Investigation of the feasibility of relative 3D dosimetry in the Radiologic Physics Center Head and Neck IMRT phantom using presage/optical-CT

Abstract

This study presents the application of the Presage/optical-CT 3D dosimetry system for relative dosimetry in the Radiologic Physics Center (RPC) Head and Neck (H&N) IMRT phantom. Performance of the system was evaluated by comparison with the "gold-standard" RPC credentialing test. A modified Presage cylindrical insert was created that extended the capability of the RPC H&N phantom to 3D dosimetry. The RPC phantom was taken through the entire treatment planning procedure with both the standard RPC insert and the modified Presage insert. An IMRT plan was created to match the desired dose constraints of the credentialing test. This plan was delivered twice to the RPC phantom: first containing the standard insert, and then again containing the Presage insert. After irradiation, the standard insert was sent for routine credentialing analysis; including point dose measurements (TLD) and planar Gafchromic EBT film measurement. The 3D dose distribution from Presage was read out at Duke using the OCTOPUS 5X optical-CT scanner. The Presage distribution was compared with gold-standard EBT measurement (determined by the RPC) and the calculated Eclipse distribution. The agreement between the normalized EBT, Presage, and Eclipse distributions, in the central axial plane was evaluated using profiles and gamma-map comparisons (4% dose difference and 3 mm distance to agreement). Profiles showed good agreement between EBT, Presage, and Eclipse distributions. 2D gamma-map comparisons between all three modalities showed at least 98% pass rate. The excellent agreement between Presage and EBT in the central plane established Presage as a standard against which to evaluate the accuracy of the 3D calculated Eclipse distribution. A gamma comparison between normalized Presage and Eclipse 3D distributions gave an overall pass rate of approximately 94%. In conclusion, the Presage/optical-CT system was found to be feasible for relative 3D dosimetry in the RPC IMRT H&N phantom. The potential to extend the RPC IMRT credentialing procedure to 3D may be feasible provided accurate calibration to dose (Gy) and robustness to shipping stress are demonstrated.

Figure 1

Treatment planning for RPC phantom containing the imaging and dosimetry insert. (a) A photograph of the phantom. (b) A photograph of the standard RPC insert. (c) Central slice (EBT film plane) of the x-ray CT scan of the phantom with RPC insert showing the primary PTV, secondary PTV, and OAR. (d) Isodose distributions in sagittal, coronal, and axial planes.

Figure 2

Treatment planning for the RPC phantom containing the customized Presage 3D dosimetry insert. (a) A photograph of the customized insert. (b) Central slice of the x-ray CT scan of the combined phantom. (c) A plot of CT numbers along dotted lines in B. (d) Isodose distribution in sagittal, coronal, and axial planes. The same treatment plan used to irradiate the RPC phantom with the standard insert was also used to irradiate the phantom with Presage insert with the exception that the prescription dose was reduced from 6.6 to 4 Gy to avoid over exposing the dosimeter.

Figure 3

Changing prescription dose from 6.6 to 4 Gy did not change the relative dose distribution in the Presage insert based on Eclipse calculations. Dose profiles from Eclipse along the dotted lines in (a) are shown in (b) for both the standard RPC insert and the Presage insert. The normalized dose distributions were identical (as shown in the lower panels in (b)), thereby enabling a direct comparison of Presage dose measurement with EBT dose measurement and Eclipse dose calculations despite the different density of the Presage insert.

Figure 4

Presage/optical-CT dose measurement. (a) Prescan central slice. (b) Postscan central slice. (c) Relative dose distribution. (d) Profiles of OD and relative dose along dotted lines in (a)–(c) confirm low noise and reduced edge artifact.

Figure 5

Dose profile comparison of measurement (Presage and EBT film) and calculation (Eclipse TPS) in the central axial plane, which contains the EBT film. Top panel shows color wash images of relative dose distributions. Dose profiles in the lower panel are along dotted lines in the upper panel.

Figure 6

Gamma comparative analysis (4% dose difference and 3 mm DTA criteria) of Presage dose, EBT dose, and Eclipse dose shows agreement (98% pass rate) in the axial plane containing EBT film. A multislice comparison was possible between Presage dose and Eclipse dose (94% pass rate). The positions of the slices [1, 2, and 3 in (c)] correspond to the positions marked by horizontal dotted lines in the coronal view of Fig. 2d.