Investigation of the relative conformity and efficiency for ... - Wcenter.de

Results• The results from the prostate phantom plan QI scores are reported in theTable below.• The prostate phantom QI’s indicated a significant sensitivity to beam size.• For the prostate case a low QI was only achieved by the 3 and 6 mm σbeams.• This result is supported by the target and rectal DVH’s that indicate similaritybetween the 3 and 6 mm σ beams, but compromise with larger beams.Conformity OAR Hit QIProstate Phantom3mm - 2 Field 1 1.0 16mm - 2 Field 2 1.1 29mm - 2 Field 5 1.1 512mm - 2 Field 17 1.2 14Highly conformal 3 mm σ 2 field plan result1293,61293,6% dose difference between 3 mm and 12 mm σplans indicating excess dose to the simulatedrectal structure

Results• The results from the lung phantom plan QI scores are reported in the Tablebelow.• Beam σ from 3-12 mm were capable of providing QI scores between 1-3.• However, to achieve this result, a higher total number of beams wererequired as sigma increased.• Because 2-3 proton beams are commonly used in clinical lung treatments,therefore this was not considered to be problematic.Lung Phantom Conformity OAR Hit QI3mm - 1 Field 1 1.0 16mm - 1 Field 2 1.3 26mm - 2 Field 1 1.0 19mm - 1 Field 4 1.6 29mm - 2 Field 3 1.1 29mm - 3 Field 2 1.1 212mm - 1 Field 7 1.7 412mm - 2 Field 5 1.2 512mm - 3 Field 5 1.4 3Right: Axial dose differenceof single 3 and 12 mm σbeams surrounding target(interior dashed line).Right: 2 fields of6 mm σdelivered to aspherical target(gray) within anorgan at risk(green)

Conclusions• This study indicates that the sensitivity of dose conformity to beam size for IMPTdepends on the proximity of critical structures, their dose constraints, and the numberof beams delivered.• With the geometrically tight prostate phantom and only 2 fields, acceptable resultswere obtained with 3 and 6 mm beam σ, but not greater.• With the more forgiving lung phantom geometry, satisfactory results were obtainedwith beam σ up to 12 mm, but only with the addition of more fields.• Theoretically the time difference between 3 and 12 mm beam σ delivered fields in thetransverse direction is (3/12)^12=0,0625 a factor of 16 times. A longitudinal timedifferential also exists where the number of energy layers required scales linearly withbeam spot size.• This study suggests that there may be some clinical cases, such as prostate, wheredelivery time may be required to be longer to maintain conformity, and others, suchas lung, where a more efficient delivery of larger beam spots may be desirable. Thismay also reduce delivery sensitivity to target motion.• However, only two limited geometries were considered for this study. Hence, furtherinvestigation is suggested also with clinical cases before general conclusions can bemade regarding suggested beam σ’s for IMPT.