Static field intensity modulation to treat a dominant intra-prostatic lesion to 90 Gy compared to seven field 3-dimensional radiotherapy.

PURPOSE/OBJECTIVE: Recent studies supported by histopathological correlation suggest that the combined use of endorectal magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) allows differentiation of normal and carcinomatous prostate. The goal of this study was to use static field intensity modulated three-dimensional conformal radiotherapy (SF-IMRT) to treat the entire prostate to a total dose of >70 Gy, while concurrently treating a dominant intraprostatic lesion (DIL) defined by MRI+MRS to 90 Gy while not exceeding normal tissue tolerances.

MATERIALS AND METHODS: For the example chosen, the DIL consisted of a large portion of the peripheral zone of the left lobe of the prostate. University of Michigan (UM-PLAN) three-dimensional treatment planning software was used to design a partially shielded 7 field conformal isodose plan that would treat the entire prostate to >70 Gy at 1.8 Gy per day (80% isodose line), while concurrently treating the DIL to 2.25 Gy per day for a total dose of 90 Gy. Dose volume histograms (DVH) were used to compare the rectal doses to rectum and other adjacent normal tissues using these two techniques.

RESULTS: SF-IMRT as described, allowed a total dose of 90 Gy to encompass the DIL, while the rectal dose was slightly lower than that using the standard 7 field technique to the prostate alone. For example, the dose to 30 cm3 of the rectum was 40 Gy using SF-IMRT and 48 Gy for the standard 7 field technique. Because of differences in the dose per fraction the biologic advantages of the SF-IMRT technique are likely to be even greater.

CONCLUSIONS: This study demonstrates the feasibility of using SF-IMRT to treat a DIL involving a single lobe of the prostate, as defined by MRI/MRS, to 90 Gy, while simultaneously treating the prostate to >70 Gy without increasing the dose to surrounding normal tissues. A similar approach could be used to treat multifocal disease. This method of treatment is an alternative to dynamic intensity modulation. It is less expensive, and can be adapted to any radiation therapy department without the use of an inverse treatment planning programs.