Abstract
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 cm3) 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 cm3 PTV, but a 10% underdose was observed for the 0.4 cm3 PTV. For the RapidArc® plans, the measured dose for the 9.9 cm3 PTV was in good agreement with the calculated one. However, for the 0.4 cm3 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.
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