CalcNTCP for computation of NTCP

The benefits of cancer radiotherapy are potentially threatened by the adverse effects of radiation on normal tissue. A safer compromise between the effect of a radiation dose regimen on cancerous and normal tissue is very important. The satisfactory outcome of radiotherapy depends on maximizing the effect of radiation on the local disease (target area) and minimizing the radiation-induced complications to the surrounding healthy tissues. Normal tissue complication probability (NTCP) is a useful parameter to determine the complication probability for normal tissue which will be affected by the radiation treatment. Lyman put forth an important model for quantitative evaluation of NTCP of an organ or tissue for uniform partial volume irradiation to a particular dose of radiation. Although the methodology involved in Lyman model is straightforward the main difficulty arises while computing the normal distribution error function for cumulative probability. Hence, computation of NTCP tends to be very complex or practically impossible without the aid of a suitable computer program or statistical tables. We have developed a simple and user-friendly software, CalcNTCP, for quick and accurate calculation of NTCP [1]. The software CalcNTCP computes NTCP in accordance with Lyman model using the following formulas:

The above formulas (Eqs. 1-4) are given in the sequence as they are executed by the software. The abbreviations for different parameters used in the above formulas are as follows: D = radiation dose; v = volume fraction of the organ receiving the dose D; V = volume of the organ receiving the dose D; Vref = reference volume of the organ, usually the total volume of the organ; TD50(v) = tolerance dose for 50% of the population when irradiating a volume fraction v; TD50(1) = tolerance dose for 50% of the population for whole organ (or reference volume) irradiation; n = the volume parameter defining the impact of volume fraction v; m = the slope parameter with impact on the steepness of the dose-response curve; t = normal standard deviate.

**Figure 1.** The screen of CalcNTCP software showing the computation of NTCP for lung associated with a radiation dose of 40 Gy to a partial volume (0.40). The pre-stored values of n, m and TD_{50}(1) are displayed automatically once the organ of interest has been selected from the list. The right side panel of the screen is designed for use with customized values of n, m and TD_{50}(1). The disabled ‘Calculate’ button shown at the right side (Mode 2) becomes activated when the values of all the respective parameters in 5 text boxes have been entered.

Mode 1 of the software utilizes the reported values of ‘n’, ‘m’ and ‘LD_{50}(1)’ for 27 organs including bladder, brachial plexus, brain, brain stem, cauda equina, colon, ear, esophagus, femoral head/neck, heart, kidney, larynx, lens, liver, lung, optic nerve, optic chiasma, parotid, rectum, retina, rib cage, skin, small intestine, spinal cord, stomach, thyroid and mandible. Mode 2 of the software is designed for customized entry of all the parameters on user’s discretion (Figure 1, right panel). The computation methodology is same as in case of Mode 1 except that there are 5 input boxes for the entries of ‘n’, ‘m’ and ‘LD_{50}(1)’, ‘v’ and ‘D’.

**Figure 2.** Demonstration of NTCP as a function of radiation dose for 3 partial volumes (1.00, 0.67 and 0.33) of the liver (upper plot) and lung (lower plot). NTCP values were computed using CalcNTCP software.

The use of CalcNTCP significantly shortens the time of calculation as compared to manual procedure. The chances of errors in manual computation of NTCP are always high due to various factors including loss of user’s concentration, more number of key presses and wrong/imperfect data entries and mistakes while referring to statistical tables. On the other hand, the display of all the data inputs on CalcNTCP screen renders the software to be more safe and reliable. CalcNTCP is a simple, user-friendly and time-saving tool for the computation of NTCP. This software could be of potential application by assisting the clinicians in quick evaluation or optimization of the radiotherapy treatment plans.

**Khan HA **(2007) CalcNTCP: A simple tool for computation of normal tissue complication probability (NTCP) associated with cancer radiotherapy. *Int. J. Radiat. Biol.* 83 (10), 717-720.