Abstract Book

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ESTRO 37

statistically significant level for the Eyes , Lenses , Mandible and Parotids, while CTV dose coverage remained similar: 97.7% (standard deviation SD:2.1) vs. 97.6% (SD:2.2%) for PTV and ROB respectively. Differences remained advantageous with ROB for the delivery simulations for all but SpinalCord . 18/20 plans and all simulated deliveries showed adequate target coverage (Figure 1). Table: Comparison of organs at risk average (standard deviation) dose parameters between the PTV and ROB. Bold values = (p<0.05)

experience with stereotactic ablative radiotherapy (SABR), with typical doses of 55Gy/5#, has generated toxicity data, used to guide the development of dose constraints for SYSTEMS-2. Material and Methods UK consensus guidelines outlining normal tissue dose constraints for SABR were used to determine maximum doses to OARs. To generate comparative data across different radiotherapy schedules, maximum doses were converted into equivalent doses in 2Gy per fraction (EQD2) for 55Gy/5# and 36Gy/6#. Constraints for a 6Gy/# regime could subsequently be calculated. Where there were concerns about acute toxicity, OAR constraints did not exceed the EQD2 of the SABR regime. Where organ delineation was required for inverse planning purposes only, the maximum dose was 36Gy/6#. Feasibility of the constraints was assessed in re-planning studies using 5 patients from the original SYSTEMS study, selected for the close proximity of their treatment volumes to critical OARs. Results Constraints are quoted either as maximum dose (Dmax) to 0.5cc or mean organ dose (Table 1). Contralateral lung doses correspond to the volume receiving 5, 10 and 20Gy. Planning studies confirmed that the constraints were completely achievable in 3 patients. V5 was unattainable in 1 patient whilst oesophageal and bronchial doses were not achieved in 1 patient with bulky para-oesophageal disease. Table 1. Achieved OAR doses for 5 patients re-planned to 36Gy/6# from the SYSTEMS study

Pati ent 2 6.6G y 13.7 Gy 22.4 Gy 19.1 Gy 13.4 Gy

Pati ent 3 21.9 Gy 15.2 Gy 26.7 Gy 22.6 Gy 21.4 Gy 73.6 %

Pati ent 4 14.2 Gy 19.1 Gy 35.7 Gy 33.6 Gy

Pati ent 5 26.9 Gy 32.3 Gy 32.3 Gy 36.8 Gy

Constr aint

Do se Patient 1

Organ

Spinal canal

Dmax (0.5cc) Dmax (0.5cc) Dmax (0.5cc)

27 Gy 27Gy

Oesopha gus

30 Gy 36 Gy 36 Gy 36 Gy

30Gy

Figure1: Average CTV dose coverage for the plans, landmark (LM) and eNAL based delivery simulations. Conclusion Treatment planning using robust optimization approach results in significant OAR dose reductions for bilateral irradiation using SIB-VMAT for head and neck patients. With similar and adequate target coverage, the gained OAR dose reduction seems to be maintained for the treatment delivery using LM or eNAL verification strategies. OC-0620 Dose constraints for hypofractionated, dose escalated radiotherapy in malignant pleural mesothelioma M. Ashton 1 , N. O'Rourke 1 , S. Harrow 1 , S. Currie 2 , D. Kearns 2 , R. Valentine 2 1 Beatson West of Scotland Cancer Centre, Clinical Oncology, Glasgow, United Kingdom 2 Beatson West of Scotland Cancer Centre, Radiotherapy Planning, Glasgow, United Kingdom Purpose or Objective SYSTEMS-2 is a multicentre, phase II, randomised trial of radiotherapy dose escalation for pain control in malignant pleural mesothelioma (MPM), in which a hypofractionated, dose escalated regime (36Gy/6#), is being compared with the standard dose (20Gy/5#). To achieve dose escalation with acceptable toxicity, advanced methods of radiotherapy planning and delivery (e.g. intensity modulated radiotherapy: IMRT) are being employed to spare organs at risk (OARs). The generation of dose constraints for OARs was prioritised because inverse planning for IMRT is associated with a risk of excessive doses to organs without specified limits. Limited data exists on normal tissue tolerances for hypofractionated regimes. However, accumulating

Heart

18.9Gy

22.9Gy

Bronchus Dmax (0.5cc)

Great vessels

Dmax (0.5cc)

26.2Gy

36Gy 27.4 Gy

Contrala teral lung

67.1 %

21.5 %

<70% V5 67.2% 13.4 %

2.8% 7.4% 0.1%

<50% V1 0

10.3% 0

<10% V2

0

0

0

0

0 0

Mean dose

16 Gy 0.2Gy 2.5G y

0.4G y

1.6G y 14.1 Gy 0.7G y

5.3G y 10.1 Gy 1.5G y

Liver

30 Gy 30 Gy

Stomach Dmax (0.5cc)

0.4Gy 14Gy 1.8G y

Small bowel

Dmax (0.5cc)

0.8G y

0.3Gy 6.4G y

Conclusion The dose constraints are being used to guide the planning and safe delivery of dose escalated hypofractionated radiotherapy in the SYSTEMS-2 study (Figure 1) which is currently recruiting across the UK.