Clinical Investigation
Fast Arc Delivery for Stereotactic Body Radiotherapy of Vertebral and Lung Tumors

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Purpose

Flattening filter–free (FFF) beams with higher dose rates and faster delivery are now clinically available. The purpose of this planning study was to compare optimized non-FFF and FFF RapidArc plans for stereotactic body radiotherapy (SBRT) and to validate the accuracy of fast arc delivery.

Methods and Material

Ten patients with peripheral lung tumors and 10 with vertebral metastases were planned using RapidArc with a flattened 6-MV photon beam and a 10-MV FFF beam for fraction doses of 7.5–18 Gy. Dosimetry of the target and organs at risk (OAR), number of monitor units (MU), and beam delivery times were assessed. GafChromic EBT2 film measurements of FFF plans were performed to compare calculated and delivered dose distributions.

Results

No major dosimetric differences were seen between the two delivery techniques. For lung SBRT plans, conformity indices and OAR doses were similar, although the average MU required were higher with FFF plans. For vertebral SBRT, FFF plans provided comparable PTV coverage, with no significant differences in OAR doses. Average beam delivery times were reduced by a factor of up to 2.5, with all FFF fractions deliverable within 4 min. Measured FFF plans showed high agreement with calculated plans, with more than 99% of the area within the region of interest fulfilling the acceptance criterion.

Conclusion

The higher dose rate of FFF RapidArc reduces delivery times significantly, without compromising plan quality or accuracy of dose delivery.

Introduction

Stereotactic body radiotherapy (SBRT) is an established treatment approach for both curative and palliative indications. High control rates with minimal toxicity have been reported by studies in early-stage non-small-cell lung cancer (NSCLC) (1) and vertebral metastases 2, 3. These excellent outcomes are achieved by precise delivery of high radiation doses to the target in a single or a few fractions, while sparing the surrounding normal tissues. To achieve such high dose conformity and steep dose fall-off outside the lesion, SBRT for lung tumors is commonly delivered using several coplanar and noncoplanar static beams, or volumetric modulated arc therapy. Stereotactic body radiotherapy for vertebral metastases is often delivered using seven or more static intensity-modulated radiotherapy beams or volumetric modulated arc therapy with multiple arcs (4).

Stereotactic body radiotherapy delivery times are often prolonged owing to the high dose per fraction, limited dose rate, use of multiple treatment beams, and intensity-modulated radiotherapy delivery. For typical SBRT treatments, the monitor units (MU) required for a fraction dose in excess of 10 Gy are in the range of 2000–10,000 MU. Total setup and treatment times, inclusive of all time spent with patient on the couch for SBRT, can extend up to 90 min (2) and 60 min (5) for SBRT of vertebral metastases and NSCLC, respectively. Extended treatment times can increase the risk of tumor displacement during delivery (6) and necessitate extra imaging for position verification. Therefore, it is logical to investigate faster delivery as one component of reducing overall treatment time and facilitating treatment accuracy.

Different volumetric modulated arc therapy approaches have been used to deliver SBRT 4, 7. One of these is RapidArc (Varian Medical Systems, Palo Alto, CA), which permits efficient delivery of highly conformal dose distributions (8). However, in particular for high fraction doses, the minimum treatment time may be substantially influenced by the maximum dose rate. The dose rate of a beam can be increased by removal of the flattening filter. The resulting flattening filter–free (FFF) beams have a cone-shaped dose profile (9) and up to a fourfold higher dose rate in the center of the beam. The use of inverse planning that takes into account the basic beam profile facilitates the use of FFF beams. Flattening filter–free delivery techniques have been reported for three-dimensional conformal radiotherapy (10) and intensity-modulated radiotherapy (11).

In this retrospective study, we evaluated plan quality and beam delivery time for RapidArc plans for SBRT of NSCLC and vertebral metastases generated using an FFF beam, and compared them with plans generated using a standard flattened beam. To validate the accuracy of fast arc delivery for clinical use, several FFF plans were measured using film dosimetry.

Section snippets

Methods and Materials

Ten patients with Stage I NSCLC and 10 patients with vertebral metastases, all of whom had undergone SBRT at our center, were replanned for this study. One patient each with a vertebral metastasis and a lung tumor were clinically treated using FFF plans, whereas all others were treated using flattened beams. All plans were created using the Eclipse treatment-planning system (version 8.9.08; Varian Medical Systems), and dose calculations were carried out using the anisotropic analytical

Results

The dosimetric analyses averaged for each technique are summarized in Table 1. For lung cases, the average differences between the FF and FFF plans for the mean ITV and PTV dose were ≤1%. The CIs for the prescription dose, CI80%, and 75% of the prescription dose, CI60%, were similar between the two techniques, as were the OAR doses. The FFF plans required on average 8.3% more MU (p = 0.008). The average beam delivery times for plans with fractional dose of 18 Gy and 11 Gy were reduced from 4.8

Discussion

The main finding of our study was that use of FFF beams significantly reduces beam delivery times for RapidArc lung and vertebral SBRT plans, without compromising plan quality. A vertebral SBRT plan with a mean PTV dose of 19 Gy could be delivered in 3.8 min, as opposed to 10.8 min when using flattened beams. Although it has been suggested that FFF beams are suitable for small fields because of the relatively flat profile in the middle of the beams (17), we found that they are also suitable for

Conclusion

The beam-on time for lung and vertebral SBRT can be shortened with the combination of RapidArc and an FFF beam. Plan quality and measured accuracy were considered clinically comparable between 10-MV FFF and 6-MV FF plans.

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Conflict of interest: The VU Medical Center has research collaborations with Varian Medical Systems and Brainlab.

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