Search

Introduction: Stereotactic body radiotherapy(SBRT) for early lung cancer results in excellent local control and potentially improved survival, but demands high levels of dose conformality. To achieve this with 3D-conformal radiotherapy(3D-CRT) and intensity modulated radiotherapy (IMRT) large numbers of beams, and non-coplanar techniques are often required. Hence treatment times are long, increasing the risk of patient motion. Volumetric arc therapy(VMAT) may result in improved dosimetric outcomes, and reduce treatment time. We report on a planning study comparing VMAT to coplanar (CP) and non-coplanar (NCP) 3D-CRT and IMRT, and our initial clinical results. Methods: Pinnacle VMAT, and CP and NCP 3D-CRT and IMRT plans for delivery on Elekta Linacs of 5 patients were compared. All plans were single arc, with 4 degree spacing and 1 cm MLC. The following dosimetric characteristics were assessed: target coverage (coverage with the prescribed dose), dose conformality (100% conformity indices, D2cm and volume of tissue outside PTV receiving high dose, defined as >105% of prescription dose) and V20 (volume of lung receiving 20 Gy). Results: Dosimetric Outcomes VMAT achieved equivalent radiotherapy target coverage to 3D-CRT and IMRT (>98.2%). However, 3D-CRT plans had a large volume of surrounding normal tissue receiving a high dose (24.0% and 21.3% of the PTV volume for CP and NCP respectively) when compared to VMAT and IMRT plans (3.9% and 4-5% respectively). 100% conformity indices were lowest for VMAT and NCP IMRT (1.1 for both) and highest for 3D-CRT (1.5 and 1.4 for CP and NCP respectively). D2cm was also lowest for VMAT and NCP IMRT (26.1 Gyand 25.3 Gy) and highestfor3D-CRT plans (29.4 Gy and 28.7 Gy for CP and NCP respectively). V20 was between 4.4 and 6.1% for all plans. Clinical outcomes VMAT treatment time takes 35 minutes, and beam-on time is 8 minutes. With short follow-up (range 4 weeks to 6 months), 1 patient has developed worsening dyspnoea and grade 3 pneumonitis. No other Grade 2 or greater toxicity was seen in the other 4 patients. Conclusions: Clinical implementation of VMAT was successfully demonstrated, and has resulted in fast treatment times for our SBRT lung patients. VMAT plans are similar in quality to NCP IMRT, but are much faster to deliver due to reduced gantry and couch movements. 3D-CRT plans were unable to deliver equivalent dose conformality, and a much larger region of normal tissue was exposed to a high dose when equivalent PTV coverage was achieved. Disclosure: No significant relationships. [ABSTRACT FROM AUTHOR]

Purpose: Stereotactic body radiotherapy (SBRT) has been demonstrated to provide durable local control, and palliation in patients with vertebral metastases1. SBRT spine requires highly conformal dose distributions and the complexity of plans may lead to long treatment times. Prior to the implementation of SBRT spine we assessed whether volumetric modulated arc therapy (VMAT) could deliver satisfactory dosimetric outcomes, as it may potentially result in shorter treatment times. Methods: Pinnacle VMAT plans were generated on a volume created on the imaging dataset of a patient previously treated for vertebral metastatic disease. All plans were dosed to deliver 24 Gy in 3 fractions to the planning target volume (PTV), with the dose to the thecal sac to receive 17.5 Gy point maximum dose. PTV coverage was such that at least 90% of the PTV received 90% of the prescribed dose (21.6 Gy).Constraints for other organs at risk (OAR) were as for RTOG 0618.5mm MLC leaves with a minimum gap of 3mm were used. Negative margins, double arcs and 2 degree gantry spacing were implemented in a stepwise manner. Plans were assessed for PTV coverage, dose to the thecal sac, and other OAR. Results: The technical characteristics and dosimetric outcomes of each planning trial are listed below. For each plan new technical approaches were trial led to improve dosimetry. Technical Characteristics Plan 1: Single arc plan with 4 degree spacing. Plan 2: PTV adjusted with negative margin around thecal sac, double arc plan with 4 degree spacing. Plan 3: PTV adjusted with negative margin around thecal sac. Double arc plan with 2 degree spacing. Plan 4: PTV adjusted with negative margin around thecal sac and superior and inferior edges. Double arc plan with2 degree spacing. Dosimetric outcomes: Coverage of the PTV by the 90% isodose was 98%, 92.2%, 90.7% and 90.4% for plans 1, 2,3 and 4 respectively. Point maximum dose to the thecal sac was 23.4 Gy for plan 1, which exceeded our planning requirements. Point maximum dose for plans 2, 3 and 4 were 16.9,16.1 and 15.0 Gy. Conclusion VMAT planning for spine SBRT patients is feasible. Double arc rather than single arc planning, reducing gantry spacing to 2 degrees, and the use of negative margins appears to successfully reduce dose to the thecal sac, and permit adequate coverage. 1.Gerszten, P. C. et al. Spine 32, 193-9(2007). Disclosure: No significant relationships. [ABSTRACT FROM AUTHOR]