Session Item

Saturday
August 28
14:15 - 15:30
Room 2.2
ESTRO-AAPM: A roadmap for the use of quantitative imaging in radiation oncology
James O'Connor, United Kingdom;
Marianne Aznar, United Kingdom
0390
Joint symposium
Physics
Inhomogeneous dose escalation in pancreatic SBRT:feasibility and impact of anatomical configuration
Mauro Loi, Italy
PO-1226

Abstract

Inhomogeneous dose escalation in pancreatic SBRT:feasibility and impact of anatomical configuration
Authors:

Mauro Loi1, Raffaela Doro2, Sara Lucidi1, Pierluigi Bonomo1, Gabriele Simontacchi1, Daniela Greto1, Andrea Allegra1, Vanessa Di Cataldo1, Giulio Francolini1, Ivano Bonucci2, Lorenzo Livi3, Laura Masi2

1Azienda Ospedaliero-Universitaria Careggi, Radiation Oncology Unit, Florence, Italy; 2IFCA, Department of Medical Physics, Radiation Oncology, Florence, Italy; 3Azienda Ospedaliera Universitaria Careggi, Radiation Oncology Unit, Florence, Italy

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Purpose or Objective

In non-resectable locally advanced pancreatic cancer, adjunction of Stereotactic Body Radiotherapy (SBRT) following chemotherapy has been proposed to improve outcome. However, prescription of more intensive schedules has been traditionally limited by risk of severe injury to nearby organs at risk (OARs), particularly in case of substantial overlap between planning volume and critical structures. To achieve higher tumor control probability, a biologically effective dose (BED) of  100 Gy10 , corresponding to a dose 50 Gy in 5 fractions of 10 Gy assuming α/β=10, is desirable.  The aim of our in-silico study is to assess the feasibility of dose escalation using inhomogeneous dose prescription for pancreatic SBRT and to determine which patients may be eligible for this strategy based on anatomical proximity between target volumes and OARS.

Material and Methods

Data from 14 locally advanced pancreatic cancer patients treated at our Institution were collected. For each patient, a CyberKnife (CK) Synchrony treatment plan was optimized for fiducial-guided pancreatic SBRT aiming at a planned dose of 50 Gy and 40 Gy in 5 fractions to the gross tumor volume (GTV) and the planning target volume (PTV), respectively. PTV was created by 5 mm GTV isotropic expansion. Acceptable target coverages were: 1) a dose of 50 Gy and 47.5 Gy to at least 90% and 95% of the GTV, and  2) a dose of 40 Gy to 95% of the PTV. Planned doses to the target regions and OARs (duodenum, stomach and bowels) were evaluated and statistically analyzed. For each plan, the intersection volume between the PTV and OARs expanded by 5 mm was defined as Expansion-Intersection Volume (EIV), as reported by Tomatis et al. 

Results

Median GTV and PTV volumes were 40.8 (range 22.3-205.3) cc and 73.7 (range 36.1-266.7) cc , respectively. V35 to duodenum, stomach and bowel was maintained below 0.5 cc in all cases. Median EIV was 12.9 (3.9-25.1) cc. Median V50 and V47.5 for GTV was 91.0% (range 82.4%-97.8%) and 96.8% (range 92.5%-99.9%), respectively: GTV coverage was acceptable in 10 out of 14 cases. Median V40Gy for PTV was 96.8% (range 90.0%-99.8%): PTV coverage was acceptable in 11 out of 14 cases. Spearman correlation showed a significant association between EIV and V47.5Gyfor GTV (rho -0,77228, p<0.001) and V40Gyfor PTV (rho -0,68352, p<0.001), respectively.


Conclusion

Inhomogenous dose escalated prescription using fiducial-based SBRT with Cyberknife respiratory tracking is a feasible strategy in selected patients with locally advanced pancreatic cancer.  EIV,  but GTV size, is significantly correlated with target coverage probability and may provide a simple tool to identify patients eligible for dose escalation.