Proceedings of the 40th Annua l A S T R O Meet ing

2287 POTENTIAL FOR INTENSITY MODULATED RADIATION THERAPY (IMRT) TO PROVIDE OPTIMIZED CONCOMITANT BOOST PLANS FOR BRAIN TREATMENTS

Stanley H. Benedict, Ph.D., Robert D. Zwicker, Ph.D., Qiuwen Wu, Ph.D., Robert M. Cardinale, M.D., Brian D. Kavanagh, M.D., Rupert K.A. Schmidt-Ullrich, M.D., and Radhe Mohan, Ph.D.

Department of Radiation Oncology, Medical College of Virginia Hospitals, Virginia Commonwealth University, Richmond, Virginia, 23298

Purpose/Objective: In this work the use of IMRT to create an integrated concomitant boost (ICB) field within the standard treatment volume is investigated for selected brain treatments. The ICB may be regarded as a special form of accelerated treatment which includes a standard large field regimen with a concomitant elevated dose to high risk reduced field volumes, usually only encompassing clinical (CTV) or gross (GTV) tumor volumes. As opposed to the standard large field/boost irradiation sequence, the ICB incorporates an increased dose to the high risk volume from the onset of treatment, thus increasing the biological effectiveness of the small field dose and reducing the potential for tumor re-population. The present work compares radiobiologieal parameters, including equivalent uniform dose (EUD), biological effective dose (BED), TCP, and normal tissue complication probability (NTCP) for standard sequential treatment regimes and ICB plans designed using IMRT.

Materials & Methods: The conditions for comparison of techniques were defined by first identifying the standard large field/boost field irradiation regimen, determining the EUD and BED for the large field over the whole irradiation sequence, and designing IMRT plans with a similar BED for the large field, but with improved TCP in the boost volume. For the sample brain case considered here the boost volume is the GTV with a 0.5 cm margin, while the CTV for the large field encompasses the GTV with a 3-4 cm margin. The standard regime for the brain irradiation includes parallel opposed large fields followed by non-coplanar fields for the boost. The standard plan is 25 fractions at 2 Gy each plus 10 fractions to reduced field at 2 Gy each. The ICB Plans use either 25 or 35 fractions to both large and reduced fields. Biological effectiveness was calculated using the linear-quadratic (L-Q) model with or/15 - 10 for tumor and 2.5 for normal tissues. Potential doubling times for ceils varied from 0.5 to 2 weeks for tumor cells, with no proliferation assumed for normal cells.

Results: The potential of lCB in reducing the effects of cell proliferation was examined by determining the boost volume dose which would give the same late effects in 25 fractions as the standard irradiation in 35 fractions, and comparing the resulting tumor cell kill for a range of doubling times. The calculations showed that for the L-Q parameters used, the ICB, with no other improvements, would result in an increase in tumor cell kill for cases in which the doubling time was less than about one week. This demonstrates a substantial potential benefit for ICB regimes in controlling tumors with a rapid cell proliferation. The studies also showed that with careful planning using highly conformal fields, lMRT-based ICB can lead to further improvements in TCP in the boost volume with no corresponding increase in NTCP to tissues at risk in the large irradiation field.

Conclusion: Concomitant boost can be used to improve tumor control by reducing the risk of cell proliferation during the course of treatment. IMRT-based ICB has the further potential of delivering improved conformal doses which allow higher boost volume doses to be given without increasing the risks of complication in the large irradiation field.

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2288 SELECTION OF OPTIMAL NONCOPLANAR BEAM ORIENTATIONS AND RATIONALE FOR THEIR USE IN THE TREATMENT OF INTRACRANIAL LESIONS

SK Das, CS Whiddon, LB Marks

Department of Radiation Oncology, Duke University Medical Center, Durham, NC 27710

Purnose: The treatment of intracranial lesions with circularly collimated arcs is ideally suited to spherically shaped lesions, since the isodose surfaces are nearly spherical. For irregular shaped lesions, multiple isocenters might be needed to achieve adequate dose conformation. Recent studies suggest that conformation to irregularly shaped lesions might be more easily achieved with multiple fixed shaped fields. The goal of this work is to present an optimization strategy for the design of fixed conformal noncoplanar fields in the treatment of intracranial lesions, and dosimetrically compare such fixed fields to conventionally utilized arc arrangements for regular and irregular lesions.

• " An optimization strategy is presented for the design of fixed, conformal fields, incorporating the goals of (1) maximum beam separation to produce a steep dose gradient at the target edge, (2) minimum volumes of normal tissue above specified isodose levels, (3) maximum target dose coverage, (4) dose limits on critical structures. Field arrangements from the optimization strategy are compared to arc arrangements, for a real patient set, and for idealized spherical and non-spherical lesions placed in a real patient data set. The dosimetric endpoints used are dose volume histograms, normal tissue complication probability, dose heterogeneity (maximum dose/tumor coverage dose), and conformity index (planning isodose volume/treatment volume),

Resultsl For both the real patient data set and the study with idealized lesions, five to seven beam orientations selected using the optimization strategy were better than 480 ° circularly collimated arc arrangements for non-spherical lesion shapes, and were similar to 480 ° continuously conformal arc arrangements (e.g., continuous multileaf collimation), as judged by the different dosimetric endpoints.

Dose Heterogeneity idealized idealized Patient lesion #1 lesion #2

(spherical) (2:1 ellipsoid)*

5 fixed shaped beams 1.13 1.11 1.27 7 fixed shaped beams 1.12 ' 1.13 ' 1.25 "

480 des circularly collimated arc 1.12 1.14 1.39 480 deg continuously, conformal arc 1.12 1.17 ' 1.28

idealized lesion #1

(spherical)

1.12 1.07 1.50 1.50

Conformity, Index idealized lesion #2

(2:1 ellipsoid)*

1.07

Patient

1.19 1.18 1.27 3.23 3.15 1.30 1.45

ellipsoid: 2:1 is the ratio of the long axis to short axes of the ellipsoid.

Conclusions: The optimization strategy for beam selection incorporates some of the goals important to physicians. It is flexible in that the beam orientations supplied by the optimization program can be adjusted by the physician, based on their judgment. The fixed beam arrangements from the optimization are shown to be dosimetrically similar to the best arc arrangement investigated, i.e., 480 ° continuously conformal arc, in both a real patient and an investigation with idealized lesion shapes. This approach provides an ahemative method to deliver highly conformal treatment delivery to intracranial lesions.