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

Dosimetric Comparison of Parallel Opposed-Laterals versus Parallel Opposed-Laterals plus an Anterior Field for Proton Therapy Prostate Plans

Meeting Abstract

  • M. Kamrava - Radiation Oncology Branch, National Cancer Institute, Bethesda, Maryland, USA
  • K. Kramer - Henry M. Jackson Foundation for the Advancement of Military Medicine, Rockville, Maryland, USA
  • D. Erickson - Radiation Oncology Service, Walter Reed Army Medical Center, Washington, D.C., USA
  • R. Kiteley - Division of Radiation Oncology, National Naval Medical Center, Bethesda, Maryland, USA
  • A. Belard - Henry M. Jackson Foundation for the Advancement of Military Medicine, Rockville, Maryland, USA
  • J. O'Connell - Radiation Oncology Service, Walter Reed Army Medical Center, Washington, D.C., 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. Doc09ptcog104

DOI: 10.3205/09ptcog104, URN: urn:nbn:de:0183-09ptcog1049

Published: September 24, 2009

© 2009 Kamrava et al.
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Outline

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Background: Parallel opposed lateral fields (LATS) have been the preferred beam angles for proton therapy prostate plans. LATS allow generous proximal and distal coverage of the target volume, and limit high doses to the rectum but do not optimally incorporate the steep dose gradient of the Bragg peak due to range uncertainty concerns. We investigated whether an anterior field (AF) added to LATS could reduce the normal tissue complication probability (NTCP) for the rectum.

Methods: Proton plans were created with anonymized CT data from 8 patients previously treated definitively with photons for prostate cancer. A series of plans was generated with LATS and an AF to a dose of 78 CGE in 39 fractions. Within each series the AF weighting (AFW) was varied from 0% to 100% in 7 equal intervals while maintaining equally weighted lateral fields. Similarly, the AF distal edge margin (DM) was independently varied from 0.25 to 1.0 cm in 4 equal intervals. Each plan was calculated with an unadjusted CT number to proton mass stopping power calibration curve (CC) and with CCs that were adjusted by ±3.5%. Standard models were used to predict rectal NTCPs using the parameters of Burman, et al. (NTCPB) and Rancati, et al. for grade ≥2 (NTCPR2) and ≥3 (NTCPR3). Plans with non-zero AFW that achieved minimal target coverage less than LATS for all 3 CCs were eliminated. Remaining plans were analyzed for the existence of a class solution (CS) with a constant AFW and DM that minimized the mean NTCPs. Predicted NTCPs were compared for each CC between the LATS and the CS and the LATS and each patient's best plan (BP).

Results: A CS for all patients was achieved with a 50% AFW and 0.25 cm DM. Differences between the mean NTCPB, NTCPR2 and NTCPR3 for LATS and the CS were -5.7%, -4.4% and -10.3% respectively and were all statistically significant. Differences for the LATS and the BPs for the mean NTCPs as ordered above were -7.5%, -5.9% and -12.6% and were all statistically significant. Statistically significant differences of larger magnitude for all NTCPs were found when analyzing all plans using the CC adjusted by -3.5%. Using the CC adjusted by +3.5%, the differences in mean for all NTCPs remained negative but most were not statistically significant.

Conclusions: Adding an AF to LATS can achieve equal or better target coverage with the potential to significantly decrease the risk of late term rectal toxicity. Individualization of the AFW and DM can potentially further reduce the rectal NTCP. Use of an anterior field warrants clinical investigation in treating prostate cancer patients with proton therapy.

This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17-W81XWH-04-2-0022. The views, opinions, interpretations, conclusions and recommendations expressed in this abstract are the authors and do not reflect the official policy of the Department of Army, Navy, Department of Defense, or US government.