A treatment planning method for sequentially combining radiopharmaceutical therapy and external radiation therapy

Abstract

Purpose: Effective cancer treatment generally requires combination therapy. The combination of external beam therapy (XRT) with radiopharmaceutical therapy (RPT) requires accurate three-dimensional dose calculations to avoid toxicity and evaluate efficacy. We have developed and tested a treatment planning method, using the patient-specific three-dimensional dosimetry package 3D-RD, for sequentially combined RPT/XRT therapy designed to limit toxicity to organs at risk.

Methods and materials: The biologic effective dose (BED) was used to translate voxelized RPT absorbed dose (D(RPT)) values into a normalized total dose (or equivalent 2-Gy-fraction XRT absorbed dose), NTD(RPT) map. The BED was calculated numerically using an algorithmic approach, which enabled a more accurate calculation of BED and NTD(RPT). A treatment plan from the combined Samarium-153 and external beam was designed that would deliver a tumoricidal dose while delivering no more than 50 Gy of NTD(sum) to the spinal cord of a patient with a paraspinal tumor.

Results: The average voxel NTD(RPT) to tumor from RPT was 22.6 Gy (range, 1-85 Gy); the maximum spinal cord voxel NTD(RPT) from RPT was 6.8 Gy. The combined therapy NTD(sum) to tumor was 71.5 Gy (range, 40-135 Gy) for a maximum voxel spinal cord NTD(sum) equal to the maximum tolerated dose of 50 Gy.

Conclusions: A method that enables real-time treatment planning of combined RPT-XRT has been developed. By implementing a more generalized conversion between the dose values from the two modalities and an activity-based treatment of partial volume effects, the reliability of combination therapy treatment planning has been expanded.

Conflict of interest statement

Conflict of Interest: The authors have no conflict of interest to disclose.

Copyright © 2011 Elsevier Inc. All rights reserved.

Figures

Figure 1

dose rate plots for the tumor VOI. The blue triangles show the calculated dose rates and the blue lines represent the fit used to calculate the DRPT. The red dotted line shows the monoexponential fit. The resulting DRPT, BED, and NTDRPT values are also shown.

Figure 2

cumulated dose volume histograms for the NTDRPT values in the tumor and spinal cord calculated in 3D-RD from 153Sm.

Figure 3

cumulated dose volume histograms for the (a) baseline IMRT NTDXRT values, and (b) the combined therapy NTDsum, in the tumor and spinal cord. In figure 3b, the blue dotted lines show that 90% of the tumor VOI receives 55.3 Gy of combined dose.

Figure 4

Isodose contours in Pinnacle showing the combined therapy treatment plan. Pink is the planning tumor volume (PTV) and the volume used in the 3D-RD calculation, blue is the additional gross tumor volume (GTV), green is the contour identifying the spinal cord as the sensitive volume, and yellow an artificial VOI used to confine the DXRT to the GTV, often called a “ring”.

Figure 5

cumulated dose volume histograms for the NTDRPT values in the tumor and spinal cord from a 3D-RD calculation with no activity spill-out correction.