Introduction: Nowadays, cancer is one of the most important concerns in human being's societies, more than one-third of people in around the world will get cancer during their lives. Physical dose depth distribution characteristic of protons in a tissue is determined with a low dose in the entrance region, maximum dose in the Bragg region, and rapid decline takes place near the end of their range. The ability to treat internal tumors, the ability of the broadening of the Bragg peak, and the small size of the particles are the other advantages of this approach which cause less damage to healthy tissues surrounding the tumor as compared to other therapy approaches such as radiation therapy with X-ray. At present, fast algorithm is generally used to obtain treatment plan (dose distribution in the patient).
Materials and Methods: At present, fast algorithm is generally used to obtain treatment plan (dose distribution in the patient). This means that at first absorbed dose in water phantom is calculated and then the necessary changes on the beam, equipment and location of the patient are applied, but this approach does not consider the dose resulting from radioactive decay in the tissue. Although radioactive products such as 3H(T1/2=12/32 a), 7Be(T1/2=53/3 d), 14C(T1/2=5730 a), and 22Na(T1/2=2/6 a) which are produced by interaction between protons and tissues' constituent elements have high longevity, they are produced in small quantities, but nuclei that decay to the ground state with positron emission and have low longevity time such as11 C(T1/2=20.3 min), 13N(T1/2=9.96 min), and 15O(T1/2=2.03 min) are produced in amount that calculation of their dose is an integral part of plan treatment. In this study, absorbed dose of protons and secondary particles such as neutrons and positrons in a head phantom have been calculated using the MCNPX 2.6 simulation code. The rate of the production of positron-emitter elements in different interactions have been studied.
Results: Result shows these short-longevity radioactive products reach their maximum amount along the proton path and particularly in Bragg region. In addition, a portion of produced photons from the annihilation of positronium atoms are absorbed by tissues and may cause unwanted dose to be applied to the surrounding treated tissues. Although, their produced dose is low, but they are not negligible and their amount should be calculated on the treatment plan. And amoung them 11C has maximum reaction rate and its production distribution has good agreement to proton dose distribution.
 
Conclusion: this particles not only are important because of their extera dose but also they are important because of they can be used during online PET scan just after proton therapy to monitor the progress of treatment