doi:10.1016/j.meddos.2007.09.002 IMPACT OF DIFFERENT CT SLICE THICKNESS ON CLINICAL TARGET VOLUME FOR 3D CONFORMAL RADIATION THERAPY RAMACHANDRAN PRABHAKAR,PH.D., THARMAR GANESH,PH.D., GOURA K. RATH, M.D., PRAMOD K. JULKA, M.D., PAPPIAH S. SRIDHAR, M.D., RAKESH C. JOSHI,PH.D., and SANJAY THULKAR, M.D. Departments of Radiation Oncology and Radiology, Institute Rotary Cancer Hospital, All India Institute of Medical Sciences, New Delhi, India (Received 25 April 2007; accepted 18 September 2007) Abstract—The purpose of this study was to present the variation of clinical target volume (CTV) with different computed tomography (CT) slice thicknesses and the impact of CT slice thickness on 3-dimensional (3D) conformal radiotherapy treatment planning. Fifty patients with brain tumors were selected and CT scans with 2.5-, 5-, and 10-mm slice thicknesses were performed with non-ionic contrast enhancement. The patients were selected with tumor volume ranging from 2.54 cc to 222 cc. Three-dimensional treatment planning was performed for all three CT datasets. The target coverage and the isocenter shift between the treatment plans for different slice thickness were correlated with the tumor volume. An important observation from our study revealed that for volume < 25 cc, most of the cases were underdosed by 18% with 5-mm slice thickness and 27% with 10-mm slickness. For volume > 25 cc, the target underdosage was less than 6.7% for 5-mm slice thickness and 8% for 10-mm slice thickness. For 3D conformal radiotherapy treatment planning (3DCRT), a CT slice thickness of 2.5 mm is optimum for tumor volume < 25 cc, and 5 mm is optimum for tumor volume > 25 cc. © 2009 American Association of Medical Dosimetrists. Key Words: Computed tomography, Tumor volume, 3D-CRT. INTRODUCTION The success of tumor control depends on treating the full extent of disease and on avoiding the surrounding normal tissue. Balancing these complementary goals has always been recognized as the most difficult task to the treating clinicians. The dose-response relationship between the tumor and the normal tissues provides the basis to arrive at an acceptable treatment plan. Experimental and clini- cal evidence shows that a small change in dose of 7% to 15% can reduce local tumor control significantly and increase normal tissue complications. 1,2 The Interna- tional Commission on Radiation Units and Measure- ments (ICRU) recommends an accuracy of  5% or better in the absorbed dose delivery to the target volume, based on the clinical and radiobiological data. 3 Mijnheer et al. 4 stated more precisely that the total error (both random and systematic) in absorbed dose delivered should be no more than  3.5% at 1 standard deviation (SD) level (68% confidence level). Volume definition is a prerequisite for meaningful 3D treatment planning and for accurate dose reporting. ICRU reports 50 and 62 define and describe several targets and critical structure volumes that aid in the treatment planning process and that provide a basis for comparison of treatment outcomes. 5,6 In radiotherapy treatment planning, proper estimation of the tumor vol- ume is essential to avoid tumor remission. 7 It plays a crucial role in complete eradication of the disease. It was formulated that tumor volume is an indirect measure of clonogen number and has a direct impact on the local disease control by radiotherapy. 8,9 Accurate volume def- inition may provide a useful measure for tumor bulk and in the future analysis of treatment response. 10 Although other imaging modalities such as magnetic resonance imaging (MRI) have been shown to decrease interob- server variation, CT remains an essential imaging mo- dality for dose calculation process in treatment planning. Slice thickness and interslice spacing between CT sec- tions impact nearly every phase of the treatment. It affects the definition of structures; the quality of recon- structed images in sagittal, coronal, arbitrary planes; digitally reconstructed radiographs (DRR) used for both localization and dose display; beam’s-eye view (BEV), and dose-volume histogram (DVH). Volume definition is critically important in intensity modulated radiation ther- apy (IMRT), which is more sensitive to geometric un- certainties because of its sharper dose gradients around the target volume and organs at risk. The importance of target definition was studied by several authors. 11–18 The Photon Treatment Planning Collaborative Work Group (PTPCWG, 1991) recommended that accu- rate definition of the inferior and superior borders of the treatment volume for localization requires a close spacing Reprint requests to: Ramachandran V. Prabhakar, Ph.D., Depart- ment of Radiation Oncology, Institute Rotary Cancer Hospital, All India Institute of Medical Sciences, New Delhi – 110 029, India. E-mail: prabhakar_smr@hotmail.com Medical Dosimetry, Vol. 34, No. 1, pp. 36-41, 2009 Copyright © 2009 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/09/$–see front matter 36