Background: Photoneutrons are produced during the radiotherapy treatment, when high energy X rays interacts with structures of the head of the linear accelerator (linac). The present day TPS are not taking into account the photo-neutron dose and the biological effects associated with it. The late induction of cancer and recurrence of the disease in old cancer patients are being frequently reported. Materials and Methods: Patients undergoing radiotherapy treatment with 15 MV X rays from a Siemens Primus Plus linear accelerator was considered for the study. In the present work, photoneutron spectrum from the linac head is measured using CR 39 SSNTD and the corresponding dose is calculated using Geant4. The composite photoneutron spectrum from the linac head and the corresponding dose was calculated using the kerma evaluation method in a human equivalent tissue phantom. The repeated calculation outcomes and the covariance error analysis in the nuclear data give consistency and an accuracy of 2 % in the results. Results: The result shows that significant amount of photoneutron dose was deposited during radiotherapy treatment when high energy X rays are used. The photoneutron production from the patient itself is yet another major issue which will cause out off field dose. Conclusion: This work gives importance in considering the photoneutron dose during radiotherapy planning and protection. This extra dose might be a factor that contributes to the induction of cancer and also to the recurrence of cancer to previously cured patients.