TLD verification of dose delivery in VMAT plan
PO-1768
Abstract
TLD verification of dose delivery in VMAT plan
Authors: Alessia Embriaco1, Paola Martucci2, Vanessa De Coste1, Maria Pimpinella1, Serenella Russo3, Christian Fiandra4, Laura Masi5, Pierino De Felice1, Michele Stasi6
1INMRI, ENEA, Rome, Italy; 2University Tor Vergata, ENEA-INMRI, Rome, Italy; 3AUSL Toscana Centro, Health Physics, Florence, Italy; 4University of Turin, Oncology, Turin, Italy; 5IFCA, Medical Physics Unit, Florence, Italy; 6AO Ordine Mauriziano, Health Physics, Turin, Italy
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Purpose or Objective
The Italian Association of Medical Physics (AIFM) in cooperation with the Italian National Institute of Ionizing Radiation Metrology (ENEA-INMRI) offers a certified audit service to radiotherapy centres for reference dosimetry in photon beams. Connected to the service, a technical protocol for verification of dose delivery in VMAT treatments is being developed. Here we report on results obtained using our audit dosimetry system in a set of non-reference irradiation conditions.
Material and Methods
The absorbed dose is measured in a cylindrical water phantom with a diameter of 30 cm and a length of 50 cm. The prototype of the phantom has two different inserts for a Farmer ionization chamber and/or TLD dosimeters and for a micro ionization chamber. The measurements were performed with a 6 MV photon beam (Elekta Synergy Linac). Thermoluminescent dosimeters (TLD-100), assembled in a waterproof PMMA holder, are applied to measure the delivered dose. TLD are calibrated at ENEA-INMRI in terms of absorbed dose to water in a reference 60Co γ-beam. The measurement signal is the average value of 10 TLD chips.
Different configurations are examined and planned with Monaco TPS on the CT images of the phantom:
a) 4 box fields 10 cm x 10 cm at different angles (0°, 90°, 180° and 270°) to delivery an uniform dose in the target volume;
b) 360° dynamic conformal arc shaped on target volume;
c) VMAT plans for Head and Neck (HN) treatment with different levels of complexity.
The target volume in condition a) and b) is a sphere of 6 cm diameter and an homogeneous dose prescription of 2 Gy to the 98% of the target volume is applied.
The VMAT plan simulated a Simultaneous Integrated Boost HN treatment for two target volumes with two different dose prescriptions (2-1.5 Gy) to the 95% of each target volume. Several Organs at Risk are considered with proper dose constraints. A ROI simulating the TLD volume is also defined and dose homogeneity within 2 % is requested. Three different VMAT plans are planned and delivered:
1. 2 arcs with a total number of 316 CP and 944 MU;
2. 2 arcs with a total number of 286 CP and 784 MU;
3. 4 arcs with a total number of 333 CP and 870 MU.
TLD measurements are compared with calculated dose on the TLD ROI.
Results
Ionization chamber measurements show that planned and delivered absorbed dose in conditions a) and b) agree within 0.6%. In the same conditions, the agreement between TLD and ionization chamber measurements is around 1.0% that is well within the TLD measurement uncertainty. In VMAT plans, the percentage difference between TLD measured dose and TPS dose on the TLD ROI is up to about 2.7%.
Conclusion
Our TLD dosimetry system has proven to be suitable for dose measurements in full VMAT conditions. The final cylindrical phantom will also have a microDiamond insert and fiducial markers for CyberKnife plan verifications. A concomitant pilot study involving some Italian RT centers started to evaluate the complexity of VMAT treatment planned by different Linac-TPS systems.