Development and evaluation of an end-to-end test for head and neck IMRT with a novel multiple-dosimetric modality phantom

Abstract

A comprehensive end-to-end test for head and neck IMRT treatments was developed using a custom phantom designed to utilize multiple dosimetry devices. Initial end-to-end test and custom H&N phantom were designed to yield maximum information in anatomical regions significant to H&N plans with respect to: (i) geometric accuracy, (ii) dosimetric accuracy, and (iii) treatment reproducibility. The phantom was designed in collaboration with Integrated Medical Technologies. The phantom was imaged on a CT simulator and the CT was reconstructed with 1 mm slice thickness and imported into Varian's Eclipse treatment planning system. OARs and the PTV were contoured with the aid of Smart Segmentation. A clinical template was used to create an eight-field IMRT plan and dose was calculated with heterogeneity correction on. Plans were delivered with a TrueBeam equipped with a high definition MLC. Preliminary end-to-end results were measured using film, ion chambers, and optically stimulated luminescent dosimeters (OSLDs). Ion chamber dose measurements were compared to the treatment planning system. Films were analyzed with FilmQA Pro using composite gamma index. OSLDs were read with a MicroStar reader using a custom calibration curve. Final phantom design incorporated two axial and one coronal film planes with 18 OSLD locations adjacent to those planes as well as four locations for IMRT ionization chambers below inferior film plane. The end-to-end test was consistently reproducible, resulting in average gamma pass rate greater than 99% using 3%/3 mm analysis criteria, and average OSLD and ion chamber measurements within 1% of planned dose. After initial calibration of OSLD and film systems, the end-to-end test provides next-day results, allowing for integration in routine clinical QA. Preliminary trials have demonstrated that our end-to-end is a reproducible QA tool that enables the ongoing evaluation of dosimetric and geometric accuracy of clinical head and neck treatments.

Figure 1

End‐to‐end phantom with film plane locations highlighted in red.

Figure 2

Superior axial film plane: (a) photograph of phantom, observed inferior‐superior. Red structures indicate bone‐equivalent material, numbers correspond to OSLD locations, with location 5 and 13 showing a tissue‐equivalent plug; (b) CT image of same plane in phantom with PTV and cord structure overlay; (c) CT image with isodose distribution.

Figure 3

Inferior axial film plane: (a) photograph of phantom, observed superior‐inferior; (b) CT image of same plane in phantom with PTV and cord structure overlay; (c) CT image with isodose distribution.

Figure 4

Coronal film plane: (a) photograph of phantom, observed anterior‐posterior; (b) CT image of same plane in phantom with PTV and cord structure overlay;)c) CT image with isodose distribution (film plane highlighted in each CT image).

Figure 5

Flowchart for end‐to‐end test.

Figure 6

Plot of normalized OSLD response vs. angle of incidence of the primary beam.

Figure 7

Plot of OSLD percent difference from plan dose for each OSLD location with error bars indicating SD of readings. Pink square symbols represent locations which were suspected to be high‐dose‐gradient regions.