A study of the effects of internal organ motion on dose escalation in conformal prostate treatments.
OBJECTIVE
To assess the effect of internal organ motion on the dose distributions and biological indices for the target and non-target organs for three different conformal prostate treatment techniques.


METHODS
We examined three types of treatment plans in 20 patients: (1) a six field plan, with a prescribed dose of 75.6 Gy; (2) the same six field plan to 72 Gy followed by a boost to 81 Gy; and (3) a five field plan with intensity modulated beams delivering 81 Gy. Treatment plans were designed using an initial CT data set (planning) and applied to three subsequent CT scans (treatment). The treatment CT contours were used to represent patient specific organ displacement; in addition, the dose distribution was convolved with a Gaussian distribution to model random setup error. Dose-volume histograms were calculated using an organ deformation model in which the movement between scans of individual points interior to the organs was tracked and the dose accumulated. The tumor control probability (TCP) for the prostate and proximal half of seminal vesicles (clinical target volume, CTV), normal tissue complication probability (NTCP) for the rectum and the percent volume of bladder wall receiving at least 75 Gy were calculated.


RESULTS
The patient averaged increase in the planned TCP between plan types 2 and 1 and types 3 and 1 was 9.8% (range 4.9-12.5%) for both, whereas the corresponding increases in treatment TCP were 9.0% (1.3-16%) and 8.1% (-1.3-13.8%). In all patients, plans 2 and 3 (81 Gy) exhibited equal or higher treatment TCP than plan 1 (75.6 Gy). The maximum treatment NTCP for rectum never exceeded the planning constraint and percent volume of bladder wall receiving at least 75 Gy was similar in the planning and treatment scans for all three plans.


CONCLUSIONS
For plans that deliver a uniform prescribed dose to the planning target volume (PTV) (plan 1), current margins are adequate. In plans that further escalate the dose to part of the PTV (plans 2 and 3), in a fraction of the cases the CTV dose increase is less than planned, yet in all cases the TCP values are higher relative to the uniform dose PTV (plan 1). Doses to critical organs remain within the planning criteria.
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021, USA.
Laura
L
Happersett
Gig S
GS
Mageras
Michael J
MJ
Zelefsky
Chandra M
CM
Burman
Steven A
SA
Leibel
Chen
C
Chui
Zvi
Z
Fuks
Sarah
S
Bull
C Clifton
CC
Ling
Gerald J
GJ
Kutcher
D006801:Humans; D008297:Male; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000379:methods*; D012007:Rectum / Q000528:radiation effects; D016896:Treatment Outcome; D001743:Urinary Bladder / Q000528:radiation effects