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

Doses at shallow depths of modulated proton beams delivered by double-scatterers and uniform scanning methods

Meeting Abstract

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  • W. Hsi - Radiation Oncology, University Florida Proton Therapy Institute, Jacksonville, Florida, USA
  • G. Zhoa - Radiation Oncology, University Florida Proton Therapy Institute, Jacksonville, Florida, USA
  • Z. Li - Radiation Oncology, University Florida Proton Therapy Institute, Jacksonville, Florida, USA
  • L. Zuofeng - Radiation Oncology, University Florida Proton Therapy Institute, Jacksonville, Florida, 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. Doc09ptcog093

DOI: 10.3205/09ptcog093, URN: urn:nbn:de:0183-09ptcog0939

Published: September 24, 2009

© 2009 Hsi et al.
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Outline

Text

Same modulation with of proton beams can be delivered by either double-scatterers (DS) or uniform-scanning (US) methods with similar momentum spread at entrance of nozzle. In principle, the characteristics of modulated proton beam should be similar between two delivery methods. However, doses at shallow depths from 5 mm to 50 mm are varied between two methods. Causes of dose variation are studied by comparing measured depth doses at central axis in water phantom for four modulated beams at four beam ranges. Four different beam-line settings; referred option, were used for four beam ranges. Complete depth doses were obtained for DS while only discrete depths measured for US due to discontinuously layer energy-stacking technique. Same aperture with same air gap was used for all of measurement to avoid introducing any difference on scattering from aperture. Measured depth doses in water phantom of range 280 mm with 140 mm modulation (denote as R280M140), R210M110, R150M75, and R80M40 were measured. For those depth doses, required ranges at nozzle entrance for DS are 323 mm, 284mm, 214mm and 173 mm while for US are 287 mm, 216mm, 156mm and 124 mm. Required ranges at nozzle entrance for US are lower than for DS, because less materials were placed into beam line for spreading proton laterally by US method to have same distal end of modulated protons in water. A 5% lower dose at 5 mm depth was found at measured R80M40 for US in comparing to for DS while the difference is less for higher beams. Doses at shallow depths are similar for R280M110 for both DS and US. Because a 60-80 mm water-equivalent depth of materials placed into beam line on DS in comparing to US results a large energy spread at entrance of water phantom, the wider width of Bragg peak (BP) is seen in DS. Therefore, the weight of each pull-back BP for forming modulated proton beam is different between DS and US methods. And, the ratio between the high of BP and entrance dose is also different. Differences of ratio and weight can be the causes of reduced shallow depth doses. In addition, amount of scattered protons generated by beam-line materials can be different between US and DS. Effects of field size, scanning size and scattered protons may also contribute to observed difference and need be further investigated.