LATERAL RECTAL SHIELDING REDUCES LATE RECTAL MORBIDITY FOLLOWING HIGH-DOSE 3-DIMENSIONAL CONFORMAL RADIATION-THERAPY FOR CLINICALLY LOCALIZED PROSTATE-CANCER - FURTHER EVIDENCE FOR A SIGNIFICANT DOSE-EFFECT

Purpose: Using conventional treatment methods for the treatment of cli nically localized prostate cancer central axis doses must be limited t o 65-70 Gray (Gy) to prevent significant damage to nearby normal tissu es. A fundamental hypothesis of three-dimensional conformal radiation therapy (3DCRT) is that, by defining the target organ(s) accurately in three dimensions, it is possible to deliver higher doses to the targe t without a significant increase in normal tissue complications. This study examines whether this hypothesis holds true and whether a simple modification of treatment technique can reduce the incidence of late rectal morbidity in patients with prostate cancer treated with 3DCRT t o minimum planning target volume (PTV) doses of 71-75 Gy. Methods and Materials: The 257 patients with clinically localized prostate cancer who completed 3DCRT by December 31, 1993 and received a minimum PTV do se of 71-75 Gy are included in this report. The median follow-up time was 22 months (range: 4-67 months); 98% of patients had follow-up of l onger than 12 months. The calculated dose at the center of the prostat e was < 74 Gy in 19 patients, 74-76 Gy in 206 patients, and > 76 Gy in 32 patients. Late rectal morbidity was graded according to the Late E ffects Normal Tissue (LENT) scoring system. Eighty-eight consecutive p atients were treated with a rectal block added to the lateral fields. In these patients the posterior margin from the prostate to the block edge was reduced from the standard 15 to 5 mm for the final 10 Gy, whi ch reduced the dose to portions of the anterior rectal wall by approxi mately 4-5 Gy. Estimates of rates for rectal morbidity were determined by Kaplan-Meier actuarial analyses. Differences in morbidity percenta ges were evaluated by the Pearson chi-square test. Results: Grade 2-3 rectal morbidity developed in 46 out of 257 patients (18%) and in the majority of cases consisted of rectal bleeding. No patient has develop ed Grade 4 or 5 rectal morbidity. The actuarial rate of Grade 2-3 morb idity is 23% at 24 months and the median time to the development of Gr ade 2-3 complications is 15 months. A statistically significant dose e ffect is evident. The incidence of Grade 2-3 rectal morbidity increase d as the dose at the center of the prostate increased (p = 0.05). In p atients receiving minimum PTV doses of less than or equal to 76 Gy the use of a rectal block significantly reduced the incidence of Grade 2- 3 toxicity; 6 out of 88 (7%) with a block vs. 30 out of 137 (22%) with out a block, (p = 0.003). Conclusion: The incidence of late rectal mor bidity with 3DCRT to minimum PTV doses of 71-75 Gy is acceptable and t o date no Grade 4-5 rectal morbidities have been observed. In our expe rience, higher doses to the center of the prostate are associated with an increased likelihood of developing Grade 2-3 rectal morbidity but treatment techniques that reduce the total dose to the anterior rectal wall have reduced the incidence of late rectal morbidity. If clinical studies indicate improved tumor control with minimum PTV doses above 71 Gy, then dose escalation above 76 Gy to the center of the prostate should be pursued cautiously with treatment techniques that limit the total dose to the anterior rectal wall.