Abstract
An inverse treatment planning (ITP) module on a commercial treatment planning system (TPS) (Helax AB, Uppsala, Sweden) is being used for an in‐house clinical trial for treatment of nasopharyngeal cancer with contralateral parotid sparing. Intensity modulated radiation therapy (IMRT) fields are delivered by step and shoot multileaf collimator (MLC) with a DMLC enabled Varian 2300 CD (Varian Associates, Palo Alto, CA). A series of testing procedures have been devised to quantify the modeling and delivery accuracy of routine clinical inverse planned IMRT using Helax TMS and the Varian step and shoot MLC delivery option. Testing was done on specific aspects of the TPS modeling germane to DMLC. Measured relative dose factors (head scatter plus phantom scatter) for small MLC fields, normalized to a 10×10cm2 non‐MLC field, were found to differ by 2–3% from the TPS values for the smallest of the fields tested. Relative distributions for small off axis fields were found to be in good agreement. A process for the routine clinical verification of IMRT fields has been implemented. Each IMRT field in an inverse plan is imported into a flat water tank plan and a “beam's eye view” (BEV) dose distribution is generated. This is compared to the corresponding measured BEV dose distribution. The IMRT verification process has also been performed using an anthropomorphic phantom. Large clinical fields (i.e., greater than 14.5 cm in the leaf direction) caused difficulties due to a vendor specific machine restriction, and several techniques for dealing with these were examined. These techniques were (i) the use of static stepping of closed junctions, (ii) the use of two separate IMRT fields for a given gantry angle, and (iii) restricting the overall maximum field size used. The overall process has allowed implementation of an in‐house protocol for IMRT use on an initial clinical site. Results of the verification measurements for the first ten patients treated at this center reveal an average maximum dose per IMRT field delivered of 71.0 cGy, with a mean local deviation from the planned dose of – 1.2 cGy, and a standard deviation of 2.4 cGy.PACS number(s): 87.53.Dq, 87.53.Tf
Highlights
One of the goals of external beam radiotherapy is the delivery of a homogeneous tumourcidal dose to a planning treatment volumePTVwhile avoiding organs at riskOARs
A number of studies have recently been published on IMRT QA, but most focus on experience with a commercially available serial tomotherapy device and its associated TPS.[6–10]
While step and shoot IMRT deliveries do not have to contend with the same junctioning issues of serial tomotherapy11 ͑i.e., the junctioning of adjacent transverse dose slices, both must contend with the delivery of small fields
Summary
One of the goals of external beam radiotherapy is the delivery of a homogeneous tumourcidal dose to a planning treatment volumePTVwhile avoiding organs at riskOARs. The optimal realization of such distributions requires fields with nonuniform energy fluence distributions. The optimal realization of such distributions requires fields with nonuniform energy fluence distributions The use of such fields is referred to as intensity modulated radiation therapyIMRT. MacKenzie et al.: Dosimetric verification of inverse planned step and. While step and shoot IMRT deliveries do not have to contend with the same junctioning issues of serial tomotherapy11 ͑i.e., the junctioning of adjacent transverse dose slices, both must contend with the delivery of small fields. Verification of relative dose factorshead scatter, phantom scatterand relative dose distributions associated with small MLC fields is essential
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