LETTER TO EDITOR Re: Clinical Application of Computed Tomography on Prostate Volume Esti- mation in Patients with Lower Urinary Tract Symptoms Sir, We have read this article with interest due to a sparse literature data regarding a comparison of prostatic volume (PV) measurement by computed tomography (CT) scan and transrectal ultrasonography (TRUS).(1) The authors have to be congratulated for implementation of Bonaventura Cavalieri’s principle for measurement of irregular bodies volume using the 3D reconstruction of CT scan images. However, several concerns regarding this study should be raised. Prostatic glands don't have unique shape. Therefore, there is no a single formula for accurate volume calculation for different prostates. An ellipsoid formula, which has been used in this paper, is associated with overestimation of volumes in larger glands as well as underestimation of volume in smaller glands.(2) Rodriguez and colleagues have shown that ellipsoid formula consistently underestimates the actual gland size in radical prostatectomy specimen. (3) Previously, McMahon and colleagues have found that a formula for bullet shape (V = Height (H) × Length (L) × Width (W) × 5π/24) is more convenient for prostates with volume < 55 cm3.(4) Therefore, the authors have not used more accurate formula for prostate volume calculation in more than 75% of samples in the study. Usage of CT scan for prostate volume measurement can’t be justified in clinical settings due to significant dose of irradiation (approximately 20-30 mSv). However, CT scan performed for unrelated reasons could serve for measurement of prostatic volume. As the authors rightfully stated, this technique also has some drawbacks due to difficulties in determinations of appropriate borders, particularly at the base and the apex of prostate. 3D CT scan prostate volume measurement using Cavalieri’s principle is time consuming procedure, and surprisingly is more inaccurate than measurement using ellipsoid formula. Probably, small movements of prostate during data acquisition could be one of reasons for overestimation of prostatic volume by CT scan. On the other side, further refinements of computer program enabling tracing of borders in two planes might be required for more accurate tracing in the apical and basal area of the gland. Unfortunately, this study has not been designed to enable judgement of intrapersonal nor interpersonal variation of measurement. Various authors have shown significant intrapersonal and interpersonal variability of prostate contour tracing on CT scan as well as measurement of dimensions and volume of prostate.(5,6) Certainly, accurate measurement of prostatic volume will remain significant challenge waiting for definite solution. Jovo Bogdanović, Vuk Sekulić, Senjin Djozić Clinical Center of Vojvodina, Clinic of Urology and Faculty of Medicine, University of Novi Sad, Novi Sad, Serbia. REFERENCES 1. Kang TW, Song JM, Kim KJ, et al. Clinical application of computed tomography on prostate volume estimation in patients with lower urinary tract symptoms. Urol J. 2014;11:1980-3. 2. Bienz, M, Hueber PA, Al-Hathal N, et al. Accuracy of Transrectal Ultrasonography to Evaluate Pathologic overestimation of volume in larger prostates and underestimation of Prostate Weight: Correlation with Various Prostate Size Groups. Urology. 2014;84:169-74. 3. Rodriguez E Jr, Skarecky D, Narula N, Ahlering TE. Prostate volume estimation using the ellipsoid formula consistently underestimates actual gland size. J Urol. 2008;179:501-3. 4. MacMahon PJ, Kennedy AM, Murphy DT, Maher M, McNicholas MM. Modified prostate volume algorithm improves transrectal US volume estimation in men presenting for prostate brachytherapy. Radiology. 2009;250:273-80. 5. Fiorino C, Reni M, Bolognesi A, Cattaneo GM, Calandrino R. Intra- and inter-observer variability in contouring prostate and seminal vesicles: implications for conformal treatment planning. Radiother Oncol. 1998;47:285-92. 6. Berthelet E, Liu MC, Agranovich A, Patterson K, Currie T. Computed tomography determination of prostate volume and maximum dimensions: a study of interobserver variability. Radiother Oncol. 2002;63:37-40. Letter to Editor 2124 Vol 12. No 02 March-April 2015 1995Vol 12. No 02 March-April 2015 2125 Reply by Author Dr. Bogdanovic and colleagues raised two issues for our article entitled “Clinical Application of Computed Tomography on Prostate Volume Estimation in Patients with Lower Urinary Tract Symptoms”. First of all, they mentioned that there is no a single formula for accurate volume calculation for different prostates. Furthermore, they recommended that modified prostate volume (PV) algorithm developed by McMahon and colleagues is more convenient for prostates with volume < 55 cm3.(1) I agree with their opinion. In 2010, Yang and colleagues reported modified ellipsoid formula at different stage of benign prostatic hyperplasia.(2) They also pointed out inaccuracy of volume estimation by ultrasound like Dr. Bogdanovic and colleagues. However, the authors desinged eccentricity parameter to improve accuracy of PV measurement by ultrasoud. Interestingly, bullet formular was also as inaccutate as ellipsoid formular in smaller gland. Furthermore, we suggested PV determination by CT scan using formula as preferable alternative in the clinics for quick volume measurement. Actually, we think that many physicians and urologists are familiar with ellipsoid formula. Secondly, we could not show intrapersonal and interpersonal variation of measurement because of retrospective study design. In addition, we just focused on the reliability and possibility of clinical use of PV measured by CT scan using the ellipsoid formula. Therefore, further studies are needed to validate intrapersonal and interpersonal variation of measurement. Thanks for all the suggestions and tips. Jae Hung Jung, MD Department of Urology and Radiology, Yonsei University, Wonju College of Medicine, Wonju, Republic of Korea. REFERENCES 1. MacMahon PJ, Kennedy AM, Murphy DT, Maher M, McNicholas MM. Modified prostate volume algorithm improves transrectal US volume estimation in men presenting for prostate brachytherapy. Radiology. 2009;250:273-80. 2. Yang CH, Wang SJ, Lin AT, Jen YM, Lin CA. Evaluation of prostate volume by transabdominal ultrasonography with modified ellipsoid formula at different stages of benign prostatic hyperplasia. Ultrasound Med Biol. 2011;37:331-7.