Aims & Objectives
The aim of this treatment planning study is to compare the techniques of 3D conformal radiotherapy (3DCRT) and IMRT to determine the feasible advantages for prostate cancer patientsof using a new direct step & shoot (DSS) IMRT module.
For the optimisation of the IMRT, Nucletron offers as a part of the optimising process their IM-optimisation software or their new module DSS. The earlier IM-optimisation software searches first for the ideal fluence for each beam, and this is then followed by the segmentation. The new DSS module integrates the segmentation into the optimisation process.
Materials & Methods
Between March 2006 and November 2006, four patients with a mean age of 71 years were enrolled for primary EBRT for localised prostate cancer. Three of these patients received antiandrogen therapy either before or during radiotherapy. All four patients had 3D CT treatment planning with a slice thickness of 5 mm and with immobilisation in a vacuum mattress (BlueBAG BodyFIX, Medical Intelligence).
As an initial step, it was planned (using Oncentra MasterPlan) to deliver 60 Gy to the planning target volume (PTV), calculated using data for a Siemens Primus linear accelerator (15 MV photons, with multileaf collimator leaf width of 1cm at the isocentre). The preselected gantry angles were 25, 90, 120, 240, 270 and 335 degrees. The rectal volume and urinary bladder were delineated as organs at risk (OARs). Additional structures were also contoured in order to help (we term them ‘Help Contours’) avoid hot spots in normal tissues surrounding the PTV to a distance of 1cm. The dose-volume objectives were defined by two schemes. After optimisation the plans were re-normalised to the average of PTV, giving 30 fractions with a fractional dose of 2 Gy. The 3DCRT plan used identical gantry angles with the beams weighted by experience.
Results
Both IMRT optimisation schemes reduced the doses received by the OARs when compared to the 3DCRT plan. Using the Nucletron IM-optimisation software the first weighting scheme of the objectives resulted in satisfactory dose-volume histograms (DVHs) for the OARs, and an obviously 'softened' DVH for the PTV (when compared to the 3DCRT plan). The DSS optimisation produced a steeper DVH for the PTV, but worse results for the OARs when compared to the IM-optimisation.
Scheme 2 improved the DVHs for the OARs using the DSS process, to about the same level as the IM-optimisation with scheme 1, the PTV DVH staying nearly unchanged. The IM-optimisation produced the worst DVH for the PTV of the five different plans we considered. In Figure 1 the DVHs are shown of a characteristic 3DCRT plan, the IM plan (Scheme 1) and the DSS plan (Scheme 2). Table 1 presents the mean values, averaged over the four patients, for the PTV and for the two OARs.
Conclusions
In every case the DSS optimisation resulted in a steeper DVH for the PTV than found using the IM process. The DVHs for the OARs are worse with scheme 1 but improve to about the same level with scheme 2. The patients benefit from IMRT by reduced doses to the OARs, keeping a very steep DVH for the PTV with the DSS optimisation. The user should note that weighting schemes based on the experience with IM-optimisation are not retained for the new DSS optimiation without control.