ESTRO 36 Abstract Book

S811 ESTRO 36 2017 _______________________________________________________________________________________________

evaluate the RP quality relative to the clinical plans (CP). Secondary, through normal tissue complication probability (NTCP) estimations, the possible effective clinical benefit in planning with RP is evaluated. Material and Methods 83 patients presenting AHNC were selected from the department database. The patients were chosen as their plans were considered as dosimetrically optimal. All plans were optimized for VMAT technique (RapidArc), with 2-4 arcs, 6 MV beam quality, treated on a department linac equipped with Millennium 120-MLC or HD-MLC. Inverse planning used the PRO optimizer, and final calculations were with AAA. Dose prescription was to 69.96 Gy and 54.45 Gy to PTV2 and PTV1, respectively, in 33 fractions. A RP model was generated for the OARs: spinal cord, brain stem, oral cavity, parotids, submanidbular glands, larynx, constrictor muscles, thyroid, eyes. To constrain the uninvolved healthy tissue, the ‘body’ with all the targets subtracted was included in the model. The optimization objectives in the model included the line objective for all OARs with generated priority. For serial organs, an upper objective was added with generated dose at 0% volume with a fixed priority of 90. For parotids and oral cavity, a mean objective was added with generated dose and fixed priority of 60. Targets upper and lower objectives were placed in a very narrow interval, with priority 110 and 120. The automatic Normal Tissue Objective NTO was added with priority 280. The model was validated on a set of 20 similar patients selected from the clinical database. The possible clinical benefit was evaluated through NTCP estimation for some of the OARs, using the biological evaluation availabile in Eclipse, based on LQ-Poisson Regarding target dose homogeneity, the standard deviation was reduced by 0.3 Gy with RP (p<0.05). The mean doses to parotids, oral cavity, and larynx were reduced with RP of 2.1, 5.2, and 7.0 Gy, respectively. Maximum doses to spinal cord and brain stem were reduced of 7.0, and 6.9 Gy, respectively (p<0.02). NTCP reductions of 11%, 16%, and 13% were estimated for parotids, oral cavity, and larynx, respectively, with RP planning. Conclusion Model validation confirmed the better plan quality with RP plans. NTCP estimation suggests that this dosimetric effect could positively affect also the toxicity profiles for patients receiving RP planning with an adequate model. EP-1529 Reducing total Monitor Units in RapidArc™ plans for prostate cancer K. Armoogum 1 , M. Hadjicosti 1 1 Derby Hospitals NHS Trust, Department of Radiotherapy, Derby, United Kingdom Purpose or Objective A retrospective planning study was performed on prostate cancer RapidArc (RA) plans to evaluate the use of the ‘MU Objective’ optimization tool in Varian Eclipse (v 13.6) incorporating the Photon Optimizer algorithm (v 13.6.23). The RA approach currently used in this study implements two complete arcs to deliver at least 95% of the prescribed dose to the Planning Target Volume (PTV) while minimizing dose to the surrounding Organs at Risk (OAR). In general, RA tends to use fewer MUs per treatment fraction than Intensity Modulated Radiation Therapy (IMRT) with an associated reduction in the risk of secondary induced cancers. The MU Objective tool offers the possibility to further decrease total Monitor Units while maintaining clinically acceptable plan quality. Material and Methods Thirty clinically approved RA plans (prostate only n=22, prostate and nodes n=8) were selected for re-optimization using the MU Objective tool. This tool allows variation of the Minimum MU, Maximum MU and Strength (S). The ‘S’ model. Results

parameter weights the optimizer to reach the MU goal within the defined Min MU and Max MU limits. Based on a previous study [1], the Min MU was set to 0%, the Max MU to 50% of the total clinical plan MUs for the non-optimized RA plan and ‘S’ was set to the maximum value of 100. The prescribed doses were either 74Gy in 37 Fractions (or 60 in 20), collimator angles were 30 ⁰ and 330 ⁰ to minimize the tongue-and-groove effect, jaw tracking was enabled and all plans were treated at 6MV and 600 MU/minute maximum dose rate. The dose/volume objectives for the PTV and OAR were unchanged. Dose calculations were performed using the Anisotropic Analytic Algorithm (v 13.6.23) with a calculation grid size of 2.5 mm, taking into account inhomogeneity correction and disregarding air cavity correction. To determine the quality of the absorbed dose distributions resulting from smoothing, the Paddick Conformity Index (CI PAD ) and the International Commission on Radiation Units (ICRU) Homogeneity Index (HI) were calculated for all plans [2]. CI PAD = (TV PI ) 2 /(PI x TV) Where PI is the volume of the prescription isodose line (95%), TV PI is the target volume within the PI, and TV is the target volume. HI = (D 2% -D 98% )/D 50% Where D 50% is the dose received by 50% of the target volume and so on. Results The MU Objective tool resulted in a reduction of total prostate RA plan MUs by approximately 29%. The average ICRU HI for the prostate patients varied from 0.055 to 0.111 (σ = 0.015, CI: 0.07-0.08). The CI PAD varied overall from 0.617 to 0.860 (σ = 0.067, CI: 0.72-0.78).

Conclusion The MU Objective tool facilitates the reduction of total prostate RA plan MUs with PTV coverage and OAR sparing maintained. A lower total MU number should translate to lower leakage from the linear accelerator and less scatter within the patient. EP-1530 Validation of RayStation Fallback Planning dose-mimicking algorithm L. Bartolucci 1 , M. Robilliard 1 , S. Caneva- Losa 1 , A. Mazal 1 1 Institut Curie, Radiotherapy, Paris, France