ESTRO 36 Abstract Book

S503 ESTRO 36 2017 _______________________________________________________________________________________________

PO-0916 Feasibility and potential for treating loca lly advanced non-sma ll cell lung cancer with a MR-linac M.J. Menten 1 , H. Bainbridge 2 , M.F. Fast 1 , S . Nill 1 , F. McDonald 2 , U. Oelfke 1 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Physics, Sutton, United Kingdom 2 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Lung Unit, Sutton, United Kingdom Purpose or Objective Treatment plans for MRI-guided radiotherapy delivered with an MR-linac vary from those designed for conventional linacs due to differing technical specifications of dose delivering systems and the presence of a static magnetic field. This study investigated this issue for radiotherapy of locally advanced non-small cell lung cancer (LA NSCLC) by comparing treatment plans for a conventional Versa HD linac (Elekta AB, Stockholm, Sweden) and the Elekta 1.5 T MR-linac. Furthermore, the effect of reducing planning target volume (PTV) margins on the MR-linac was examined. Material and Methods Ten patients with LA NSCLC were retrospectively re- planned six times using the Monaco treatment planning system, research version 5.19.00. Three plans were designed according to our institution’s protocol for conventionally fractionated treatment (55 Gy/ 20 fractions) and three plans following guidelines for isotoxic dose escalation up to 79.2 Gy/ 44 fractions (NCT01836692). In each case, two plans were designed for the MR-linac, using IMRT with nine equidistant, coplanar beams, either with standard (7 mm) or reduced (3 mm) PTV margins, while one plan was created for a conventional linac using VMAT with standard margins. Treatment plan optimization and dose calculation were conducted under consideration of magnetic field effects. Potential to escalate tumour dose was quantified for the isotoxic plans, and differences in dose-volume metrics were analysed for conventionally fractionated treatment plans. Statistical significance was evaluated using a paired t- test after confirming normal distribution and correcting for multiple endpoints. Results All generated treatment plans fulfilled their respective planning constraints and would have been clinically acceptable. With the conventionally fractionated schedule small differences in dose-volume metrics could be identified with statistical significance (see table). Mean lung doses were similar between conventional and MR- linac plans, whereas high lung doses were reduced and low lung doses increased on the MR-linac (graphically illustrated in the figure). In terms of dose-escalation, the mean achievable doses were 75.4, 74.0, and 76.9 Gy for Versa HD, MR-linac (standard margins) and MR-linac (reduced margins) respectively, with inferiority of the standard margin MR-linac plans versus the Versa HD plans ( p =0.003).

Conclusion It is feasible to generate conventionally fractionated treatment plans for LA NSCLC patients on a 1.5 T MR-linac with minor differences in dose-volume metrics, which are unlikely to be clinically meaningful. When using standard PTV margins, isotoxic dose escalation was limited on the MR-linac. However, reducing margins alleviates these observed effects. This study only represents an early indicator of the treatment implications of MRI-guided radiotherapy. It is conceivable that the availability of MRI- guidance will result in further benefits through inter- and intrafractional treatment adaptation. PO-0917 Nationwide audit of small fields output calculations in Poland W. Bulski 1 , K. Chelminski 1 1 The Maria Sklodowska-Curie Memorial Cancer Center, Medical Physics Department, Warsaw, Poland Purpose or Objective The delivery of accurate intensity-modulated radiation therapy (IMRT) or stereotactic radiotherapy depends on a multitude of steps in the treatment delivery process. Within the treatment planning system’s (TPS) dose calculation algorithm, various unique small field dosimetry parameters are essential, such as multileaf collimator modeling and field size dependence of the output. One of the most considerable challenges in this process is to determine accurate small field size output factors. Modern radiotherapy routinely involve s the use of small radiation fields as components of IMRT. Because of the difficulties in commissioning small field data, a set of field size dependent output factors could prove to be an invaluable tool to confirm the validity of an individual institution’s dosimetry parameters. Such a set of data has been prepared by the Radiological Physics Center (RPC), M. D. Anderson Cancer Center, Houston. The RPC has gathered multiple small field size output factor datasets for X-ray beam qualities, ranging from 6 to 18 MV, from Varian, Siemens and Elekta linear accelerators. These datasets were measured at 10 cm depth and ranged from 10×10 cm 2 to 2×2 cm 2 . Within the framework of the IAEA CRP E2.40.16 project "Development of Quality Audits for Radiotherapy Dosimetry for Complex Treatment Techniques, a methodology of the audit of small field output performance was established. Material and Methods The participants had to calculate t he output factors for the beams formed by the multi-leaf collim ator (MLC). The results of their calculations were compared with the reference RPC data. 32 Polish radiotherapy departments took part in the audit. In total, 65 beams were audited. The participants of the audit were asked to calculate the number of monitor units (MU) for the delivery of a prescribed dose to water with square fields of different sizes. A dose of 10 Gy was prescribed to a reference point at 10 cm depth on the central axis, at 100 cm source-to- phantom distance (SFD). The output factors for five field sizes, 10×10, 6×6, 4×4, 3×3 and 2×2 cm 2 , shaped by a multileaf collimator (MLC), were calculated. Results