(P 92) DOSIMETRIC CHARACTERISTICS, OUTCOMES AND PROGNOSTIC FACTORS OF PATIENTS WITH EXTRACRANIAL CHORDOMAS AND CHONDROSARCOMAS TREATED WITH HIGH DOSE RADIOTHERAPY: A LARGE SINGLE INSTITUTION COHORT

Objective: Chordomas (Ch) and chondrosarcomas (CS) are radio-resistant tumours requiring high dose radiotherapy (RT) for local control. Most published studies describe prescribed RT doses which could be different to received doses in the final RT plan due to compromises required for critical organs and implants.

Methods: We retrospectively analysed 71 adult patients from University College London Hospitals who were treated with high-dose (>45Gy EqD2) proton or photon RT for extracranial Ch and CS between 2002 and October 2017. Electronic medical records were reviewed and RT data, including prescribed and received doses, were extracted. Patients with extra-skeletal mesenchymal CS and skull base tumours were excluded. This study was approved as a service evaluation with the local radiotherapy governance directorate at UCLH.

Median age was 59 years (range 22-85 years). Median follow-up duration was 42 months (range 4-121 months). RT modalities used were protons (PBT) only (31.0%), mixed PBT/Intensity Modulated RT (IMRT) (12.7%), IMRT (23.9%) and 3D-Conformal RT (3D-CRT) (32.4%). More patients received PBT, including mixed plans, for Ch (50%, n=31) compared with CS (34.5%, n=10). 68 patients (95.8%) received RT to the primary site of disease, of which 75% (n=51) were treated at the initial presentation and 25% (n=17) at recurrence. 43.7% (n=31) of patients had macroscopic residual disease, and 18.3% (n=13) had metastases at the time of RT. Sites treated included: spine (46.5%), sacrum (40.9%), extremity (5.6%), paraspinal (4.2%) and pelvis (2.8%). Further demographics are detailed in Table 1.

40 patients had two RT treatment dose levels prescribed, split into PTV low dose (LD) and high dose (HD) volumes. Otherwise, patients were considered to have PTV-HD level only.

Results: 30 (42.3%) local failures at 2-57 months were observed; 16 (55.2%) within the CS cohort and 14 (33.3%) within the Ch cohort. The 4-year local control (LC) rate was 47.2%; CS 37.5% and Ch 54.5%.

On univariate analysis, LC was significantly worse (p≤0.05) for: CS histology, poorly differentiated/de-differentiated subtype, tumour diameter 5cm, pelvic anatomical site, local recurrence prior to RT and presence of macroscopic disease at time of RT. There was a trend towards improved LC if received PTV-HD D95% >65Gy was achieved (Fig 1).

Overall survival (OS) and distant progression free survival (DPFS) analyses were performed for the non-metastatic cohort (n=58). 4-year OS was 66.8%, with CS at 57.4% and Ch at 72.1%. 4-year estimated DPFS was 64.6% and 80.8% for CS and Ch, respectively. OS was significantly worse if local (p < 0.001) or distant (p < 0.002) recurrence occurred post RT.

RT was well tolerated, with 4 patients (5.6%) experiencing >Grade 2 acute and late toxicity.

Median PTV-HD volume was 516.4cc (range 63.5-7849.9cc) for CS and 456.6cc (65.3-3025.6cc) for Ch. Received dose PTV-HD V95% was 94.9% for CS and 94% for Ch. Median prescribed and received PTV-HD D95% were similar for most modalities, except the mixed PBT/IMRT group (11.6Gy difference) (Fig 2). High PTV-HD D95% doses were achieved for IMRT plans in the Ch group (66.7Gy) but not in the CS group (44.1Gy) (Table 1). This is likely due to the anatomical distribution differences observed; all extremity sites were in the CS group.

Larger PTV-HD volume and lower received PTV-HD D95% were associated with poorer LC when assessed as continuous variables. Similar LC was observed for patients treated with IMRT or PBT only modality but a trend towards worse LC was observed with 3D-CRT and mixed PBT/IMRT technique (Fig 3). This could be explained by choice of mixed plans for patients with metalwork in situ.

Conclusion: High dose RT for Ch and CS was well tolerated and effective. Similar LC was observed with IMRT or PBT-only techniques. High dose dosimetry to PTV-HD volumes were achieved for Ch with IMRT plans, suggesting that advanced photon RT may remain an appropriate treatment option for patients unable to access PBT.