Interfractional variations in the setup of pelvic bony anatomy and soft tissue, and their implications on the delivery of proton therapy for localized prostate cancer.
OBJECTIVE
To quantify daily variations in the anatomy of patients undergoing radiation therapy for prostate carcinoma, to estimate their effect on dose distribution, and to evaluate the effectiveness of current standard planning and setup approaches employed in proton therapy.


METHODS
We used series of computed tomography data, which included the pretreatment scan, and between 21 and 43 in-room scans acquired on different treatment days, from 10 patients treated with intensity-modulated radiation therapy at Morristown Memorial Hospital. Variations in femur rotation angles, thickness of subcutaneous adipose tissue, and physical depth to the distal surface of the prostate for lateral beam arrangement were recorded. Proton dose distributions were planned with the standard approach. Daily variations in the location of the prescription isodose were evaluated.


RESULTS
In all 10 datasets, substantial variation was observed in the lateral tissue thickness (standard deviation of 1.7-3.6 mm for individual patients, variations of >5 mm from the planning computed tomography observed in all series), and femur rotation angle (standard deviation between 1.3° and 4.8°, with the maximum excursion exceeding 10° in 6 of 10 datasets). Shifts in the position of treated volume (98% isodose) were correlated with the variations in the lateral tissue thickness.


CONCLUSIONS
Analysis suggests that, combined with image-guided setup verification, the range compensator expansion technique prevents loss of dose to target from femur rotation and soft-tissue deformation, in the majority of cases. Anatomic changes coupled with the uncertainties of particle penetration in tissue restrict possibilities for margin reduction in proton therapy of prostate cancer.
Department of Radiation Oncology, Massachusetts General Hospital, Boston, MA 02114, USA. atrofimov@partners.org
Alexei
A
Trofimov
Paul L
PL
Nguyen
Jason A
JA
Efstathiou
Yi
Y
Wang
Hsiao-Ming
HM
Lu
Martijn
M
Engelsman
Scott
S
Merrick
Chee-Wai
CW
Cheng
James R
JR
Wong
Anthony L
AL
Zietman
D015992:Body Mass Index; D005269:Femur / Q000000981:diagnostic imaging; D006801:Humans; D007103:Immobilization / Q000295:instrumentation / Q000379:methods; D008297:Male; D009068:Movement; D009929:Organ Size; D010384:Pelvic Bones / Q000000981:diagnostic imaging; D011467:Prostate / Q000000981:diagnostic imaging / Q000473:pathology; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy*; D011859:Radiography; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012399:Rotation; D050151:Subcutaneous Fat / Q000033:anatomy & histology / Q000000981:diagnostic imaging