Influence of dose point and inverse optimization on interstitial cervical and oropharyngeal carcinoma brachytherapy.
OBJECTIVE
Evaluation of the use of optimization methods in interstitial cervical and oropharyngeal brachytherapy; evaluation of the conformal index (COIN) and the natural dose ratio (NDR) to quantify the implant quality.


METHODS
CT-based dose distributions were obtained for seven implants according to the Paris system. CT-based implants were used to assess the dose point and inverse optimization methods. To compare the results of these planning methods, the coverage index (CI), normal tissue irradiation (NTI), and the protection of organs at risk (OARs) were evaluated using cumulative dose volume histograms (CDVH).


RESULTS
In regular cervical implants, a CI of 94 and 96%; a NTI of 35 and 28% resulted for non-optimized and optimized implants, respectively. In irregular cervical implants, a CI of 88, 96, and 90%; a NTI of 44, 37, and 44% resulted for non-optimized, dose point optimized, and inverse optimized implants, respectively. Compared to the non-optimized implants; both optimization methods resulted in better protection for the bladder wall. As for the protection of the rectal wall, only the inverse optimization gave a better result. In oropharyngeal implants, a better CI resulted after dose point optimization. Irradiation of the contralateral parotid were improved after both optimization methods. The maximum change in COIN that could have been achieved by optimization was 3%, as CI and NTI increased similarly. For the same value of COIN, an underdosage of PTV was avoided by the optimization methods as NDR increased from 0.86 to 1.01.


CONCLUSIONS
CT-based optimized implant allows conformation of the dose distribution to the PTV while sparing normal tissue and organs at risk. COIN and NDR should be used together to evaluate both doses to normal tissue and organs at risk, and an under- or overdose inside the PTV.
Ahmad
Faten
F
Medical Physics Department, Centre Alexis Vautrin, av. de Bourgogne, 54511 Vandoeuvre-les-Nancy Cedex, France.
Aletti
Pierre
P
Charra-Brunaud
Claire
C
Van der Laarse
Rob
R
Lapeyre
Michel
M
Hoffstetter
Sylvette
S
Peiffert
Didier
D
Nöel
Alain
A
D001918:Brachytherapy / Q000379:methods*; D002277:Carcinoma / Q000532:radiotherapy*; D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D020714:Maximum Tolerated Dose; D009959:Oropharyngeal Neoplasms / Q000532:radiotherapy*; D010347:Patient Care Planning; D011786:Quality Control; D011832:Radiation Injuries / Q000517:prevention & control; D020266:Radiotherapy, Conformal; D012307:Risk Factors; D014057:Tomography, X-Ray Computed; D002583:Uterine Cervical Neoplasms / Q000532:radiotherapy*