Influence of MLC leaf width on biologically adapted IMRT plans.
BACKGROUND
High resolution beam delivery may be required for optimal biology-guided adaptive therapy. In this work, we have studied the influence of multi leaf collimator (MLC) leaf widths on the treatment outcome following adapted IMRT of a hypoxic tumour.


METHODS
Dynamic contrast enhanced MR images of a dog with a spontaneous tumour in the nasal region were used to create a tentative hypoxia map following a previously published procedure. The hypoxia map was used as a basis for generating compartmental gross tumour volumes, which were utilised as planning structures in biologically adapted IMRT. Three different MLCs were employed in inverse treatment planning, with leaf widths of 2.5 mm, 5 mm and 10 mm. The number of treatment beams and the degree of step-and-shoot beam modulation were varied. By optimising the tumour control probability (TCP) function, optimal compartmental doses were derived and used as target doses in the inverse planning. Resulting IMRT dose distributions and dose volume histograms (DVHs) were exported and analysed, giving estimates of TCP and compartmental equivalent uniform doses (EUDs). The impact of patient setup accuracy was simulated.


RESULTS
The MLC with the smallest leaf width (2.5 mm) consistently gave the highest TCPs and compartmental EUDs, assuming no setup error. The difference between this MLC and the 5 mm MLC was rather small, while the MLC with 10 mm leaf width gave considerably lower TCPs. When including random and systematic setup errors, errors larger than 5 mm gave only small differences between the MLC types. For setup errors larger than 7 mm no differences were found between non-uniform and uniform dose distributions.


CONCLUSIONS
Biologically adapted radiotherapy may require MLCs with leaf widths smaller than 10 mm. However, for a high probability of cure it is crucial that accurate patient setup is ensured.
Rødal
Jan
J
Department of Medical Physics, The Norwegian Radium Hospital, Oslo University Hospital, Nydalen, Oslo, Norway. jan.rodal@radiumhospitalet.no
Søvik
Aste
A
Malinen
Eirik
E
D000818:Animals; D002277:Carcinoma / Q000000981:diagnostic imaging / Q000378:metabolism / Q000532:radiotherapy*; D004283:Dog Diseases / Q000000981:diagnostic imaging / Q000378:metabolism / Q000532:radiotherapy*; D004285:Dogs; D019583:Dose Fractionation, Radiation; D005260:Female; D008962:Models, Theoretical; D009669:Nose Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism / Q000532:radiotherapy*; D019047:Phantoms, Imaging; D011859:Radiography; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation* / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods; D047368:Tumor Burden / Q000528:radiation effects