ESTRO 36 Abstract Book

S570 ESTRO 36 2017 _______________________________________________________________________________________________

radiotherapy treatment position with immobilisation devices, and had subsequently received neck radiotherapy following chemotherapy. CTVs were delineated according to the principles of modern ISRT guidelines. A CTV INRT (involved node radiotherapy) was delineated following coregistration of the radiotherapy position PET- CT to the planning CT scan. A CTV diagPET was delineated by side-by-side assessment of the diagnostic PET-CT; no additional CTV expansion was made to account for uncertainties introduced by the absence of optimal pre- chemotherapy imaging. CTV INRT and CTV diagPET were compared using multiple positional metrics. Repeat coregistrations and delineations were undertaken for 3 patients to determine the effect of intra-observer variation. Figure 1 shows the variation in CTV when using the diagnostic and radiotherapy position PET-CT data respectively during the delineation process.

coverage was 2.6mm (range 1-4.8) and 7.4mm (range 1.5- 14.3) respectively. Conclusion In the absence of treatment-position PET-CT, CTV expansion cranially and caudally by 10mm and 18mm respectively, along with generous contouring in the axial plane, was required to encompass pre-chemotherapy disease. EP-1040 Identifying risk factors for L’Hermitte’s syndrome after chemo-IMRT for head and neck cancer H. Laidley 1 , D. Noble 2 , G. Barnett 2 , R. Jena 2 , N. Burnet 2 1 Cambridge University Hospitals, School of Clinical Medicine, Cambridge, United Kingdom 2 Cambridge University Hospitals, Oncology, Cambridge, United Kingdom Purpose or Objective Studies suggest that L’Hermitte’s syndrome (LS) after chemo-radiotherapy for head and neck cancer patients is related to higher spinal cord doses and younger age. IMRT plans limit spinal cord dose, but the incidence of LS remains high. We aimed to identify other risk factors. Material and Methods 128 patients treated with TomoTherapy™ between 2008 and 2015 prospectively completed a side-effect questionnaire 3, 6 and 12 months post-treatment. 45 (35.2%) reported typical LS symptoms (consistent with Grade 1 CTCAE v4.03 myelitis) at least once, and graded severity of tingling/electric shock sensations down their spine. Data on age, diabetes, hypertension, concurrent systemic therapy, and unilateral versus bilateral neck irradiation (UNI vs BNI) were collected. Radiotherapy plans were assessed to compare maximum dose, mean dose, and absolute and partial volumes receiving 10, 20, 30 and 40 Gy to the spinal cord in LS and non-LS patients. Univariate analyses of baseline parameters against LS incidence were assessed with Fisher’s exact test and student’s t-test. Box and whisker plots were used to inspect dosimetric parameters against LS incidence. Variables reaching or trending towards significance were included in a binary logistic regression model. Results The only significant variable on univariate analysis was diabetes (p = 0.032). 13 patients in our cohort were diabetic (9 on metformin); only 1 developed LS (OR = 0.13). Concurrent weekly cisplatin did not increase LS risk; 23/61 (38%) patients receiving cisplatin developed LS, compared with 22/67 (33%) who did not (p = 0.58). V 40Gy was the only dose parameter showing a difference between LS and non-LS patients, so others were excluded from logistic regression to prevent co-linearity. The binary logistic regression showed that higher absolute volume receiving 40 Gy (V 40Gy ) was significant (p = 0.037, OR = 1.55), despite only 29% of patients with LS, and 28% of patients without LS receiving any dose to the spinal cord over 40 Gy. There was also a trend for LS patients to be slightly younger (mean age 56.3 vs 59.4, p = 0.074), and a protective effect of diabetes was again seen (p = 0.035). Patients receiving UNI (p = 0.015, OR = 2.82) were more likely to develop LS; 42% of LS patients received UNI, compared to 25% of non-LS patients.

Figure 1: Comparison of CTV INRT . A) represents pre-chemotherapy FDG PET-CT acquired in the radiotherapy treatment-position, B) represents routine diagnostic pre-chemotherapy PET-CT with arms up, C-F) planning CT scan with CTVINRT (blue)(contoured using co- registered pre-chemotherapy radiotherapy treatment- position PET-CT) and CTVdiagPET (red) (contoured using side-by-side assessment of diagnostic PET-CT) in the coronal plane (C), in the sagittal plane (D), in the axial plane at the inferior extent of the CTVs (E), in the axial plane at the superior extent of the CTVs (F). Results Intra-observer variability was limited, with delineation of CTV INRT highly reproducible and slightly lower for CTV diagPET (mean DICE 0.88 and 0.8 respectively). Superiorly, CTV diagPET varied by -10 to + 15mmfrom CTV INRT , with a mean difference of +0.5mm. Inferiorly, CTV diagPET varied by -18 to +6mm compared with CTV INRT , with a mean difference of +3.8mm. Comparing CTV INRT and CTV diagPET in the axial plane, the mean DICE was 0.74. Mean sensitivity index was 0.75 (range 0.56-0.91), showing that on average 75% of the CTV INRT was encompassed by the CTV diagPET . The average and maximum ‘mean distance to conformity’ (MDT) under- and CTV diagPET