The influence of interpatient and intrapatient rectum variation on external beam treatment of prostate cancer

Purpose: The rectal dose/volume relationship and inherent variations thereo f are fundamental parameters to guide dose escalation in prostate cancer tr eatment. This study evaluates the effect of rectal dose/volume variation on the risk of rectal complication for different planning target volume (PTV) constructions. Methods and Materials: Thirty prostate patients with multiple daily CT scan s obtained during the treatment course were included in this retrospective study. The dose distribution was calculated based on the pretreatment CT im age alone. Treatment plans were generated by applying the four-field-box be am arrangement to each of three different PTVs: PTVs with 0.5-cm and 1.0-cm uniform margins, and a patient-specific PTV constructed using treatment im aging feedback. For each of the 30 patients, the rectal wall as manifested on each of multiple CT images was delineated after image bony registration to the pretreatment CT image, and applied to the corresponding treatment pl an to obtain the rectal wall dose-volume histogram (DVH). Interpatient and intrapatient rectal dose/volume variations were quantified accordingly. The corresponding uncertainty and sensitivity of the risk of rectal complicati on to the variations were evaluated for each of the three PTVs. Finally, th e efficacy of using multiple CT images to reduce uncertainty in planning ev aluation was examined. Results: Sensitivity of the risk of rectal complication to rectal dose/volu me variation strongly depends on the clinical tar, A volume (CTV)-to-PTV ma rgin or prescription dose, or both. Compared to the conventional two-dimens ional (2D) prostate cancer treatment, the sensitivity for a conformal treat ment can be 3 times higher or more. Due to the interpatient rectal dose/vol ume variation, the individual normal tissue complication probability (NTCP) was distributed from 10% to 37% when a common prescription dose was applie d for all patients. The intrapatient rectal dose/volume variation introduce s at least +/- 25% uncertainty in the NTCP calculation for at least 10% or 25% of the patients treated with the PTV of 1.0- or 0.5-cm margin, respecti vely. These uncertainties were larger for the smaller PTV, with the standar d deviation up to 20%. By applying multiple CT image feedback, the NTCP unc ertainty could be reduced by a factor of 2. Conclusions: Shape and position variation of rectum has less influence on t reatment planning in the conventional 2D treatment of prostate cancer. Howe ver, this influence is quickly growing with high treatment dose or small CT V-to-PTV margins. To reduce the variation and uncertainties in the treatmen t planning evaluation associated with the inter- and intrapatient rectal do se/volume variation, the iso-NTCP model and treatment image feedback techni que can be applied in dose escalation trials of prostate cancer treatment. (C) 2001 Elsevier Science Inc.