PMID- 17440246
OWN - NLM
STAT- MEDLINE
DCOM- 20070712
LR  - 20070418
IS  - 0031-9155 (Print)
IS  - 0031-9155 (Linking)
VI  - 52
IP  - 9
DP  - 2007 May 7
TI  - Improvement of field matching in segmented-field electron conformal therapy using 
      a variable-SCD applicator.
PG  - 2459-81
AB  - The purpose of the present study is to demonstrate that the use of an electron 
      applicator with energy-dependent source-to-collimator distances (SCDs) will 
      significantly improve the dose homogeneity for abutted electron fields in 
      segmented-field electron conformal therapy (ECT). Multiple Coulomb scattering 
      theory was used to calculate and study the P(80-20) penumbra width of off-axis 
      dose profiles as a function of air gap and depth. Collimating insert locations 
      with air gaps (collimator-to-isocenter distance) of 5.0, 7.5, 11.5, 17.5 and 19.5 
      cm were selected to provide equal P(80-20) at a depth of 1.5 cm in water for 
      energies of 6, 9, 12, 16 and 20 MeV, respectively, for a Varian 2100EX radiation 
      therapy accelerator. A 15 x 15 cm(2) applicator was modified accordingly, and 
      collimating inserts used in the variable-SCD applicator for segmented-field ECT 
      were constructed with diverging edges using a computer-controlled hot-wire 
      cutter, which resulted in 0.27 mm accuracy in the abutted edges. The resulting 
      electron beams were commissioned for the pencil-beam algorithm (PBA) on the 
      Pinnacle(3) treatment planning system. Four hypothetical planning target volumes 
      (PTVs) and one patient were planned for segmented-field ECT using the new 
      variable-SCD applicator, and the resulting dose distributions were compared with 
      those calculated for the identical plans using the conventional 95 cm SCD 
      applicator. Also, a method for quality assurance of segmented-field ECT dose 
      plans using the variable-SCD applicator was evaluated by irradiating a 
      polystyrene phantom using the treatment plans for the hypothetical PTVs. 
      Treatment plans for all four of the hypothetical PTVs using the variable-SCD 
      applicator showed significantly improved dose homogeneity in the abutment regions 
      of the segmented-field ECT plans. This resulted in the dose spread (maximum 
      dose-minimum dose), sigma, and D(90-10) in the PTV being reduced by an average of 
      32%, 29% and 32%, respectively. Reductions were most significant for abutted 
      fields of nonadjacent energies. Planning segmented-field ECT using the 
      variable-SCD applicator for a patient with recurrent squamous cell carcinoma of 
      the left ear showed the dose spread, sigma, and D(90-10) of the dose distribution 
      in the PTV being reduced by an average of 38%, 22% and 22%, respectively. The 
      measured and calculated dose in a polystyrene phantom resulting from the 
      variable-SCD, segmented-field ECT plans for the hypothetical PTVs showed good 
      agreement; however, isolated differences between dose calculation and measurement 
      indicated the need for a more accurate dose algorithm than the PBA for 
      segmented-field ECT. These results confirmed our hypothesis that using the 
      variable-SCD applicator for segmented-field ECT results in the PTV dose 
      distribution becoming more homogenous and being within the range of 85-105% of 
      the 'given dose'. Clinical implementation of this method requires variable-SCD 
      applicators, and the design used in the present work should be acceptable, as 
      should our methods for construction of the inserts. Dose verification 
      measurements in a polystyrene phantom and the recommended improvements in dose 
      calculation should be appropriate for quality assurance of segmented-field ECT.
FAU - Richert, John D
AU  - Richert JD
AD  - Department of Physics and Astronomy, Louisiana State University, 202 Nicholson 
      Hall, Baton Rouge, LA 70803-4001, USA.
FAU - Hogstrom, Kenneth R
AU  - Hogstrom KR
FAU - Fields, Robert S
AU  - Fields RS
FAU - Matthews, Kenneth L 2nd
AU  - Matthews KL 2nd
FAU - Boyd, Robert A
AU  - Boyd RA
LA  - eng
PT  - Journal Article
PT  - Research Support, Non-U.S. Gov't
DEP - 20070410
PL  - England
TA  - Phys Med Biol
JT  - Physics in medicine and biology
JID - 0401220
SB  - IM
MH  - Carcinoma, Squamous Cell/radiotherapy
MH  - Ear Neoplasms/radiotherapy
MH  - *Electrons
MH  - Humans
MH  - Neoplasm Recurrence, Local
MH  - Radiotherapy, Conformal/instrumentation/*methods
EDAT- 2007/04/19 09:00
MHDA- 2007/07/13 09:00
CRDT- 2007/04/19 09:00
PHST- 2007/04/19 09:00 [pubmed]
PHST- 2007/07/13 09:00 [medline]
PHST- 2007/04/19 09:00 [entrez]
AID - S0031-9155(07)30911-1 [pii]
AID - 10.1088/0031-9155/52/9/009 [doi]
PST - ppublish
SO  - Phys Med Biol. 2007 May 7;52(9):2459-81. doi: 10.1088/0031-9155/52/9/009. Epub 
      2007 Apr 10.