Purpose: To compare the dose coverage of planning and clinical target volume (PTV, CTV), and organs-at-risk (OAR) between intensity-modulated (3D-IMRT) and conventional conformal radiotherapy (3D-CRT) before and after internal organ variation in prostate cancer.
Methods and materials: We selected 10 patients with clinically significant interfraction volume changes. Patients were treated with 3D-IMRT to 80 Gy (minimum PTV dose of 76 Gy, excluding rectum). Fictitious, equivalent 3D-CRT plans (80 Gy at isocenter, with 95% isodose (76 Gy) coverage of PTV, with rectal blocking above 76 Gy) were generated using the same planning CT data set ("CT planning"). The plans were then also applied to a verification CT scan ("CT verify") obtained at a different moment. PTV, CTV, and OAR dose coverage were compared using non-parametric tests statistics for V95, V90 (% of the volume receiving 95 or 90% of the dose) and D50 (dose to 50% of the volume).
Results: Mean V95 of the PTV for "CT planning" was 94.3% (range, 88-99) vs 89.1% (range, 84-94.5) for 3D-IMRT and 3D-CRT (p=0.005), respectively. Mean V95 of the CTV for "CT verify" was 97% for both 3D-IMRT and 3D-CRT. Mean D50 of the rectum for "CT planning" was 26.8 Gy (range, 22-35) vs 43.5 Gy (range, 33.5-50.5) for 3D-IMRT and 3D-CRT (p=0.0002), respectively. For "CT verify", this D50 was 31.1 Gy (range, 16.5-44) vs 44.2 Gy (range, 34-55) for 3D-IMRT and 3D-CRT (p=0.006), respectively. V95 of the rectum was 0% for both plans for "CT planning", and 2.3% (3D-IMRT) vs 2.1% (3D-CRT) for "CT verify" (p=non-sig.).
Conclusion: Dose coverage of the PTV and OAR was better with 3D-IMRT for each patient and remained so after internal volume changes.