OLUME measurement in radiosurgery is a crucial function both for deciding whom to treat and for as- sessing the effectiveness of treatment at follow-up intervals. Thus, it is important to have a reliable and re- producible method to measure volumes. All currently used methods have their problems. One of the most common methods used to estimate the volume of intracranial lesions is to take the largest diame- ters seen on CT and/or MR imaging from which a geomet- ric volume assessment is then made. 7 The product of the diameters is multiplied by a factor 0.52, which results from the quotient of the equation of the volume of the sphere and the equation for the calculation of the volume of the cube: 7 (4/3  r 3 )/(2r) 3 = 3.1416 / 6 = 0.52. This approximation of volume measurement has been generally accepted by professionals in imaging departments because all imaging units are equipped with the tools for di- ameter measurements. 3,6,7 This method has limitations because most tumors are not spherical and indeed tend to be more irregular after surgery. 2 The objective of this current study was to compare ste- reotactic volume measurement when using GammaPlan (Elekta Instrument AB, Stockholm, Sweden) with geomet- ric volume measurement by using retrospective and pro- spective analyses. The aim was to determine the level of ac- curacy and the impact of the two methods on the decision to treat a given patient; we also sought to determine which method is more reliable for the evaluation of the treatment response. Materials and Methods The medical records of 100 patients who were treated with GKS and whose treatment volumes were calculated with stereotactic planning software on multiplanar MR and CT images were analyzed. The GammaPlan system (version 5.34; Elekta Instrument AB) was used to measure the longest diameters of x, y, and z lesions previously treated with GKS and they were named x’, y’, and z’ in the same way that they are measured in the imaging devices when volumes are being determined. The product of the diameters was multiplied by factor 0.52, which results from the quotient of the equation of the J Neurosurg (Suppl) 102:140–142, 2005 140 Stereotactic noninvasive volume measurement compared with geometric measurement for indications and evaluation of gamma knife treatment RAMIRO DEL V ALLE, M.D., MIGUEL PÉREZ, BS., JUAN ORTIZ, M.S., SALVADOR RUIZ, M.D., SALVADOR DE ANDA, M.D., JOSÉ JARAMILLO, M.D., MANUEL MARTÍNEZ, M.D., ROBERTO CORONA, MD., INGRID VIVAS, MD., M.A. TERESA F ACHA, M.D., ROSARIO OLMOS, M.D., ARTURO ARRIETA, R.T., AND JOSUÉ ESTRADA M.SC., NS. Gamma Radiosurgery Department and MRI Department, Hospital Médica Sur. Mexico City; and Radiosurgery and Stereotactic Radiotherapy Section of the Mexican College of Neurological Surgery, Puente de Piedra, México Object. Volume estimation is one of the most important criteria in the evaluation and follow up of radiosurgical treat- ments and outcomes; however, several limitations are involved in the calculation estimation of target volumes. Methods. Retrospective and prospective studies were conducted to evaluate the efficacy of a new noninvasive stereo- tactic method when it is compared with geometric volume calculation of intracranial tumors for planning stereotactic radio- surgery treatment as well as for follow up and outcome evaluation. Two equations were created that permit comparison of the calculated and measured volumes. These equations took lin- ear and quadratic forms, respectively. Volume estimation using the stereotactic approach compared with traditional vol- ume calculation gave more accurate results regardless of the shape and size of the lesion. Conclusions. The use of stereotactic volume calculation is highly recommended in planning, follow up, and determina- tion of the outcome in patients participating in radiosurgical treatment and should lead to more uniform reports of the response to treatment. KEY WORDS • intracranial tumor • volume measurement • stereotaxy • gamma knife surgery V J. Neurosurg. / Volume 102 / January, 2005 Abbreviations used in this paper: CT = computerized tomogra- phy; GKS = gamma knife surgery; MR = magnetic resonance.