ESTRO 36 Abstract Book

S507 ESTRO 36 _______________________________________________________________________________________________

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).

in-line MV EPID. The MPC acquires a series of MV images of an IsoCal phantom, capturing beam properties and mechanical data such as MLC and gantry accuracies. A new MPC test monitors output stability in terms of percent deviation from the baseline data of the actual measured beam. All measured data are automatically processed, analyzed, and displayed for evaluation, thus providing a reliable and fast method for routine machine performance assessment. Independent tests such as star-shots, Winston-Lutz, MLC picket fence patterns and output measurements on a daily basis were employed to benchmark the MPC test results for the prototype system. Results MPC results were collected daily for six months on both the prototype and a TB. The independent tests on the prototype system were repeated weekly to validate the MPC results. A sample comparison of the MPC results for the prototype against independent tests are shown in Table 1. The output stability of the prototype system, as measured with the MPC and a DailyQA TM 3 device, is comparable (Fig. 1), and within 0.5% of independent output measurement for the period shown. All tests performed were within the tolerances allowed by the MPC and agreed in most cases with the result of the independent tests. The prototype system performs as well as the TB system. A summary of MPC test results and comparisons with independent measurements will be shown alongside with the TB MPC results. Table 1

Figure 1

Conclusion The performance of a straight-through linac d elivering 6X- FFF (filter-free) beam with an EPID panel wa s investigated with the MPC testing tool and that method w as validated against independent tests for proof of con cept. MPC is a complete, reliable and quick test suite that monitors the performance of a treatment unit on routine basis.