Session Item

To evaluate the use of Optically Stimulated Luminescence Dosimeters (OSLDs) for the verification of Helical TomoTherapy (HT) (Accuray Inc, CA, USA) treatment plans using the dynamic jaw delivery feature.

A batch of nanoDot OSLDs (Landauer Inc, IL, USA) were used. The dosimeters were calibrated in terms of dose to water using a 6MV x-ray photon beam for doses up to 300cGy. Due to the helical treatment delivery fashion, radiation is incident to the dosimeters from variable directions. Therefore, Monte Carlo (MC) simulations were performed to assess potential directional dependence of nanoDot response using the EGSnrc MC code and the egs_chamber user code. To model the dosimeters, the C++ geometrical package available with the EGSnrc and information found in the literature and vendor manuals were used. For the measurement setup, RW3 (PTW, Freiburg, Germany) slabs of 13cm total thickness were used. The central slab was appropriately machined to hold the OSLDs in axial and coronal orientations. The phantom was CT scanned and two HT treatment plans were developed using the dynamic jaw delivery option of the HT systems. The first plan involved the irradiation of 5 targets situated along the central craniocaudal axis of the slab-phantom delivering 2Gy. In the second plan, 3 targets centered on the phantom craniocaudal axis lying across the Left-Right direction were irradiated delivering 2Gy on the side targets while a boost dose of 2.5Gy was planned for a sub-volume of the central target mimicking the delivery of simultaneously integrated boost (SIB). NanoDot measurements were performed at dose-plateau areas (plans 1&2), as well as regions of dynamic jaw movement (plan 1). Phantom alignment at irradiation position was performed using the image guidance capabilities of the HT system. 

MC simulations revealed mirror directional dependence of nanodot response, being up to 5% when irradiated from an angle of 90^o. Experimental dosimetry results are presented in figure 1 along with corresponding profile data exported from the PrecisionTM treatment planning system (TPS). As seen, nanoDot measurements agree with TPS predictions within experimental uncertainties of 5%. 

Exhibiting minimal directional dose-response dependence, in agreement with the corresponding literature, the nanoDots can provide precise dosimetry measurements allowing for the experimental verification of HT treatment plans.