Class solution in inverse planned HDR prostate brachytherapy for dose escalation of DIL defined by combined MRI/MRSI

Abstract

Purpose: To establish an inverse planning set of parameters (class solution) to boost dominant intra-prostatic lesion (DIL) defined by MRI/MRSI.

Methods: For 15 patients, DIL were contoured on CT or MR images and a class solution was developed to boost the DIL under the dosimetric requirements of the RTOG-0321 protocol. To determine the maximum attainable level of boost for each patient, 5 different levels were considered, at least 110%, 120%, 130%, 140% and 150% of the prescribed dose. The maximum attainable level was compared to the plan without boost using cumulative dose volume histogram (DVH).

Results: DIL dose escalation was feasible for 11/15 patients under the requirements. The planning target volume (PTV) dose was slightly increased, while the DIL dose was significantly increased without any violation of requirements. With slight adjustments of the dose constraint parameters, the dose escalation was feasible for 13/15 patients under requirements.

Conclusion: Using a class solution, a dose escalation of the MRI/MRSI defined DIL up to 150% while complying with RTOG dosimetric requirements is feasible. This HDR brachytherapy approach to dose escalation allows a significant dose increase to the tumor while maintaining an acceptable risk of complications.

Figures

Fig. 1

(a) Axial MR image and MRSI voxel grids with their spectral scores at the midgland of prostate I. (b) HDR brachytherapy planning axial CT image at the midgland of prostate I shows the contours of PTV, urethra, rectum. A DIL was manually drawn based in (a). As seen as black dots on planning CT image, 16 catheters were implanted to cover the entire target volume.

Fig. 2

Percent volume of the DIL receiving at least 150% of the prescribed dose (V150[%]) for 7 DIL-boost plans. Each boost plan has a different weighting factor for its penalty value imposed to the maximum dose (150% of the prescribed dose) of the DIL in comparison with a plan without boost (Ref.) under the RTOG-0321 dosimetric requirements. Parallel bars represent the maximum, 75, 50, 25 percentiles and minimum values. The black dot represents the mean value.

Fig. 3

Under the RTOG 0321 dosimetric requirements, the maximum attainable level of a DIL-boost for 15 patients (A–O).

Fig. 4

Group average PTV DVH for 11 patients who allowed a certain level of DIL-boost.

Fig. 5

For 11 patients who allowed a certain level of DIL-boost, the group average DIL DVH compares the maximum attainable DIL-boost plan (B) with the plan without boost (N). The solid line graph shows the increased DIL volume due to DIL boost. The maximum increase (41.8%) was observed at 150% of the prescribed dose.