Srijit Kamath

Leaf sequencing algorithms for segmented multileaf collimation.

The delivery of intensity-modulated radiation therapy (IMRT) with a multileaf collimator (MLC) requires the conversion of a radiation fluence map into a leaf sequence file that controls the movement of the MLC during radiation delivery. It is imperative that the fluence map delivered using the leaf sequence file is as close as possible to the fluence map generated by the dose optimization algorithm, while satisfying hardware constraints of the delivery system. Optimization of the leaf sequencing algorithm has been the subject of several recent investigations. In this work, we present a systematic study of the optimization of leaf sequencing algorithms for segmental multileaf collimator beam delivery and provide rigorous mathematical proofs of optimized leaf sequence settings in terms of monitor unit (MU) efficiency under most common leaf movement constraints that include minimum leaf separation constraint and leaf interdigitation constraint. Our analytical analysis shows that leaf sequencing based on unidirectional movement of the MLC leaves is as MU efficient as bidirectional movement of the MLC leaves.

Algorithms for optimal sequencing of dynamic multileaf collimators.

Dynamic multileaf collimator (DMLC) intensity modulated radiation therapy (IMRT) is used to deliver intensity modulated beams using a multileaf collimator (MLC), with the leaves in motion. DMLC-IMRT requires the conversion of a radiation intensity map into a leaf sequence file that controls the movement of the MLC while the beam is on. It is imperative that the intensity map delivered using the leaf sequence file be as close as possible to the intensity map generated by the dose optimization algorithm, while satisfying hardware constraints of the delivery system. Optimization of the leaf-sequencing algorithm has been the subject of several recent investigations. In this work, we present a systematic study of the optimization of leaf-sequencing algorithms for dynamic multileaf collimator beam delivery and provide rigorous mathematical proofs of optimized leaf sequence settings in terms of monitor unit (MU) efficiency under the most common leaf movement constraints that include leaf interdigitation constraint. Our analytical analysis shows that leaf sequencing based on unidirectional movement of the MLC leaves is as MU efficient as bi-directional movement of the MLC leaves.

Optimal leaf sequencing with elimination of tongue-and-groove underdosage

The individual leaves of a multileaf collimator (MLC) have a tongue-and-groove or stepped-edge design to minimize leakage radiation between adjacent leaves. This design element has a drawback in that it creates areas of underdosages in intensity-modulated photon beams unless a leaf trajectory is specifically designed such that for any two adjacent leaf pairs, the direct exposure under the tongue-and-groove is equal to the lower of the direct exposures of the leaf pairs. In this work, we present a systematic study of the optimization of a leaf sequencing algorithm for segmental multileaf collimator beam delivery that completely eliminates areas of underdosages due to tongue-and-groove or stepped-edge design of the MLC. Simultaneous elimination of tongue-and-groove effect and leaf interdigitation is also studied. This is an extension of our previous work (Kamath et al 2003a Phys. Med. Biol. 48 307) in which we described a leaf sequencing algorithm that is optimal for monitor unit (MU) efficiency under most common leaf movement constraints that include minimum leaf separation. Compared to our previously published algorithm (without constraints), the new algorithms increase the number of sub-fields by approximately 21% and 25%, respectively, but are optimal in MU efficiency for unidirectional schedules.

A comparison of step-and-shoot leaf sequencing algorithms that eliminate tongue-and-groove effects

The performances of three recently published leaf sequencing algorithms for step-and-shoot intensity-modulated radiation therapy delivery that eliminates tongue-and-groove underdosage are evaluated. Proofs are given to show that the algorithm of Que et al (2004 Phys. Med. Biol. 49 399-405) generates leaf sequences free of tongue-and-groove underdosage and interdigitation. However, the total beam-on times could be up to n times those of the sequences generated by the algorithms of Kamath et al (2004 Phys. Med. Biol. 49 N7-N19), which are optimal in beam-on time for unidirectional leaf movement under the same constraints, where n is the total number of involved leaf pairs. Using 19 clinical fluence matrices and 100000 randomly generated 15 x 15 matrices, the average monitor units and number of segments of the leaf sequences generated using the algorithm of Que et al are about two to four times those generated by the algorithm of Kamath et al.

