The effect of beam energy and number of fields on photon-based IMRT for deep-seated targets

Pirzkall, Andrea; Carol, Mark P; Pickett, Barby; Xia, Ping; Roach, Mack; Verhey, Lynn J

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Volume 53, Issue 2 , Pages 434-442, 1 June 2002

The effect of beam energy and number of fields on photon-based IMRT for deep-seated targets☆

Presented at the AAPM meeting, July 24th, 2000, in Chicago, IL.

Andrea Pirzkall, M.D.
- Reprint requests to: Andrea Pirzkall, M.D., University of California, San Francisco, Dept. of Radiation Oncology, 505 Parnassus Av., L-75, San Francisco, CA 94143-0226. Tel: (415) 353-8906; Fax: (415) 353-8679, USA http://www.ucsf.edu/radonc
- Department of Radiation Oncology, University of California, San Francisco, San Francisco, CA, USA
Mark P Carol, M.D.
- Department of Radiation Oncology, University of California, San Francisco, San Francisco, CA, USA
Barby Pickett, M.S.
- Department of Radiation Oncology, University of California, San Francisco, San Francisco, CA, USA
Ping Xia, Ph.D.
- Department of Radiation Oncology, University of California, San Francisco, San Francisco, CA, USA
Mack Roach III, M.D.
- Department of Radiation Oncology, University of California, San Francisco, San Francisco, CA, USA
Lynn J Verhey, Ph.D.
- Department of Radiation Oncology, University of California, San Francisco, San Francisco, CA, USA

Received 12 July 2001; accepted 16 January 2002.

Abstract

To examine the influence of energy and number of beams on nontarget dose when using intensity-modulated radiation therapy (IMRT) to treat deep-seated targets.

Ten patients with prostate cancer (36–226 cc) treated locally to 75.6 Gy were studied. IMRT plans were created for 6-, 10-, and 18-MV photons using 4, 6, 9, and 11 coplanar nonopposed fields. Plans, normalized to cover 95% of the target volume, were analyzed using: (a) conformity index (CI) at 105%, 100%, 95%, 90%, 80%, 70%, 50% of prescribed dose; (b) prescription isodose line (PI); (c) minimum dose to target (Tar_min); (d) maximum dose to tissue (Tis_max); (e) dose to rectum/bladder/penis bulb; (f) integral nontarget dose (ID). Because CI evaluates dose independent of location, tissue also was divided into “near region” (NR: 1-cm-thick shell surrounding target) and “far region” (FR: tissue minus NR) volumes that were evaluated at the same levels as CI.

The target and sensitive structure metrics were the same for all plans. However, although there was little difference in NR volume exposed to dose, regardless of energy or number of fields, there was a significant increase in FR volume exposed to dose, at all levels, for low energy/few field plans compared to high energy/many fields (e.g., > 50 cc ≥ 65 Gy). This effect disappeared with ≥ 9 fields regardless of energy.

With IMRT, the use of 6 MV photons with less than 9 fields may result in an increase in dose in regions distant from the target volume (e.g., near the skin surface), even though the CI and sensitive structure metrics may indicate good conformance of high dose to the target volume itself. The clinical significance of this increased dose distant from the target, in terms of complications, remains to be determined.