Implementing RapidArc into clinical routine: A comprehensive program from machine QA to TPS validation and patient QA

1. september 2011 - Eva Samsøe

Link til: Med. Phys. 38, 5146-5166 (2011)

Purpose: With the increased commercial availability of intensity modulated arc therapy (IMAT)
comes the need for comprehensive QA programs, covering the different aspects of this newly available
technology. This manuscript proposes such a program for the RapidArc (RA) (Varian Medical
Systems, Palo Alto) IMAT solution.

Methods: The program was developed and tested out for a Millennium120 MLC on iX Clinacs and
a HighDefinition MLC on a Novalis TX, using a variety of measurement equipment including Gafchromic
film, 2D ion chamber arrays (Seven29 and StarCheck, PTW, Freiburg, Germany) with inclinometer
and Octavius phantom, the Delta4 systam (ScandiDos, Uppsala, Sweden) and the portal
imager (EPID). First, a number of complementary machine QA tests were developed to monitor the
correct interplay between the accelerating/decelerating gantry, the variable dose rate and the MLC
position, straining the delivery to the maximum allowed limits. Second, a systematic approach to
the validation of the dose calculation for RA was adopted, starting with static gantry and RA specific
static MLC shapes and gradually moving to dynamic gantry, dynamic MLC shapes. RA plans
were then optimized on a series of artificial structures created within the homogeneous Octavius
phantom and within a heterogeneous lung phantom. These served the double purpose of testing the
behavior of the optimization algorithm (PRO) as well as the precision of the forward dose calculation.
Finally, patient QA on a series of clinical cases was performed with different methods. In
addition to the well established in-phantom QA, we evaluated the portal dosimetry solution within
the Varian approach.

Results: For routine machine QA, the "Snooker Cue" test on the EPID proved to be the most sensitive
to overall problem detection. It is also the most practical one. The "Twinkle" and "Sunrise"
tests were useful to obtain well differentiated information on the individual treatment delivery components.
The AAA8.9 dose calculations showed excellent agreement with all corresponding measurements,
except in areas where the 2.5 mm fixed fluence resolution was insufficient to accurately
model the tongue and groove effect or the dose through nearly closed opposing leafs. Such cases
benefited from the increased fluence resolution in AAA10.0. In the clinical RA fields, these effects
were smeared out spatially and the impact of the fluence resolution was considerably less pronounced.
The RA plans on the artificial structure sets demonstrated some interesting characteristics
of the PRO8.9 optimizer, such as a sometimes unexpected dependence on the collimator rotation
and a suboptimal coverage of targets within lung tissue. Although the portal dosimetry was successfully
validated, we are reluctant to use it as a sole means of patient QA as long as no gantry angle
information is embedded.

Conclusions: The all-in validation program allows a systematic approach in monitoring the different
levels of RA treatments. With the systematic approach comes a better understanding of both the
capabilities and the limits of the used solution. The program can be useful for implementation, but
also for the validation of major upgrades. VC 2011 American Association of Physicists in Medicine.
[DOI: 10.1118/1.3622672]

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