1. Comparison of Rectal Dose Volume Histograms for Definitive Prostate Radiotherapy Among Stereotactic Radiotherapy, IMRT, and 3D-CRT Techniques Author(s): Berit Madsen, MD; Huong Pham, MD; R. Alex Hsi, MD; Joseph Presser, MSc, DABR; Laura Esagui, CMD, Nancy Collins, CMD; Edward Lawrence, MSc; Guobin Song, MD, PhD; Kasra Badiozamani, MD; Michael Hunter, MD; Eric Taylor, MD, Section of Radiation Oncology, Virginia Mason Medical Center, Seattle, WA I ABSTRACT III FINDINGS IV CONCLUSIONS II METHODS PURPOSE To compare rectal dose volume histograms (DVH) among stereotactic radiotherapy, intensity modulated radiation therapy (IMRT), and 3D- conformal radiotherapy (3D-CRT) for prostate cancer. METHODS/MATERIALS Radiotherapy plans for 28 patients treated with stereotactic radiotherapy, IMRT, and 3D-CRT techniques were reviewed. There were 10 stereotactic plans, 9 IMRT plans, and 9 3D-CRT plans. The planning systems utilized were Northwest Medical Physics/pReference for the stereotactic plans and ADAC/Pinnacle for IMRT and 3D- CRT plans. Fused MRI and CT were used for target delineation with IMRT and stereotactic radiotherapy. CT alone was utilized for 3D-CRT. For patients treated with IMRT and stereotactic radiotherapy, fiducial markers were placed within the gland and utilized for daily target localization with electronic portal imaging. Patients treated with 3D-CRT were localized with skin marks. The CTV was expanded by 0.4 cm for stereotactic radiotherapy plans and 0.5 cm for IMRT plans based on accuracy of the localization technique and on previous data related to intrafractional stability. CTV expansion was 1-1.5 cm for the 3D-CRT plans. Stereotactic radiotherapy was delivered with 6 non-coplanar fields and custom stereotactic cones on the 2100 CD Varian linac with patients lying in a flex-prone position. IMRT was delivered with 7 coplanar beams, and 3D-CRT with 6 coplanar beams. Treatment was delivered with either 0.5 cm or 1 cm MLC on a Varian 2100 CD or 21 EX linac for IMRT and 3D-CRT in supine position without other immobilization. The dose to the prostate was 33.5 Gy/6 fractions for stereotactic radiotherapy plans, and 75-78 Gy/35-42 fractions for IMRT and 3D-CRT plans. The rectal volume was contoured as a solid organ from anal canal to the level of the sacro-iliac joint in all plans and reported in cubic centimeters. RESULTS There were notable differences in the rectal DVH among the three treatment techniques (see table). IMRT produced the most rectal sparing at the high dose levels but below the 80% level, stereotactic radiotherapy produced better sparing. 3D-CRT plans provided the least rectal sparing at all dose levels. CONCLUSION In our experience, IMRT and stereotactic radiotherapy plans resulted in minimal rectal volumes exposed to high dose radiation compared to 3D-CRT plans. Additionally, stereotactic radiotherapy resulted in significantly less rectal volume exposed to the lower radiation dose levels than IMRT or 3D-CRT plans which could be advantageous in hypofractionated treatment regimens. This analysis helps substantiate the use of this unique stereotactic technique for our phase I/II clinical trial of hypofractionated radiation therapy for prostate cancer. Rectal DVH Expressed in Cubic Centimeters STEREOTACTIC RT IMRT 3D-CRT 3 Implanted Gold Markers Daily Portal Imaging Stereotactic Localization Flex-prone Position MRI/CT Fusion 3 Implanted Gold Markers Daily Portal Imaging Stereotactic Localization MRI/CT Fusion Conformal Blocking Around Prostate for 3-D Plan IMRT Isodose Plan 3D Conformal Isodose Plan Isodose Plan Rectum CTV No Immobilization Skin Marks for localization External Beam Radiation Therapy Beam Arrangements & Stereotactic Cones 6 stationary non-coplanar fields CTV-PTV 4 mm expansion Precision conformal blocks are cast within stereotactic cones 3-D conformal dose distribution as seen on a CT image Intensity Modulated (IMRT) dose distribution Stereotactic RT & IMRT plans result in minimal rectal volume exposed to high dose radiation Stereotactic RT results in least rectal volume exposed to moderate and lower doses of radiation 6 coplanar fields CTV-PTV expansion 10-15 mm 5 mm MLC Gold markers used for registration 7 coplanar fields 35-40 control points CTV-PTV 5 mm expansion 5 mm MLC i Madsen B, Hsi RA, Pham H, Presser J, Esagui L, et al. Intrafractional Stability of the Prostate Using a Stereotactic Radiotherapy Technique. Int J Radiat Oncol Biol Phys 2003;57:1285-1291.