Quantitative assessment of range fluctuations in charged particle lung irradiation.
OBJECTIVE
Water equivalent path length (WEL) variations due to respiration can change the range of a charged particle beam and result in beam overshoot to critical organs or beam undershoot to tumor. We have studied range fluctuations by analyzing four-dimensional computed tomography data and quantitatively assessing potential beam overshoot.


METHODS
The maximal intensity volume is calculated by combining the gross tumor volume contours at each respiratory phase in the four-dimensional computed tomography study. The first target volume calculates the maximal intensity volume for the entire respiratory cycle (internal target volume [ITV]-radiotherapy [RT]), and the second target volume is the maximal intensity volume corresponding to gated RT (gated-RT, approximately 30% phase window around exhalation). A compensator at each respiratory phase is calculated. Two "composite" compensators for ITV-RT and gated-RT are then designed by selecting the minimal compensator depth at the respective respiratory phase. These compensators are then applied to the four-dimensional computed tomography data to estimate beam penetration. Analysis metrics include range fluctuation and overshoot volume, both as a function of gantry angle. We compared WEL fluctuations observed in treating the ITV-RT versus gated-RT in 11 lung patients.


RESULTS
The WEL fluctuations were <21.8 mm-WEL and 9.5 mm-WEL for ITV-RT and gated-RT, respectively for all patients. Gated-RT reduced the beam overshoot volume by approximately a factor of four compared with ITV-RT. Such range fluctuations can affect the efficacy of treatment and result in an excessive dose to a distal critical organ.


CONCLUSIONS
Time varying range fluctuation analysis provides information useful for determining appropriate patient-specific treatment parameters in charged particle RT. This analysis can also be useful for optimizing planning and delivery.
Mori
Shinichiro
S
Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA. shinshin@nirs.go.jp
Wolfgang
John
J
Lu
Hsiao-Ming
HM
Schneider
Robert
R
Choi
Noah C
NC
Chen
George T Y
GT
D000368:Aged; D000369:Aged, 80 and over; D018287:Carcinoma, Large Cell / Q000473:pathology / Q000532:radiotherapy; D002289:Carcinoma, Non-Small-Cell Lung / Q000473:pathology / Q000532:radiotherapy; D045853:Exhalation; D005260:Female; D006801:Humans; D008168:Lung; D008175:Lung Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D008297:Male; D008875:Middle Aged; D009068:Movement; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D020266:Radiotherapy, Conformal / Q000295:instrumentation / Q000379:methods*; D012119:Respiration; D014057:Tomography, X-Ray Computed / Q000379:methods*; D047368:Tumor Burden