The effect of changing technique, dose, and PTV margin on therapeutic ratio during prostate radiotherapy

To quantify the dosimetric and radiobiological changes seen when using intensity-modulated radiation therapy (IMRT) or planning target volume (PTV) margin reduction with consistent planning parameters in a representative sample of localized prostate cancer patients.

Twenty patients were randomly selected from a cohort that received 79.8 Gy using six-field conformal radiotherapy. Using the clinical contours, PTV margin, planning system, and dose constraints, five-field IMRT plans were generated for 79.8, 83.8, and 88.0 Gy. The 88.0-Gy IMRT plan was then reoptimized with a PTV margin reduced to 3 mm. These plans were then compared using various dosimetric and radiobiological endpoints calculated for various alpha/beta.

Intensity-modulated RT resulted in greater conformity to the PTV (p < 0.001). No improvement in mean normal tissue complication probabilities in the rectal wall (NTCPrw) was seen, and the modified therapeutic ratio (TR(mod)) was largely unchanged between six-field conformal and IMRT for the majority of the patients. When IMRT was used to escalate dose, NTCPrw increased by 9% at each 5% prescription increase (p < 0.001). Reducing the posterior PTV margin from 7 mm to 3 mm for an IMRT plan reduced the mean NTCPrw by 12% (p < 0.001) and resulted in a trend toward increased TR(mod)(p = 0.005). Changes in TR(mod) between conformal and IMRT planning or PTV reduction showed large interpatient variability.

Changing from conformal to IMRT, or from PTV(10-7) to PTV(3), did not produce a uniform interpatient increase in TR(mod)when the CTV contained the prostate alone. Radiobiological benefits of these two methods seem to be dependent on the particular anatomy of individual patients, supporting the use of patient-specific margin, planning, and dose prescription strategies.