A closer look at RapidArc® radiosurgery plans using very small fields

• Published in: Physics in medicine & biology 2011-03, Vol.56 (6), p.1853-1863
• Main Authors: Fog, Lotte S, Rasmussen, Jens F B, Aznar, Marianne, Kjær-Kristoffersen, Flemming, Vogelius, Ivan R, Engelholm, Svend Aage, Bangsgaard, Jens Peter
• Format: Article
• Language: English
• Subjects: Algorithms; Humans; Neoplasms - pathology; Neoplasms - radiotherapy; Neoplasms - surgery; Phantoms, Imaging; Radiation Dosage; Radiosurgery - instrumentation; Radiosurgery - methods; Radiotherapy Dosage; Radiotherapy, Computer-Assisted - methods
• ISSN: 0031-9155; 1361-6560
• DOI: 10.1088/0031-9155/56/6/020

RapidArc® has become the treatment of choice for an increasing number of treatment sites in many clinics. The extensive use of multiple subfields in RapidArc® treatments presents unique challenges, especially for small targets treated in few fractions. In this work, very small static fields and subsequently RapidArc® and conventional plans for two targets (0.4 and 9.9 cm(3)) were investigated. Doses from static fields 1-4 MLC leaves (0.25-1.00 cm) wide, and larger fields with 1-4 MLC leaves closed in their centres, were measured using the portal dosimeter-based QA system EPIQA (v 1.3) and gafchromic film. RapidArc and conventional plans for two tumours were then measured using EPIQA, gafchromic EBT2 film and the phantom-based QA system Delta4. Eclipse 8.6 and 8.9, grid spacings of 1.25 and 2.50 mm and a Varian HD linac were used. For static fields one MLC leaf wide, the dose was underestimated by Eclipse by as much as 53% (v 8.6, 2.5 mm grid). Eclipse underestimated the dose downstream from a few MLC leaves closed in the centre of a large MLC field by as much as 30%. Eclipse consistently overestimated the width of the penumbra by about 100%. For the conventional plans, there was good agreement between the calculated and measured dose for the 9.9 cm(3) PTV, but a 10% underdose was observed for the 0.4 cm(3) PTV. For the RapidArc® plans, the measured dose for the 9.9 cm(3) PTV was in good agreement with the calculated one. However, for the 0.4 cm(3) PTV about 10% overdosing was detected (Eclipse v 8.6, 2.5 mm grid spacing). EPIQA data indicated that the measured dose profiles were overmodulated compared to the calculated one. The use of small subfields, typically a few MLC leaves wide, or larger fields with one or a few MLC leaves closed in its centre can result in significant errors in the dose calculation. The detector systems used vary in their ability to detect the discrepancies. Using a smaller grid size and newer version of Eclipse reduces the discrepancies observed in this work but does not eliminate them.