A procedure for calculation of monitor units for passively scattered proton radiotherapy beams Narayan Sahoo, a X. Ronald Zhu, Bijan Arjomandy, George Ciangaru, MingFwu Lii, Richard Amos, Richard Wu, and Michael T. Gillin Department of Radiation Physics, UT MD Anderson Cancer Center, 1515 Holcombe Boulevard, Box 1150, Houston, Texas 77030 Received 11 February 2008; revised 8 September 2008; accepted for publication 9 September 2008; published 21 October 2008 The purpose of this study is to validate a monitor unit MU calculation procedure for passively scattered proton therapy beams. The output dose per MU d/MU of a therapeutic radiation beam is traditionally calibrated under specific reference conditions. These conditions include beam energy, field size, suitable depth in water or water equivalent phantom in a low dose gradient region with known relative depth dose, and source to point of calibration distance. Treatment field settings usually differ from these reference conditions leading to a different d/MU that needs to be deter- mined for delivering the prescribed dose. For passively scattered proton beams, the proton specific parameters, which need to be defined, are related to the energy, lateral scatterers, range modulating wheel, spread out Bragg peak SOBP width, thickness of any range shifter, the depth dose value relative to the normalization point in the SOBP, and scatter both from the range compensator and inhomogeneity in the patient. Following the custom for photons or electrons, a set of proton dosimetry factors, representing the changes in the d/MU relative to a reference condition, can be defined as the relative output factor ROF, SOBP factor SOBPF, range shifter factor RSF, SOBP off-center factor SOBPOCF, off-center ratio OCR, inverse square factor ISF, field size factor FSF, and compensator and patient scatter factor CPSF. The ROF, SOBPF, and RSF are the major contributors to the d/MU and were measured using an ion chamber in water tank during the clinical commissioning of each beam to create a dosimetry beam data table to be used for calculating the monitor units. The following simple formula is found to provide an independent method to determine the d/MU at the point of interest POI in the patient, namely, d / MU =ROF·SOBPF·RSF·SOBPOCF·OCR·FSF·ISF·CPSF. The monitor units for delivering the in- tended dose D to the POI can be obtained from MU= D ÷ d / MU. The accuracy and robustness of the above formula were validated by calculating the d/MU in water for many different combi- nations of beam parameters and comparing it with the corresponding measured d/MU by an ion chamber in a water or water/plastic phantom. This procedure has been in use for MU calculation for patient treatment fields at our facility since May 2006. The differences in the calculated and measured values of the d/MU for 623 distinct fields used for patient treatment during the period of May 2006 to February 2007 are within 2% for 99% of these fields. The authors conclude that an intuitive formula similar to the one used for monitor unit calculation of therapeutic photon beams can be used to compute the monitor units of passively scattered proton therapy beams. © 2008 American Association of Physicists in Medicine. DOI: 10.1118/1.2992055 Key words: proton beam therapy, monitor units I. INTRODUCTION Determination of monitor units MU for the delivery of pre- scribed dose D is an essential dosimetry task in external beam radiation therapy. The formalism or procedure for calculating the MU for photon and electron beams is well established, but is less well established for heavy particle beam therapy like proton therapy. 1,2 It is desirable to devise an MU calculation formula, based on well-understood do- simetry principles and measurements, which can be used in many of the frequently used treatment conditions. This con- servative approach is essential for any new treatment deliv- ery system. The number of different treatment conditions for the passively scattered proton beams at the University of Texas MD Anderson Cancer Center Proton Therapy Facility in Houston PTC-H is large. In passively scattered beams, a double scattering system consisting of a first and a second scatterer is used to spread the protons laterally. Each beam- line has three different snouts, namely, small, medium, and large, to hold apertures and range compensators of sizes up to 10 cm  10 cm, 18 cm  18 cm, and 25 cm  25 cm, re- spectively. Each beamline has options for eight discrete en- ergy selections, namely 100, 120, 140, 160, 180, 200, 225, and 250 MeV. For each of the above energy and maximum field size combination, a specific range modulating wheel RMW is used to spread the Bragg peak longitudinally. This results in 24 RMWs per beamline. Different second scatters are designed to be used with the three maximum field size choices and certain energy ranges. Each of the 24 RMWs is 5088 5088 Med. Phys. 35 „11…, November 2008 0094-2405/2008/35„11…/5088/10/$23.00 © 2008 Am. Assoc. Phys. Med.