On the need for tuning the dosimetric leaf gap for stereotactic treatment plans in the Eclipse treatment planning system

Laure Vieillevigne; Catherine Khamphan; Jordi Saez; Victor Hernandez

doi:10.1002/acm2.12656

On the need for tuning the dosimetric leaf gap for stereotactic treatment plans in the Eclipse treatment planning system

J Appl Clin Med Phys. 2019 Jul;20(7):68-77. doi: 10.1002/acm2.12656. Epub 2019 Jun 21.

Authors

Laure Vieillevigne^{1

2}, Catherine Khamphan³, Jordi Saez⁴, Victor Hernandez⁵

Affiliations

¹ Department of Medical Physics, Institut Claudius Regaud Institut Universitaire du Cancer de Toulouse, Toulouse, France.
² Centre de Recherches et de Cancérologie de Toulouse UMR1037 INSERM - Université Toulouse 3 - ERL5294 CNRS Oncopole, Toulouse, France.
³ Medical Physics Unit, Institut Sainte Catherine, Avignon, France.
⁴ Department of Radiation Oncology, Hospital Clınic de Barcelona, Barcelona, Spain.
⁵ Department of Medical Physics Hospital, Sant Joan de Reus, IISPV, Tarragona, Spain.

Abstract

The dosimetric leaf gap (DLG) and tongue-and-groove (T&G) effects are critical aspects in the modeling of multileaf collimators (MLC) in the treatment planning system (TPS). In this study, we investigated the dosimetric impact of limitations associated with the T&G modeling in stereotactic plans and its relationship with the need for tuning the DLG in the Eclipse TPS. Measurements were carried out using Varian TrueBeam STx systems from two different institutions. Test fields presenting MLC patterns with several MLC gap sizes (meanGap) and different amounts of T&G effect (TGi) were first evaluated. Secondly, dynamic conformal arc (DCA) and volumetric modulated arc therapy (VMAT) deliveries of stereotactic cases were analyzed in terms of meanGap and TGi. Two DLG values were used in the TPS: the measured DLG (DLG_meas ) and an optimal DLG (DLG_opt ). Measured and calculated doses were compared according to dose differences and gamma passing rates (GPR) with strict local gamma criteria of 2%/2 mm. The discrepancies were analyzed for DLG_meas and DLG_opt , and their relationships with both TGi and meanGap were investigated. DCA arcs involved significantly lower TGi and larger meanGap than VMAT arcs (P < 0.0001). By using DLG_meas in the TPS, the dose discrepancies increased as TGi increased and meanGap decreased for both test fields and clinical plans. Dose discrepancies dramatically increased with the ratio TGi/meanGap. Adjusting the DLG value was then required to achieve acceptable calculations and configuring the TPS with DLG_opt led to an excellent agreement with median GPRs (2%/2 mm) > 99% for both institutions. We also showed that DLG_opt could be obtained from the results of the test fields. We demonstrated that the need for tuning the DLG is due to the limitations of the T&G modeling in the Eclipse TPS. A set of sweeping gap tests modified to incorporate T&G effects can be used to determine the optimal DLG value.

Keywords: dosimetric leaf gap; dynamic conformal arc; stereotactic treatments; tongue-and-groove modeling; volumetric modulated arc therapy.

MeSH terms

Brain Neoplasms / pathology
Brain Neoplasms / surgery*
Humans
Lung Neoplasms / pathology
Lung Neoplasms / surgery*
Organs at Risk / radiation effects
Particle Accelerators / instrumentation*
Phantoms, Imaging*
Prognosis
Radiometry / methods
Radiosurgery / methods*
Radiotherapy Dosage
Radiotherapy Planning, Computer-Assisted / methods*
Radiotherapy, Intensity-Modulated / methods