A treatment planning study of the potential of geometrical tracking for intensity modulated proton therapy of lung cancer.
BACKGROUND
Proton therapy of lung cancer holds the potential for a reduction of the volume of irradiated normal lung tissue. In this work we investigate the robustness of intensity modulated proton therapy (IMPT) plans to motion, and evaluate a geometrical tumour tracking method to compensate for tumour motion.


METHODS
Seven patients with a nine targets with 4DCT scans were selected. IMPT plans were made on the midventilation phase using a 3-field technique. The plans were transferred and calculated on the remaining nine phases of the 4DCT, and the combined dose distribution was summed using deformable image registration (DIR). An additional set of plans were made in which the proton beam was simply geometrically shifted to the centre of the gross tumour volume (GTV), i.e. simulating tracking of the tumour motion but without on-line adjustment of the proton energies. A possible interplay effect between the dynamics of the spot scanning delivery and the tumour motion has not been considered in this work.


RESULTS
Around 97-100% of the GTV was covered by 95% of the prescribed dose (V95) for a tumour displacement of less than about 1 cm with a static beam. For the remaining three of nine targets with a larger motion the tracking method studied provided a marked improvement over static beam; raising the GTV V95 from 95 to 100%, 82 to 98% and 51 to 97%, respectively.


CONCLUSIONS
The possibility of performing DIR and summing the dose on the 4DCT data set was shown to be feasible. The fairly simplistic tracking method suggested here resulted in a marked improvement in GTV coverage for tumours with large intra-fractional motion (>1 cm displacement), indicating that on-line adjustment of the proton energies may be redundant.
Munck af Rosenschöld
Per
P
Department of Radiation Oncology, Copenhagen University Hospital (Rigshospitalet), Copenhagen, Denmark. per.munck@rh.regionh.dk
Aznar
Marianne C
MC
Nygaard
Ditte E
DE
Persson
Gitte F
GF
Korreman
Stine S
SS
Engelholm
Svend Aage
SA
Nyström
Håkan
H
D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging / Q000473:pathology* / Q000532:radiotherapy*; D004307:Dose-Response Relationship, Radiation; D056973:Four-Dimensional Computed Tomography / Q000379:methods*; D006801:Humans; D008168:Lung / Q000473:pathology / Q000502:physiology / Q000528:radiation effects; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology* / Q000532:radiotherapy*; D009068:Movement / Q000502:physiology; D009929:Organ Size; D061766:Proton Therapy; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D047368:Tumor Burden / Q000502:physiology; D011522:Protons
D011522:Protons