ESTRO 36 Abstract Book

S507

ESTRO 36

_______________________________________________________________________________________________

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

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.

PO-0916 Feasibility and potential for treating loca lly

advanced non-sma ll cell lung cancer with a MR-linac

M.J. Menten

, H. Bainbridge

, M.F. Fast

, S . Nill

, F.

McDonald

, U. Oelfke

The Institute of Cancer Research and The Royal Marsden

NHS Foundation Trust, Physics, Sutton, United Kingdom

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

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

(

=0.003).