Pulsed Low Dose Rate Radiation Therapy Using Intensity Modulated Radiotherapy

Purpose/Objective(s): Pulsed low dose rate (PLDR) radiotherapy, which utilizes the hyper-radiosensitivity of tumor cells below their transition doses (< 50 cGy) and the increased normal tissue repair at low dose rates, has the potential to reduce normal tissue toxicities while providing significant tumor control for recurrent cancers. This technique as implemented at the University of Wisconsin, Madison, employs conventional 3DCRT with forward treatment planning. This work investigates the dosimetry feasibility of PLDR treatment using intensity modulated radiation therapy (IMRT) to further improve the dose conformity.Material/Methods: Six body sites (20 cases: 8 treated, 12 evaluation purpose) were investigated in this study including prostate, pancreas, lung, head and neck, breast and pelvis. IMRT plans were generated using the Varian Eclipse and Elekta/CMS XiO treatment planning systems with 6, 10 and 15 MV photons. Each plan consisted of 10 fields to achieve a daily dose of 2 Gy. However, for breast cases, two tangential intensity modulated fields were employed to reduce the dose to the contralateral breast, lung and heart, and the plan was delivered five times to achieve the same daily dose of 2 Gy. The dosimetry requirement was to deliver approximately 20 cGy per field with a 3 min interval to achieve an effective dose rate of 6.7 cGy/min. The maximum, minimum and mean doses to the PTV were analyzed together with the dose volume histograms and isodose distributions.Results: Ten-field IMRT provided superior dose distributions and spared critical structures effectively. The preliminary clinical results were favorable. For most cases, the IMRT plans generated using routine dose optimization constrains and beam arrangements fulfilled the dosimetric requirement for the PLDR treatment. The Dmean per beam achieved an effective dose rate of 4.6-9.4 cGy/min. The Dmax per beam was less than the tumor transition dose of 50cGy. It was noticed that one field had Dmax more than 50 cGy in a large lung tumor and a large H&N tumor cases. This can be solved by delivering the field at the last or re-plan with non-coplanar beam arrangement to achieve uniform dose contributions in PTV.Conclusion:PLDR radiotherapy using the IMRT technique takes both the dosimetric benefits and radiobiological benefits, which may have great potential for those previously irradiated patients who have historically done poorly.Table 1: The median number and the range of the min., mean, and max. doses to PTV.Site, number of casesDmin (cGy)Dmean (cGy)Dmax (cGy)Prostate (n=4)4.1(2.6-17.5)22.0(16.2-26.8)32.5(26.1-47.5)Pancreas (n=3)10.6(2.6-18.2)21.1(13.9-28.3)29.4(23.4-42.2)Breast (n=2)12.8(12.3-14.3)20.1(19.6-20.8)21.7(27.6-30.6)Pelvis (n=3)5.0(2.0-16.6)20.6(16.6-24.1)30.8(28.4-45.0)Lung (n=5)4.3(2.0-22.6)19.1(12.8-36.7)30.1(21.9-52.2)H&N (n=3)9.8(1.5-16.8)21.0(12.1-31.4)33.5(25.6-51.0)

SSA22-09

Pulsed Low Dose Rate Radiation Therapy Using Intensity Modulated Radiotherapy