Flattening filter-free linac improves treatment delivery efficiency in stereotactic body radiation therapy

Abstract

Stereotactic body radiation therapy (SBRT) employs precision target tracking and image-guidance techniques to deliver ablative doses of radiation to localized malignancies; however, treatment with conventional photon beams requires lengthy treatment and immobilization times. The use of flattening filter-free (FFF) beams operating at higher dose rates can shorten beam-on time, and we hypothesize that it will shorten overall treatment delivery time. A total of 111 lung and liver SBRT cases treated at our institution from July 2008 to July 2011 were reviewed and 99 cases with complete data were identified. Treatment delivery times for cases treated with a FFF linac versus a conventional dose rate linac were compared. The frequency and type of intrafraction image guidance was also collected and compared between groups. Three hundred and ninety-one individual SBRT fractions from 99 treatment plans were examined; 36 plans were treated with a FFF linac. In the FFF cohort, the mean (± standard deviation) treatment time (time elapsed from beam-on until treatment end) and patient's immobilization time (time from first alignment image until treatment end) was 11.44 (± 6.3) and 21.08 (± 6.8) minutes compared to 32.94 (± 14.8) and 47.05 (± 17.6) minutes for the conventional cohort (p < 0.01 for all values). Intrafraction-computed tomography (CT) was used more often in the conventional cohort (84% vs. 25%; p < 0.05), but use of orthogonal X-ray imaging remained the same (16% vs. 19%). For lung and liver SBRT, a FFF linac reduces treatment and immobilization time by more than 50% compared to a conventional linac. In addition, treatment with a FFF linac is associated with less physician-ordered image guidance, which contributes to further improvement in treatment delivery efficiency.

Figure 1

A schematic representation of the crossbeam profile of a conventional (with flattening filter) 10 megavolt photon beam (dashed line) is compared to the crossbeam profile of an unflattened photon beam (solid line) of equivalent energy. The unflattened beam has approximately four times higher dose rate at central axis.

Figure 2

Treatment delivery time comparison: the overall length of the bars indicates the total immobilization time (IT) was 46.8 minutes and 21.1 minutes for the conventional and FFF cohorts, respectively. The unshaded region represents image guidance time (IGT), which was only marginally different between the two groups (13.9 vs. 9.6 minutes for conventional and FFF, respectively). The checkered portion represents beam‐on time (BOT), which was more than two times longer in the conventional group than the FFF group (5.6 vs. 2.3 minutes, respectively). The striped region represents intrafraction downtime (IFDT), which was substantially longer for the conventional group than the FFF group (27.3 minutes vs. 9.2 minutes, respectively).