gms | German Medical Science

48th Meeting of the Particle Therapy Co-Operative Group

Particle Therapy Co-Operative Group (PTCOG)

28.09. - 03.10.2009, Heidelberg

Interfractional variations in the set-up of pelvic bony anatomy and soft tissue, and their implication in proton therapy of the prostate

Meeting Abstract

  • A. Trofimov - Radiation Oncology, Massachusetts General Hospital, Boston, MA, USA
  • S. Mauceri - Radiation Oncology, Massachusetts General Hospital, Boston, MA, USA
  • P. Nguyen - Radiation Oncology, Brigham and Women's Hospital, Boston, MA, USA
  • H.-M. Lu - Radiation Oncology, Massachusetts General Hospital, Boston, MA, USA
  • M. Engelsman - Radiation Oncology, Massachusetts General Hospital, Boston, MA, USA
  • J. Efstathiou - Radiation Oncology, Massachusetts General Hospital, Boston, MA, USA
  • S. Merrick - Radiation Oncology, Morristown Memorial Hospital, Morristown, NJ, USA
  • C.-W. Cheng - Radiation Oncology, Morristown Memorial Hospital, Morristown, NJ, USA
  • J. Wong - Radiation Oncology, Morristown Memorial Hospital, Morristown, NJ, USA
  • A. Zietman - Radiation Oncology, Massachusetts General Hospital, Boston, MA, USA

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog208

DOI: 10.3205/09ptcog208, URN: urn:nbn:de:0183-09ptcog2088

Published: September 24, 2009

© 2009 Trofimov et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by-nc-nd/3.0/deed.en). You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.


Outline

Text

Purpose: Prostate carcinoma patients represent a growing share of proton therapy recipients worldwide. Notwithstanding, concerns persist about many uncertainties associated with dose delivery to such deep-seated, mobile targets, in the close proximity of sensitive critical organs. We use serial CT data to investigate the distortions in the delivered dose distributions due to daily variations in the patient anatomy and immobilization, to evaluate the effectiveness of the current planning and set-up approaches employed at the MGH proton center, and to develop recommendations for improved management of the uncertainties.

Methods: We examined series of CT data from 10 patients treated with IMRT at Morristown Memorial Hospital. Each series consisted of the pre-treatment CT scan, as well as between 23 and 43 in-room scans taken on different days throughout the course. Variations were recorded of the hip rotation angles, thickness of the lateral subcutaneous adipose tissue, and physical depth to the distal surface of the prostate for the lateral beam approach. Treatment plans were designed with the standard MGH approach, assuming ultrasound-based set-up, and confirmation with orthogonal X-ray images: namely, a 5-mm target margin expansion (for intrafractional prostate motion), 3.5% expansion of the distal and proximal spread-out Bragg peak edges (to account for the uncertainties in the conversion of CT numbers to stopping-powers), and the compensator expansion with 10-mm smear radius (for the misalignments of the heterogeneities with the target position). The daily variations in the volume treated to the prescription dose, and the target coverage were evaluated.

Results: In all 10 datasets, substantial variation was observed in the lateral tissue thickness (standard deviation between 1.7 and 3.6 mm for individual patients, variations of over 10 mm from the planning CT in 5 patients), and hip rotation angle (standard deviation between 1.4 and 4.8 degrees, with the maximum excursion exceeding 15 degrees in 6 out of 10 datasets). Shifts in the position of the treated volume (98% iso-dose) were correlated with the variations in the lateral tissue thickness and hip rotation angle.

Conclusions: Hip rotation, and soft tissue dislocation may cause substantial perturbation in the shape of the treated volume. The compensator expansion technique prevents loss of dose to target in majority of cases. In-room set-up verification is essential for precise targeting, and maintaining the target coverage. While improved target immobilization may allow one to reduce PTV and compensator expansion margins, the uncertainties in the proton penetration in tissue remain as the biggest contribution to the margin expansion. Consequentially, refinements in in-vivo proton range verification methods may potentially have the biggest impact on the margin reduction, and confirmation of the treated volume to the target.