Abstract
PurposeThis study aimed to investigate the dosimetric effect and delivery reliability of jaw tracking (JT) with increasing planning target volume (PTV) for lung stereotactic body radiation therapy (SBRT) plans. A threshold of PTV was proposed as a selection criterion between JT and fixed-jaw (FJ) techniques.MethodsA total of 28 patients with early-stage non-small-cell lung cancer were retrospectively included. The PTVs ranged from 4.88 cc to 68.74 cc, prescribed with 48 Gy in four fractions. Three-partial-arc volumetric modulated arc therapy (VMAT) plans with FJ and with JT were created for each patient with the same optimization objectives. These two sets of plans were compared using metrics, including conformity index (CI), V50%, R50%, D2cm, dose–volume parameters of organs at risk, and monitor units (MUs). The ratio of small subfields (<3 cm in either dimension), %SS, was acquired as a surrogate for the small-field uncertainty. Statistical analyses were performed to evaluate the correlation between the differences in these parameters and the PTV.ResultsThe V50%, R50%, D2cm, and V20Gy, D1,500cc, and D1,000cc of the lung showed a statistically significant improvement in JT plans as opposed to FJ plans, while the number of MU in JT plans was higher by an average of 1.9%. Between FJ and JT plans, the PTV was strongly correlated with the differences in V50%, moderately correlated with those in V20Gy of the lung, and weakly correlated with those in D2cm and D1,500cc of the lung. By using JT, %SS was found to be negatively correlated with the PTV, and the PTV should be at least approximately 12.5 cc for an expected %SS <50%, which was 15 cc for a %SS <20% and 20 cc for a %SS <5%.ConclusionsConsidering the dosimetric differences and small-field uncertainties, JT could be selected using a PTV threshold, such as 12.5, 15, or 20 cc, on the basis of the demand of delivery reliability for lung SBRT.
Highlights
Stereotactic body radiation therapy (SBRT) has been widely used as an alternative modality to surgery for the treatment of earlystage non-small-cell lung cancer (NSCLC) in recent years [1,2,3]
This study aimed to investigate the dosimetric advantages and the ratio of small subfields while implementing jaw tracking (JT) in Volumetric modulated arc therapy (VMAT) planning for lung SBRT with a wide range of tumor sizes and discuss the feasibility of using PTV as a selection criterion between JT and FJ techniques
All plans complied with the Radiation Therapy Oncology Group (RTOG) 0915 protocol [6, 7]
Summary
Stereotactic body radiation therapy (SBRT) has been widely used as an alternative modality to surgery for the treatment of earlystage non-small-cell lung cancer (NSCLC) in recent years [1,2,3]. SBRT demands a high level of delivery accuracy during the treatment, which can be affected by multiple factors, such as beam modeling and machine commissioning [8, 9]. With the extensive implementation of SBRT, interest in the use of small photon fields has been rapidly growing [9,10,11]. Small-field dosimetry is one of the main factors related to the delivery accuracy of SBRT. In the commissioning of treatment planning systems (TPSs), such as the Eclipse TPS, the beam data for small fields, i.e., below 3 × 3 cm, are mostly not included in the beam modeling, except for the carefully measured output factors, which could be used for jaw-defined fields [15]
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