SU-E-T-226: Junction Free Craniospinal Irradiation in Linear Accelerator Using Volumetric Modulated Arc Therapy : A Novel Technique Using Dose Tapering

Abstract

Purpose: Spatially separated fields are required for craniospinal irradiation due to field size limitation in linear accelerator. Field junction shits are conventionally done to avoid hot or cold spots. Our study was aimed to demonstrate the feasibility of junction free irradiation plan of craniospinal irradiation (CSI) for Meduloblastoma cases treated in linear accelerator using Volumetric modulated arc therapy (VMAT) technique. Methods: VMAT was planned using multiple isocenters in Monaco V 3.3.0 and delivered in Elekta Synergy linear accelerator. A full arc brain and 40° posterior arc spine fields were planned using two isocentre for short (<1.3 meter height) and 3 isocentres for taller patients. Unrestricted jaw movement was used in superior‐inferior direction. Prescribed dose to PTV was achieved by partial contribution from adjacent beams. A very low dose gradient was generated to taper the isodoses over a long length (>10 cm) at the conventional field junction. Results: In this primary study five patients were planned and three patients were delivered using this novel technique. As the dose contribution from the adjacent beams were varied (gradient) to create a complete dose distribution, therefore there is no specific junction exists in the plan. The junction were extended from 10–14 cm depending on treatment plan. Dose gradient were 9.6±2.3% per cm for brain and 7.9±1.7 % per cm for spine field respectively. Dose delivery error due to positional inaccuracy was calculated for brain and spine field for ±1mm, ±2mm, ±3mm and ±5 mm were 1%–0.8%, 2%–1.6%, 2.8%–2.4% and 4.3%–4% respectively. Conclusion: Dose tapering in junction free CSI do not require a junction shift. Therefore daily imaging for all the field is also not essential. Due to inverse planning dose to organ at risk like thyroid kidney, heart and testis can be reduced significantly. VMAT gives a quicker delivery than Step and shoot or dynamic IMRT.

Comparative dosimetric analysis of volumetric modulated arc therapy based craniospinal irradiation plans between Halcyon ring gantry and TrueBeam C-arm linear accelerator

“…VMAT/RapidArc based technique for Varian/Eclipse was introduced by Fogliata et al . in 2011 and subsequently for Elekta/Monaco by Sarkar et al in 2014 6 , 12 . Since the inception of the VMAT-based CSI technique, it has become a popular practice in the routine clinical practice.…”

“…The modern optimisers perform the junction dose tapering very efficiently without any additional contour, and delivery is also fast. It is now well established for over a decade that the C-arm linear accelerators capable of VMAT/RapidArc can deliver low-gradient junction based CSI efficiently 6 – 8 , 12 , 13 , 15 , 16 . This report extends the research for the newly introduced Halcyon-E linear accelerator.…”

“…First described by Fogliata et al, in 2011, different properties of the VMAT-based CSI technique were subsequently investigated by several researchers and the technique is currently well adapted in clinical practice. 6 – 8 , 12 – 16 . These investigations were limited to the c-arm linear accelerator or helical Tomotherapy (Accuray Inc., Sunnyvale, CA) treatment delivery systems 5 – 8 , 12 – 17 .…”

“… 6 – 8 , 12 – 16 . These investigations were limited to the c-arm linear accelerator or helical Tomotherapy (Accuray Inc., Sunnyvale, CA) treatment delivery systems 5 – 8 , 12 – 17 . Recently, Varian Medical Systems (Palo Alto, CA) has introduced a ring-gantry type treatment delivery system called Halcyon.…”