Beam intensity modulation for penumbra enhancement in the treatment of lung cancer.
OBJECTIVE
A treatment planning study was performed for patients with lung cancer in order to investigate the extent to which doses to critical structures could be reduced by penumbra enhancement at the superior and inferior field edges, using beam intensity modulation (BIM) with a multileaf collimator. By applying two independent published models for the prediction of the incidence of normal tissue complications, the potential for dose escalation without increasing the incidence of pneumonitis was estimated.


METHODS
For 12 patients, the standard treatment technique was compared with the BIM technique using the Cadplan 3D planning system (Varian-Dosetek). Dose distributions in the healthy lung tissue were evaluated by considering both lungs minus the tumor as one functional unit. The following parameters were compared: (i) the average normalized total dose (NTD), (ii) the lung volume receiving an NTD of more than 20 Gy, and (iii) the calculated normal tissue complication probability (NTCP).


RESULTS
Due to the applied BIM technique, the field lengths could be reduced by 1.4 cm for all patients, while achieving a minimum dose at the superior and inferior parts of the target of 95% of the isocenter dose. Compared to the standard technique, BIM reduced the patient mean of the average NTD for the healthy lung tissue from 16.5 to 15.3 Gy. The volume of healthy lung tissue receiving an NTD of 20 Gy or more was reduced by 9.7% (range 2.2 to 23.1%). The calculated NTCP reduced from 10.7% to 7.6% on average. The length of the esophagus that received a dose of 60 Gy or more could be reduced for 5 of the 6 stage III patients in this study. Based on equal lung NTCPs for the standard technique and the BIM technique, a mean dose escalation of 5.7 Gy (range 1.1 to 16.0 Gy) was possible for the 12 patients in this study. Based on equal average NTDs for the two techniques, the patient mean of the allowed dose escalation was 6.5 Gy (range 1.1 to 18.2 Gy). All dose escalations would be possible without exceeding the spinal cord tolerance dose.


CONCLUSIONS
The BIM technique reduced the dose delivery to critical tissues. Two published methods for estimating the incidence of pneumonitis both pointed to a potential for dose escalation of 6 to 7 Gy on average with the BIM technique, without increasing the incidence of pneumonitis. For 2 of the 12 patients in this study the estimated allowed dose escalation even exceeded 15 Gy.
Dirkx
M L
ML
Division of Clinical Physics and Instrumentation, University Hospital Rotterdam, Daniel den Hoed Cancer Center, The Netherlands.
Essers
M
M
van Sornsen de Koste
J R
JR
Senan
S
S
Heijmen
B J
BJ
D004947:Esophagus; D006801:Humans; D008168:Lung / Q000473:pathology; D008175:Lung Neoplasms / Q000532:radiotherapy*; D009367:Neoplasm Staging; D055585:Physical Phenomena; D010825:Physics; D017564:Radiation Pneumonitis / Q000517:prevention & control*; D020266:Radiotherapy, Conformal / Q000295:instrumentation / Q000379:methods* / Q000592:standards