Reirradiation of patients who were previously treated with radiotherapy is vastly challenging. Pulsed low-dose rate (PLDR) external beam radiotherapy has the potential to reduce normal tissue toxicities while providing significant tumor control for recurrent cancers. This work investigates treatment planning techniques for intensity-modulated radiation therapy (IMRT)-based PLDR treatment of various sites, including cases with pancreatic and prostate cancer. A total of 20 patients with clinical recurrence were selected for this study, including 10 cases with pancreatic cancer and 10 with prostate cancer. Large variations in the target volume were included to test the ability of IMRT using the existing treatment planning system and optimization algorithm to deliver uniform doses in individual gantry angles/fields for PLDR treatments. Treatment plans were generated with 10 gantry angles using the step-and-shoot IMRT delivery technique, which can be delivered in 3-minute intervals to achieve an effective low dose rate of 6.7 cGy/min. Instead of dose constraints on critical structures, ring structures were mainly used in PLDR-IMRT optimization. In this study, the PLDR-IMRT plans were compared with the PLDR-3-dimensional conformal radiation therapy (3DCRT) plans and the PLDR-RapidArc plans. For the 10 cases with pancreatic cancer that were investigated, the mean planning target volume (PTV) dose for each gantry angle in the PLDR-IMRT plans ranged from 17.6 to 22.4 cGy. The maximum doses ranged between 22.9 and 34.8 cGy. The minimum doses ranged from 8.2 to 17.5 cGy. For the 10 cases with prostate cancer that were investigated, the mean PTV doses for individual gantry angles ranged from 18.8 to 22.6 cGy. The maximum doses per gantry angle were between 24.0 and 34.7 cGy. The minimum doses per gantry angle ranged from 4.4 to 17.4 cGy. A significant reduction in the organ at risk (OAR) dose was observed with the PLDR-IMRT plan when compared with that using the PLDR-3DCRT plan. The volume receiving an 18-Gy (V-18) dose for the left and right kidneys was reduced by 10.6% and 12.5%, respectively, for the pancreatic plans. The volume receiving a 45-Gy (V-45) dose for the small bowel decreased from 653% to 45.5%. For the cases with prostate cancer, the volume receiving a 40-Gy (V-40) dose for the bladder and the rectum was reduced significantly by 25.1% and 512%, respectively. When compared with the RapidArc technique, the volume receiving a 30-Gy (V-30) dose for the left and the right kidneys was lower in the IMRT plans. For most OARs, no significant differences were observed between the PLDR-IMRT and the PLDR-RapidArc plans. These results clearly demonstrated that the PLDR-IMRT plan was suitable for PLDR pancreatic and prostate cancer treatments in terms of the overall plan quality. A significant reduction in the OAR dose was achieved with the PLDR-IMRT plan when compared with that using the PLDR-3DCRT plan. For most OARS, no significant differences were observed between the PLDR-IMRT and the PLDR-RapidArc plans. When compared with the PLDR-3DCRT plan, the PLDR-IMRT plan could provide superior target coverage and normal tissue sparing for PLDR reirradiation of recurrent pancreatic and prostate cancers. The PLDR-IMRT plan is an effective treatment choice for recurrent cancers in most cancer centers. (C) 2014 American Association of Medical Dosimetrists.