COMPARISON OF TREATMENT TECHNIQUES FOR CONFORMAL RADIOTHERAPY OF THE PROSTATE USING DOSE-VOLUME HISTOGRAMS AND NORMAL TISSUE COMPLICATION PROBABILITIES

The aim of this study was to evaluate the relative merits of the copla nar field arrangements most frequently used for conformal radiotherapy of the prostate using dose-volume histograms and normal tissue compli cation probabilities (NTCPs). Twelve patients with early prostate canc er underwent a planning CT scan of the pelvis. Isocentric plans for ea ch patient were devised using three, four, six and eight conformal fie lds and beam-weights optimised using fast simulated annealing to give a dose homogeneity across the planning target volume of +/-5% or bette r while minimising irradiation of the relevant organs at risk. The pla ns were then evaluated using dose-volume histograms of the organs at r isk (bladder, rectum and both femoral heads) and the Lyman model of no rmal tissue complication probability for the rectum. Analysis of dose- volume histogram data averaged over the 12 patients indicates an advan tage for six (p = 0.002) and eight (p = 0.0001) fields with respect to the percentage volume of the femoral heads receiving > 50% of the pre scribed dose compared with three fields. There was a similar advantage for six (p = 0.0007) and eight (p = 0.0001) fields compared with four fields. Ranking of the treatment techniques indicates that the four-f ield technique is the worst with respect to femoral head irradiation b ut the best with respect to reducing rectal irradiation. A higher dose can be prescribed to the isocentre with the four-field technique for a 5% rectal NTCP. The six-field technique led to sparing of the bladde r when the different treatment techniques were ranked using bladder do se-volume histogram data. We conclude that none of the techniques stud ied consistently proved to be superior when applied to this sample of patients with prostate cancer with respect to sparing all the organs a t risk. The absolute differences between techniques are small and woul d be very difficult to detect with respect to clinically relevant endp oints.