear accelerators. FFF beams have higher dose rates than flattened beams of equivalent energy which can lead to increased efficiency of treatment delivery, especially in conjunction with increased FFF beam energy and arc‐based delivery configurations. The purpose of this study is to quantify and assess the implications of improved treatment efficiency for several FFF delivery options on common types of linac applicable radiotherapy. Eleven characteristic cases representative of a variety of clinical treatment sites and prescription doses were selected from our patient population. Treatment plans were generated for a Varian TrueBeam linear accelerator. For each case, a reference plan was created using DMLC IMRT with 6 MV flat beams. From the same initial objectives, plans were generated using DMLC IMRT and volumetric‐modulated arc therapy (VMAT) with 6 MV FFF and 10 MV FFF beams (max. dose rates of 1400 and 2400 MU/min, respectively). The plans were delivered to a phantom; beam‐on time, total treatment delivery time, monitor units (MUs), and integral dose were recorded. For plans with low dose fractionations (1.8–2.0 & 3.85 Gy/fraction), mean beam‐on time difference between reference plan and most efficient FFF plan was 0.56 min (41.09% decrease); mean treatment delivery time difference between the reference plan and most efficient FFF plan was 1.54 min (range: 0.31–3.56 min), a relative improvement of 46.1% (range: 29.2%‐59.2%). For plans with high dose fractionations (16–20 Gy/fraction), mean beam‐on time difference was 6.79 min (74.9% decrease); mean treatment delivery time difference was 8.99 min (range: 5.40–13.05 min), a relative improvement of 71.1% (range: 53.4%‐82.4%). 10 MV FFF VMAT beams generated the most efficient plan, except in the spine SBRT case. The distribution of monitor unit counts did not vary by plan type. In cases where respiratory motion management would be applicable, 10 MV FFF DMLC IMRT reduced beam‐on time/field to less than 12 sec. FFF beams significantly reduced treatment delivery time. For radiosurgical doses, the efficiency improvement for FFF beams was clinically significant. For conventional fractionation, a large improvement in relative treatment delivery time was observed, but the absolute time savings were not likely to be of clinical value. In cases that benefit from respiratory motion management, beamon/field was reduced to a time for which most patients can comfortably maintain deep inspiratory breath hold.PACS numbers: 87.55.D‐, 87.55.de, 87.56.bd, 87.56.N‐

Highlights

  • Flattening filters were incorporated in linear accelerator beam lines to increase the dose homogeneity,(1) which was important for minimizing normal tissue toxicity.[2]. As the clinical use of cranial radiosurgery and extracranial stereotactic body radiation therapy (SBRT) has increased, there has been increasing interest in the use of flattening filter-free (FFF) beams to decrease treatment time and reduce scatter.[3,4] Decreased treatment time is meaningful if it can reduce the duration of treatment time slots, improve the patient’s treatment experience, or reduce the likelihood of intrafraction motion

  • Volumetric-modulated arc therapy (VMAT).(6) The first reported clinical use of a FFF beam was in 1991 to reduce the long treatment time for high-dose radiosurgical cases.[7]. A variety of other treatment planning studies have since examined the implications of removal of the flattening filter for its potential to reduce beam-on time and, patient treatment time.[3,6,8,9,10] Mancosu et al[11] have recently reported in detail on their institutional experiences using FFF beams in volumetric-modulated arc therapy (VMAT) delivery configurations for liver SBRT treatments

  • In a recent study at our institution, we demonstrated that clinical use of a FFF linear accelerator for lung and liver SBRT reduces treatment delivery time by 50% compared to conventional dose rates.[12]

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Summary

Introduction

Flattening filters were incorporated in linear accelerator beam lines to increase the dose homogeneity,(1) which was important for minimizing normal tissue toxicity.[2]. Volumetric-modulated arc therapy (VMAT).(6) The first reported clinical use of a FFF beam was in 1991 to reduce the long treatment time for high-dose radiosurgical cases.[7] A variety of other treatment planning studies have since examined the implications of removal of the flattening filter for its potential to reduce beam-on time and, patient treatment time.[3,6,8,9,10] Mancosu et al[11] have recently reported in detail on their institutional experiences using FFF beams in VMAT delivery configurations for liver SBRT treatments. TrueBeam (Varian Medical Systems, Palo Alto, CA) has two high-intensity FFF modes: a 6 MV photon beam (1400 MU/min) and a 10 MV photon beam (2400 MU/min) These high-dose rates have the potential to improve treatment efficiency for computer optimized treatment plans. In a recent study at our institution, we demonstrated that clinical use of a FFF linear accelerator for lung and liver SBRT reduces treatment delivery time by 50% compared to conventional dose rates.[12]

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