The potential of the variable-aperture collimator (VAC) in intensity-modulated radiation therapy (IMRT) has been evaluated by comparing its performance with that of the multi-leaf collimator (MLC). This comparison used a decomposition algorithm to find the series of collimator segments that would treat a given intensity-modulated beam (IMB). Collimator performance was measured using both the number of segments required to complete the IMB and the monitor-unit efficiency of the treatment. The VAC was modelled with aperture sizes from 4 x 4 cm to 20 x 20 cm, and these apertures were allowed to be located anywhere within the IMB. To enable a direct comparison, a similar scanning MLC was modelled at the same range of aperture sizes. Using both collimators, decompositions were run on 10 x 10 and 20 x 20 random IMBs with integer bixel values ranging from 1 to 10. Clinical IMBs from lung, head and neck, and pelvic patients were taken from a Pinnacle treatment-planning system and tested in the same manner. It was found that for all treatment sites, a small, scanning MLC performs as well or better than an equivalent sized VAC in both number of segments and monitor-unit efficiency, and would be an efficient choice for centres looking for a simple collimator for IMRT.

The objective of this paper is an investigation of the feasibility and applicability of a new Co-60 irradiation facility concept. The concept is in particular appropriate for developing countries to provide access to advanced treatment modalities such as conformal radiotherapy (RT) or radiosurgery (RS).A concept for a new irradiation facility has been developed. The concept consists of using a small number of Co-60 sources (typically 5-7), mounted on a swivel arc. This system, named CoRA (Cobalt Radiotherapy Arrangement), would be suited for performing both fractionated or conformal RT and RS with simple Co-60 technology. The unit is designed as an isocentric device enabling simultaneous irradiation from a number of different directions. If additionally equipped with multi-leaf-collimators (MLCs), the system would be capable of advanced conformal RT techniques. Irradiations of typical indications have been simulated using an in-house treatment planning system. For this purpose typical treatment situations for two different treatment techniques - a fixed beam (RT) and a moving arc technique (RS) - using CoRA Were investigated. Results were compared with those for a conventional linear accelerator (Linac) and evaluated using dose volume histograms (DVH).For the fixed beam technique with extracranial lesions it is shown that acceptable dose distributions can be achieved within the,planning target volume and the gross tumor volume. For these examples not more than eight angular positions of the swivel arc were used together with irregular field shaping and field weighting. In comparison to the standard Linac treatment plans, only minor differences are obtained for the organs at risk. For the moving arc technique with cranial lesions no significant differences between the moving arc technique and the CoRA arc technique were found.The technical design of the new Co-60 therapy facility CoRA and its arc technique provide dose distributions for treatments of cranial and extracranial target volumes that compare well with modem irradiation techniques using Linacs. The concept of the CoRA system and its multifunctional usage could be particularly advantageous for radiation oncology centers in developing countries.

This paper extends some earlier concepts of using a tertiary mask plus jaws for delivering IMRT without a multileaf collimator. The new concept is to sweep a variable-aperture collimator (VAC) across the space of the intensity-modulated beam (IMB) to be delivered and to strip this IMB down into multiple-static-field components, each deliverable with the VAC. The stripping algorithm is described and it is shown, for several designs of VAC, that the mean number of field components and mean number of monitor units is less using the VAC than would be required for a jaws-only (JO) decomposition. The VAC would be simpler to construct than several previously suggested jaws-plus-mask (J+M) combinations. As well as describing a simple VAC for the use with jaws, we propose a design concept of a hybrid VAC. We also show that adding the potential to rotate the simple or hybrid VAC for some components relative to the field to be modulated is advantageous.