Optimal field splitting for large intensity-modulated fields.

The multileaf travel range limitations on some linear accelerators require the splitting of a large intensity-modulated field into two or more adjacent abutting intensity-modulated subfields. The abutting subfields are then delivered as separate treatment fields. This workaround not only increases the treatment delivery time but it also increases the total monitor units (MU) delivered to the patient for a given prescribed dose. It is imperative that the cumulative intensity map of the subfields is exactly the same as the intensity map of the large field generated by the dose optimization algorithm, while satisfying hardware constraints of the delivery system. In this work, we describe field splitting algorithms that split a large intensity-modulated field into two or more intensity-modulated subfields with and without feathering, with optimal MU efficiency while satisfying the hardware constraints. Compared to a field splitting technique (without feathering) used in a commercial planning system, our field splitting algorithm (without feathering) shows a decrease in total MU of up to 26% on clinical cases and up to 63% on synthetic cases.

An image quality comparison study between XVI and OBI CBCT systems

The purpose of this study is to evaluate and compare image quality characteristics for two commonly used and commercially available CBCT systems: the X‐ray Volumetric Imager and the On‐Board Imager. A commonly used CATPHAN image quality phantom was used to measure various image quality parameters, namely, pixel value stability and accuracy, noise, contrast to noise ratio (CNR), high‐contrast resolution, low contrast resolution and image uniformity. For the XVI unit, we evaluated the image quality for four manufacturer‐supplied protocols as a function of mAs. For the OBI unit, we did the same for the full‐fan and half‐fan scanning modes, which were respectively used with the full bow‐tie and half bow‐tie filters. For XVI, the mean pixel values of regions of interest were found to generally decrease with increasing mAs for all protocols, while they were relatively stable with mAs for OBI. Noise was slightly lower on XVI and was seen to decrease with increasing mAs, while CNR increased with mAs for both systems. For XVI and OBI, the high‐contrast resolution was approximately limited by the pixel resolution of the reconstructed image. On OBI images, up to 6 and 5 discs of 1% and 0.5% contrast, respectively, were visible for a high mAs setting using the full‐fan mode, while none of the discs were clearly visible on the XVI images for various mAs settings when the medium resolution reconstruction was used. In conclusion, image quality parameters for XVI and OBI have been quantified and compared for clinical protocols under various mAs settings. These results need to be viewed in the context of a recent study that reported the dose‐mAs relationship for the two systems and found that OBI generally delivered higher imaging doses than XVI. (1) PACS numbers: 85.57.C‐, 85.57.cj, 85.57.cm, 85.57.cf

Generalized field-splitting algorithms for optimal IMRT delivery efficiency

Intensity-modulated radiation therapy (IMRT) uses radiation beams of varying intensities to deliver varying doses of radiation to different areas of the tissue. The use of IMRT has allowed the delivery of higher doses of radiation to the tumor and lower doses to the surrounding healthy tissue. It is not uncommon for head and neck tumors, for example, to have large treatment widths that are not deliverable using a single field. In such cases, the intensity matrix generated by the optimizer needs to be split into two or three matrices, each of which may be delivered using a single field. Existing field-splitting algorithms used the pre-specified arbitrary split line or region where the intensity matrix is split along a column, i.e., all rows of the matrix are split along the same column (with or without the overlapping of split fields, i.e., feathering). If three fields result, then the two splits are along the same two columns for all rows. In this paper we study the problem of splitting a large field into two or three subfields with the field width as the only constraint, allowing for an arbitrary overlap of the split fields, so that the total MU efficiency of delivering the split fields is maximized. Proof of optimality is provided for the proposed algorithm. An average decrease of 18.8% is found in the total MUs when compared to the split generated by a commercial treatment planning system and that of 10% is found in the total MUs when compared to the split generated by our previously published algorithm.

