The effect of seed orientation deviations on the quality of 125I prostate implants.

We quantified the effect of seed orientation deviations on five prostate seed implant cases at our institution. While keeping their positions fixed, the iodine-125 seeds were assigned orientations sampled from a realistic probability distribution derived from the post-implant radiographs of ten patients. Dose distributions were calculated with both a model that explicitly includes anisotropy (TG43 anisotropy function) and a point source model (TG43 anisotropy factor). Orientation deviations had only a small influence on prostate dose-volume histograms: the 95% confidence intervals on the volumes receiving 100%, 150% and 200% dose were at most +/-0.8%, +/-1.1% and +/-0.6% of the prostate volume, respectively. The dose-volume histograms of anisotropic seed distributions were marginally better than those with isotropic point-source seeds. Anisotropy caused a displacement of cold spots (regions receiving <100% of the prescribed dose) in <1% of the prostate volume. Our results indicate no net benefit to prostate dosimetry in using more isotropic seeds. Furthermore, we propose a new 'weighted anisotropy function' to better account for the effects of anisotropy when seed orientation is unknown. Conceptually, the TG43 anisotropy factor described in AAPM TG43 averages the effect of anisotropy over all solid angles, with the implicit assumption that all seed orientations are equally probable. In prostate implants, however, seeds are preferentially oriented parallel to the needle axis. The proposed weighted anisotropy function incorporates this non-uniform probability.

A practical approach to inverse planning for high-precision dose escalated conformal prostate radiotherapy.

The problem of choosing the best gantry angles and beam weights for dose-escalated conformal prostate treatment planning is formulated using a mixed-integer linear programming approach, to account for tumour dose homogeneity and dose-volume constraints. The formulation allows the number of beams to be restricted and for some of the beams to be compulsory. The present planning algorithm interfaces with and utilizes the three-dimensional planning capabilities of a commercial treatment planning system. A case study is illustrated, which represents a particularly challenging planning problem due to a large planning target volume and an unusually small bladder. Treatment plans with different numbers of beams are generated to compare with each other and with the standard six-field plan. Significant improvement is shown in the reduction of hot regions within the femoral heads and rectal wall, while not unduly compromising homogeneity constraints for the tumour.

Extending the concept of primary and scatter separation to the condition of electronic disequilibrium.

A major deficiency of current photon calculation methods that are based on the concept of primary and scatter separation is their inability to handle the condition of electronic disequilibrium. This deficiency is examined and it is shown that the limitation is not inherent in the algorithms themselves but is, at least in part, in the data which the algorithms use. A new concept of primary and scatter separation is developed to cover the condition of electronic disequilibrium. This new concept requires little change to the existing algorithms and only additional data are required, which are generated using Monte Carlo calculation methods. The new concept is tested using programs in the Theratronics Theraplan treatment-planning system, and two calculation examples illustrate the ability to model electron transport and also the improvement over the existing algorithms. Close analogy of the extended concept with the convolution/superposition method of dose calculation is also indicated.