Session Item

To develop a verification technique for lung SABR treatment delivery using silica TLD beads within a respiratory motion phantom that provides a novel method for assessment of 3D dose distribution inside and outside the target.

Silica bead TLD detectors (Trueinvivo Ltd, UK) with 1.1 mm thickness and 1.6 mm diameter were characterised in previous studies for the MV photon range used in radiotherapy plan deliveries [1,2]

A respiratory motion phantom (QUASAR™, Modus QA, US), comprised a Perspex body section with custom-designed moving insert consisting of a cedar wood cylinder to represent normal lung tissue, with three milled cavities with diameters of 3, 2 and 1 cm filled with water-equivalent material infills (Fig.1). Inside infills a range of ~1.7 mm cavities were drilled to accommodate TLDs (23 in total). The Phantom was scanned with 4DCT technique (4cm sup-inf amplitude and 4 second period) with dummy beads located in cavities for accurate localisation on the CT images

A SABR VMAT treatment plan for largest infill (55Gy in 5#) was created in Pinnacle™ (TPS) on the Average Intensity Phase (AIP) image following UK Consortium SABR guidelines. Plan comprised of two 180 degrees arcs with 10MV FFF beam.

TLDs were calibrated and individual bead sensitivities were established prior to experiment [1,2].

Two single fractions and one full 5# of SABR treatment plan was delivered to the moving phantom with separate sets of TLDs inserted in the prepared cavities for each delivery.

TLDs were readout using a TOLEDO 654 Vinten TLD reader.

The treatment plan was transferred to each respiratory phase of 4DCT dataset. TLDs locations were identified separately on each phase and TPS calculated doses were noted for these points. Dose from each phase for the particular TLD were added and compared with the measured doses

References:

1. Jafari SM, et all: Radiation Physics and Chemistry, 97, 95-101
2. Jafari SM, et all: Physics in Medicine and Biology, 59: 6875-6889

Comparison between measured dose and TPS dose is shown on Fig 2. For points located in target measured dose difference ranged from -6.8% to +5.8% for single fraction and from -2% to +4% for the 5# delivery. For points outside target range was from -7.1% to +10.8% for single fractions and -1.8% to +2.7% for 5# delivery. 

A novel method of assessment of the lung SABR treatment delivery was proposed. Accuracy of the dose delivery was assessed inside and outside of the target. The 5# delivery produced best agreement between TPS calculated and TDLs measured dose, both in target and out of field. The discrepancy between the predicted and measured doses for single fraction deliveries are likely the result of the interplay between the target and the delivery system respective motions. The measurements results gave confidence in TPS beam model and confirmed viability of using TLD beads to test the delivery.