Algorithms for sequencing multileaf collimators

In delivering radiation therapy for cancer treatment, it is desirable to deliver high doses of radiation to the target tumor, while permitting a low dosage on the surrounding healthy tissues. In recent years, the development of intensity modulated radiation therapy (IMRT) has made this possible. IMRT may be delivered by several techniques. The delivery of IMRT with a multileaf collimator (MLC) requires the delivery of radiation from several beam orientations. The intensity profile for each beam direction is described as a MLC leaf sequence, which is developed using a leaf sequencing algorithm. Important considerations in developing a leaf sequence for a desired intensity profile include maximizing the monitor unit (MU) efficiency (equivalently minimizing the beam-on time) and minimizing the total treatment time subject to the leaf movement constraints of the MLC model. In this work, we present a systematic study of the optimization of leaf sequencing algorithms and provide rigorous mathematical proofs of optimized leaf sequence settings in terms of MU efficiency under most common leaf movement constraints that include minimum and maximum leaf separation, leaf interdigitation and tongue-and-groove. We also develop algorithms to split large intensity modulated fields into two or three subfields.

SU‐CC‐J‐6C‐06: A Generalized Field Splitting Algorithm for Optimal IMRT Delivery Efficiency

Purpose: The purpose of this work is to develop an algorithm for optimally splitting a large intensity‐modulated field for delivery with a MLC into two or more adjacent subfields that maximizes MU efficiency, and to provide mathematical proofs that the algorithm is optimal in the most general cases. Method and Materials: The field split can be stated as the following mathematical problem: given a fluence matrix which exceeds the largest field size limitation of the delivery system, find two or three subfields, each of which satisfies the field size limitation constraint, that combine to give the original fluence map, and with the additional constraint that the sum of the delivered MUs of the subfields be minimized. In this general formalism, the subfields can overlap and the field split does not have to be in a straight line. We first construct an optimal MU leaf sequence for the large field ignoring the width constraint. The optimal field split is then generated by appropriate partitioning of the optimal leaf sequence into leaf sequences for the subfields. The overlapping region of the subfields creates a natural feathering area which is clinically desirable. Results: Compared to a simple field splitting that cuts through the center of a fluence map, our algorithm showed an average decrease in total MU of about 19% on 32 clinical fluence maps with the largest decrease in total MU of 45%. In many cases, the total MU of the split fields does not increase from the MU of the original sequence when the width constraint is ignored. Conclusion: We have developed an algorithm that solves the most general version of the field splitting problem for large intensity modulated fields. We provide rigorous mathematical proofs that the proposed algorithm for field splitting is optimal in MU efficiency.

SU-FF-T-221: Efficient Registration of Dose Distributions

Purpose: To develop efficient algorithms to compute the optimal alignment between two high resolution 2D dose distributions (usually one calculated, one measured), with one of them as a reference. This will help eliminate 2D dosimeter setup errors and/or misalignments between radiation and light fields, which both affect the integrity of the IMRT‐QA process. We feel that these algorithms will also serve as a useful tool for other physics QA procedures involving calculation/measurement of planar dose/fields. Method and Materials: The mathematical objective of alignment is to maximize the percentage of points that pass a similarity test. Two commonly used similarity tests, viz., the composite test (DTA/dose difference) and the gamma test are considered. The algorithms work by efficiently and exhaustively examining a search space of alignments that are in a predefined neighborhood (say, 10 mm in each direction) of the initial alignment, for translational transformations that maximize the objective. The resulting alignments are guaranteed to be optimal provided that the optimal solution lies within that neighborhood. Mathematical properties of the objective function are utilized for efficient algorithm design. The program execution time depends on the selection of size of the neighborhood to be searched, which may be reduced by visually or automatically (using image registration) picking a good starting alignment. Results: For registering two 140*140 distributions with 1 mm spatial resolution and with the 5%/5 mm gamma criterion, the algorithm executes within a few seconds for a search within 10 mm of the initial alignment on a 1.7 GHz processor. Conclusion: We present efficient and optimal algorithms to determine the best alignment of two 2D dose distributions for IMRT‐QA. New or enhanced algorithms are needed to correct rotational misalignments and to extend the work to 3D dose distributions that may be measured using 3D diode arrays or gel dosimeters.