Rapid Delivery of Stereotactic Radiotherapy for Peripheral Lung Tumors using Volumetric Intensity Modulated Arcs

Article Outline

• Purpose/Objective(s)
• Materials/Methods
• Results
• Conclusions
• Copyright

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Purpose/Objective(s) 

The delivery of high dose conventional stereotactic body radiotherapy (SBRT) for peripheral lung tumors can take >20 minutes per fraction, and optimal dose distributions are difficult to achieve for nonspherical tumors. Faster treatment delivery is more patient-friendly, and can reduce intrafraction shifts in tumor position. We evaluated the use of RapidArc (RA), a novel planning and delivery technique for Volumetric Intensity Modulated Arcs, in SBRT.

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Materials/Methods 

RA (version 8.2.23) was used for planning and delivery of SBRT in 17 patients with Stage I lung tumors. Three risk-adapted fractionation schemes were used, prescribed on the 80% isodose, 3 × 18 Gy, 5 × 11 Gy, or 8 × 7.5 Gy, dependent on size of the PTV and proximity of organs at risk (OAR). The PTV sizes ranged from 6.2–275 cm³. Clinical delivery was performed using multiple arcs, as the present RA version allows maximum 999 MU per arc. Setup was performed using a tumor match on CBCT, which was repeated after 2 arcs. For dosimetric verification, patient-specific QA was performed by comparing GafChromic EBT film dose measurements in the Quasar phantom with calculations, the Quasar phantom moving in a sinusoidal pattern with the observed tumor amplitude (if >5 mm). For purpose of comparison, conventional SBRT plans using 10 noncoplanar static fields were also generated for all patients, except those receiving 8 × 7.5 Gy. Conformity indices (CI_{80, 60,} the volume encompassed by the 80 or 60% isodose divided by PTV-volume receiving the same dose), volume of chest wall receiving more than 45 Gy (V_45,chest) and typical radiation delivering times were compared.

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Results 

In comparison to conventional plans, RA plans achieved superior dose conformity around the PTV and reduced chest wall doses. Average CI₈₀ and CI₆₀ were 1.12 and 2.17 for RA vs. 1.24 and 2.42 for conventional SBRT, respectively. The average V_45,chest was 1.4 cm³ for RA and 2.0 cm³ for conventional SBRT. Film dosimetry revealed that RA plans were not compromised by possible interplay between moving leaves and a moving PTV, even with a tumor motion amplitude of up to 25 mm. Typical RA delivery times (from first beam on until last beam off, excluding time for extra setup) ranged from 4.5 minutes (7.5 Gy) to 11 minutes (18 Gy). Typical conventional SBRT delivery times range between 10 and 15 minutes. In the near future, the combination of a new clinical release of RapidArc (8.6.10) which allows delivery of >999 MU per arc, and a linac allowing dose rates of 1,000 MU/minimum will allow 18 Gy to be delivered in less than 6 minutes.

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Conclusions 

An RA allows for rapid and highly conformal delivery of SBRT for Stage I lung tumors. The shorter treatment time reduces risk of intrafraction patient motion.

Acknowledgement: The VUmc has received research grants from Varian Medical Systems.