The kidneys play a very important role in xenobiotic elimination. Renal function is known to decrease with advancing age. Renal function is reduced by ∼1% per year beyond 30–40 years of age, so that by age 70, renal function may have decreased by 40% owing to shrinkage in renal mass, age-related reduction in renal blood flow, and a gradual loss of functioning nephrons. Most anticancer agents have narrow therapeutic indices, and several of them are excreted renally. Therefore, accurate estimation of glomerular filtration rate (GFR) is extremely important for optimal dosing of anticancer drugs in the elderly. Plasma clearance of inulin, iohexal and iothalamate are considered precise markers for the calculation of GFR. These tests are limited by the need for continuous intravenous infusions, and repeat blood and urine collections causing inconvenience to patients, besides being expensive, cumbersome and time consuming. The 99mTc-DTPA is a reliable method to measure GFR. It also has many advantages such as its availability, lower cost and the option of performing renal imaging alongside quantifying GFR. The Cockcroft–Gault (CG) formula was published in the 1970s as a bedside equation for estimated creatinine clearance (eCrCl). However, this equation is an imprecise estimate of true GFR in large part due to its failure to adequately compensate for several non-GFR determinants of serum creatinine (SCr), such as body composition, diet, race, tubular secretion, and extrarenal elimination of creatinine. Improved methods for determining eGFR have been developed in the last 20 years, notably the modification of Diet in Renal Disease (MDRD) Study equation (MDRD-4 and MDRD-6) and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) and the CKD-EPI Cystatin C equations. These equations were developed with standardized SCr values and iothalamate clearance as the reference, and they incorporate surrogates such as age, sex, and race to account for the effects of some non-GFR determinants of SCr. Recently, the Berlin Initiative Study (BIS) creatinine and Cystatin C equation, appears to offer better accuracy than either the MDRD or CKD-EPI equation. Cystatin C is considered more reliable than serum creatinine in the elderly population because it is not affected by changes in muscle mass. Although the use of these newer equations results in a value that is closer to the true GFR when compared to the CG formula and are considered a potential replacement for the same, their performance in geriatric cancer patients is still uncertain and has not yet been investigated. This study aimed to assess the most accurate GFR estimating equation in older Indian patients with cancer.