The impact of proton LET/RBE modeling and robustness analysis on base-of-skull and pediatric craniopharyngioma proton plans relative to VMAT

Purpose: Pediatric craniopharyngioma, adult base-of-skull sarcoma and chordoma cases are all regarded as priority candidates for proton therapy. In this study, a dosimetric comparison between volumetric modulated arc therapy (VMAT) and intensity modulated proton therapy (IMPT) was first performed. We then investigated the impact of physical and biological uncertainties. We assessed whether IMPT plans remained dosimetrically superior when such uncertainty estimates were considered, especially with regards to sparing organs at risk (OARs).Methodology: We studied 10 cases: four chondrosarcoma, two chordoma and four pediatric craniopharyngioma. VMAT and IMPT plans were created according to modality-specific protocols. For IMPT, we considered (i) variable RBE modeling using the McNamara model for different values of (α/β)x, and (ii) robustness analysis with ±3 mm set-up and 3.5% range uncertainties.Results: When comparing the VMAT and IMPT plans, the dosimetric advantages of IMPT were clear: IMPT led to reduced integral dose and, typically, improved CTV coverage given our OAR constraints. When physical robustness analysis was performed for IMPT, some uncertainty scenarios worsened the CTV coverage but not usually beyond that achieved by VMAT. Certain scenarios caused OAR constraints to be exceeded, particularly for the brainstem and optical chiasm. However, variable RBE modeling predicted even more substantial hotspots, especially for low values of (α/β)x. Variable RBE modeling often prompted dose constraints to be exceeded for critical structures.Conclusion: For base-of-skull and pediatric craniopharyngioma cases, both physical and biological robustness analyses should be considered for IMPT: these analyses can substantially affect the sparing of OARs and comparisons against VMAT. All proton RBE modeling is subject to high levels of uncertainty, but the clinical community should remain cognizant possible RBE effects. Careful clinical and imaging follow-up, plus further research on end-of-range RBE mitigation strategies such as LET optimization, should be prioritized for these cohorts of proton patients.

The impact of route of diagnosis on survival in patients with glioblastoma

INTRODUCTION

The standard of care for glioblastoma is maximal debulking surgery followed by chemo-radiotherapy (CRT). Published data show worse outcomes for patients who present with GBM as an emergency. This study investigates prognostic factors in a cohort of GBM patients treated with postoperative CRT, and compares outcomes in patients who present via emergency pathways with those who present through outpatient clinics.

METHODS

Patients with GBM operated on between 1 April 2010 and 5 October 2015 and then treated with postoperative CRT were included in the study. Data were collected from electronic patient records and radiotherapy planning systems. Survival data were censored on 22 March 2016. Univariate and multivariate analyses of prognostic factors were performed.

RESULTS

104 patients were studied; mean age 51.6 years (range 19 to 70 years). Median overall survival (OS) was 16.5 months, with 68.2% and 37.8% alive at 12 and 24 months respectively. On multivariate analysis, improved OS was associated with ECOG Performance Status of 0 (vs ≥1; p = .012), patient age <60 years (vs ≥60 years; p < .001), and surgical debulking or macroscopic complete resection (vs biopsy; p < .001). Patients who presented through emergency medical pathways had worse survival (p = .005).

CONCLUSION

This study supports published data that initial presentation through emergency pathways is associated with worse outcomes in GBM, even in patients who remain fit enough to receive post-operative CRT.