ESTRO 37 Abstract book

S1046

ESTRO 37

D50%, D2%, and the conformity and homogeneity indices (CI, HI respectively) were used for target comparison, while for OARs Dmean and Dmax were used. V5Gy, V10Gy and V70Gy were used for skin dose evaluation. A paired T test, was used to compare the dosimetric results obtained for the two groups (TP vs NP). Results A total of 96 treatments planning over the TP and NP groups were analysed. No statistical difference (p>0.05) was found for target volume D 95%, D 98% ,D 50% , D 2% , HI, CI. For what is concerning the OARs, following the finding we can state that, TOMO treatments planning resulted (P<0.05) in lower dose for brainstem (Dmax), spinal cord (Dmax), left and right parotides (D33) and oropharynx (Dmean) and higher Dmax dose for lens, optic nerve, optic chiasma and pituitary. Nevertheless, the neck skin dose was sensibly higher in TOMO treatments (Figure 1). For the three technologies a reduction of the skin dose is obtained in the NP group as reported in Table 1. We can highlight that if the skin of the neck was contoured as a sensitive structure for dose optimization, the volume of skin that received >40 Gy was further reduced by at least 29% (TOMO) or more (dIMRT and VMAT).

knowledge, based on a large library of treatment plans. Corollary is to reduce the planning time as minimum as reasonably achievable. Material and Methods An ensemble of 120 VMAT based cranial stereotactic plans were categorised on basis of following eight parameters ,PTV dose coverage OAR challenged or unchallenged, prescription dose, number of PTVs, laterality (left /right), tumour volume, shortest distance between OAR-PTV, centre to centre distance between OARs-PTV, lateral dimension of brain (fig-1). Performance of ensemble mapping technique was validated in existing library plan. Further for new patient a knowledge based planning (KBP) was created by coping most appropriate library plan with all parameters unchanged; optimization and dose calculation was carried out with no or very minimal changes in the constraint. Another independent treatment plan (IP) using the same arc configuration was generated by an experience dosimetrist and compared with KBP for PTV_V98%,conformity index (PCI),50% and 20% dose spillage (I50%, I20%) and OAR doses. Results Validation result for ensemble mapping technique shows that for an OAR challenged PTV dose patient it is appropriately picking up the library plan. However for patent having an OAR unchallenged PTV it is not very accurate and may pick up a nearby plan as well. Independent plan (IP) was better than the knowledge based plans (KBP) in PTV coverage and dose conformity. PTV volume receiving 98%dose (V98%) was 98.7±1.1% and 97.5±1.3% for IP and KBP respectively. For OAR challenged PTV’s conformity index was slightly high in IP (0.712) than KB plans (0.693). If collimator angle optimization is not done for the OAR unchallenged PTV’s PCI is higher than KBP and variation was statistically significant (p<0.04). For the largest prescription dose group (12Gy in 1#, 64 patients) brainstem 0.5 cc Volume exhibit a mean dose of 873.1±134.2 cGy and 854.5±122.4 cGy for IP and KBP respectively. Mean 0.2 cc optic chiasma dose were 734.0±67.8 cGy and 690.1±78.3 cGy for KBP and IP respectively. MU difference was very nominal with IP shows a mean excess MU of 3.7% over KBP. IP and KBP requires on 3.5 and 1.5 runs respectively costing about 3.5-5 and 1.5-2 hrs. Conclusion KBP plans validation result indicate multidimensional ensemble mapping mechanism can pick up the library plan accurately. KBP plans, although marginally inferior in the dosimetric quality, they fulfil all the required clinical condition and dose constraints. KBS plans save a considerable planning time and almost independent of the treatment planner skill and knowledge. KBP works well with Monte Carlo planning system like MONACO. EP-1923 Integration of microscopic spread and geometric uncertainties into a single target volume expansion E. Sterpin 1 , K. Haustermans 1 , M. Lambrecht 1 , X. Geets 2 , T. Mackie 3 , V. Gregoire 2 1 Katholieke Universiteit Leuven, Oncology, Leuven, Belgium 2 Universite catholique de Louvain, Molecular Imaging- Radiotherapy and Oncology, Brussels, Belgium 3 University of Wisconsin, Medical Physics, Madison, USA Purpose or Objective In external beam radiotherapy, the definition of several target volumes follows a conventional GTV-CTV-PTV margin expansion formalism. The magnitude of the GTV to CTV expansion is based on histology studies or patterns of recurrence over patient populations and typically determined by a clinician. The CTV volume is further corrected manually to exclude zones in which tumor expansion is made impossible due to anatomical barriers.

Conclusion The effect of skin injury for early-stage nasopharyngeal carcinoma treated with dIMRT and VMAT is comparable while it worsens for TOMO treatments. By taking into consideration the skin as a sensitive structure during inverse planning, it was possible to reduce the skin dose to a tolerable level without compromising tumor target coverage. EP-1922 Standardisation of VMAT frameless SRS/SRT planning using ensemble mapped knowledge based planning. B. Sarkar 1 , T. Ganesh 1 , A. Manikandan 2 , A. Munshi 1 , B.K. Mohanti 1 1 Fortis Memorial Research Institute, Radiation Oncology, Gurgaon, India 2 Nagarjuna Hospital, Radiation Oncology, Vijayvada, India Purpose or Objective Knowledge based planning (KBP) is an emerging technique in a radiation therapy planning.This study attributed to standardisation of stereotactic treatment plan by ensemble matching technique. This study divided in two part, primary aim of this study is standardisation of the treatment plan that is minimization of the influence of individual treatment planner skill and