[P236] Rapid Arc versus a new sectorial sliding window IMRT template in radiotherapy brain treatments: Lens sparing

The ICRP Publication 118 introduced new threshold doses for tissue reactions and other non cancer effects on lens:the new threshold doses resulted significantly lower than previous scientific evidences. Lenses are often involved in brain radiotherapy treatment volumes and the risk of cataratta becomes relevant. Our study compares the results obtained from treatment plans calculated with both Rapid Arc technique and a new SSW IMRT template, for each patient recruited in the study, in terms of lens maximum absorbed doses. 10 patients with brain tumours diagnosis (PTV medium size af about 90 cc) localized in different areas were treated with the new SSW IMRT template, that consists in five fields spreading over a The dose prescription for all patients was 60 Gy in 2 Gy daily fractions. On the same patients, Rapid Arc plans were conformed with equal optimization objectives. The target coverage was at least 97% of prescription dose to 95 % of volume in both calculated plans. All the patiens treated with the SSW IMRT technique received a lower maximum omolateral lens dose (range from 0.4 to 2.4 Gy; mean value 1.7 Gy) compared to doses obtained with Rapid Arc plans (range from 0.5 to 4.4 Gy; mean value 2.4 Gy). Relatively to contralateral dose lens, we obtained a mean value of max dose of 1.5 Gy (range 0.4–2.2 Gy) for SSW IMRT plans and 2.2 Gy (range 0.5–4.6 Gy) for Rapid Arc plans. Patients treated with the new SSW IMRT template showed a lens dose sparing of about the 28% compared to doses they would have received with Rapid Arc plans.The other OAR involved as chiasm, optical nerve and brainstem are widely lower than the corresponding constraints. The new SSW IMRT template could be applied to treat target with different size, shape and anatomical position. Furthermore, the lens maximum dose reduction could potentially reduce the risk of cataract.