4060 POSTER Annual ibandronic acid to prevent gonadotropin included bone loss in men with prostate cancer

Abstract

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.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).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.).Dose coverage of the PTV and OAR was better with 3D-IMRT for each patient and remained so after internal volume changes.