Impact of heterogeneity-based dose calculation using a deterministic grid-based Boltzmann equation solver for intracavitary brachytherapy

Abstract

Purpose: To investigate the dosimetric impact of the heterogeneity dose calculation Acuros (Transpire Inc., Gig Harbor, WA), a grid-based Boltzmann equation solver (GBBS), for brachytherapy in a cohort of cervical cancer patients.

Methods and materials: The impact of heterogeneities was retrospectively assessed in treatment plans for 26 patients who had previously received (192)Ir intracavitary brachytherapy for cervical cancer with computed tomography (CT)/magnetic resonance-compatible tandems and unshielded colpostats. The GBBS models sources, patient boundaries, applicators, and tissue heterogeneities. Multiple GBBS calculations were performed with and without solid model applicator, with and without overriding the patient contour to 1 g/cm(3) muscle, and with and without overriding contrast materials to muscle or 2.25 g/cm(3) bone. Impact of source and boundary modeling, applicator, tissue heterogeneities, and sensitivity of CT-to-material mapping of contrast were derived from the multiple calculations. American Association of Physicists in Medicine Task Group 43 (TG-43) guidelines and the GBBS were compared for the following clinical dosimetric parameters: Manchester points A and B, International Commission on Radiation Units and Measurements (ICRU) report 38 rectal and bladder points, three and nine o'clock, and (D2cm3) to the bladder, rectum, and sigmoid.

Results: Points A and B, D(2) cm(3) bladder, ICRU bladder, and three and nine o'clock were within 5% of TG-43 for all GBBS calculations. The source and boundary and applicator account for most of the differences between the GBBS and TG-43 guidelines. The D(2cm3) rectum (n = 3), D(2cm3) sigmoid (n = 1), and ICRU rectum (n = 6) had differences of >5% from TG-43 for the worst case incorrect mapping of contrast to bone. Clinical dosimetric parameters were within 5% of TG-43 when rectal and balloon contrast were mapped to bone and radiopaque packing was not overridden.

Conclusions: The GBBS has minimal impact on clinical parameters for this cohort of patients with unshielded applicators. The incorrect mapping of rectal and balloon contrast does not have a significant impact on clinical parameters. Rectal parameters may be sensitive to the mapping of radiopaque packing.

Figure 1

Spatial distributions of the 3 factors contributing to differences between GBBS and TG-43: source and boundary, applicator, and heterogeneity. The contrast is overridden to muscle, no override, or bone. (a) Top: contribution of source and boundary. Bottom: contribution of applicator. (b) Top row: combination of all three factors. Bottom row: contribution of the heterogeneity. From left to right, the GBBS with solid applicator with contrast overridden to muscle D(Y,N,M,M), contrast not overridden D(Y,N,N,N), or contrast overridden to bone D(Y,N,B,B).

Figure 2

Barplots showing contribution of factors to differences between the GBBS with solid applicator and no overrides D(Y,N,N,N) and TG-43. Summing source and boundary (red), applicator (green), and tissue heterogeneities (blue) yields the total percent difference (orange) from TG-43. (a) ICRU rectum (b) balloon center

Figure 2

Barplots showing contribution of factors to differences between the GBBS with solid applicator and no overrides D(Y,N,N,N) and TG-43. Summing source and boundary (red), applicator (green), and tissue heterogeneities (blue) yields the total percent difference (orange) from TG-43. (a) ICRU rectum (b) balloon center

Figure 3

Barplots showing different heterogeneity factors for different contrast-to-material mappings at (a) ICRU rectum (b) balloon center

Figure 3

Barplots showing different heterogeneity factors for different contrast-to-material mappings at (a) ICRU rectum (b) balloon center