Viability of an isocentric cobalt-60 teletherapy unit for stereotactic radiosurgery

Comparative analysis of 60Co intensity-modulated radiation therapy

In this study, we perform a scientific comparative analysis of using (60)Co beams in intensity-modulated radiation therapy (IMRT). In particular, we evaluate the treatment plan quality obtained with (i) 6 MV, 18 MV and (60)Co IMRT; (ii) different numbers of static multileaf collimator (MLC) delivered (60)Co beams and (iii) a helical tomotherapy (60)Co beam geometry. We employ a convex fluence map optimization (FMO) model, which allows for the comparison of plan quality between different beam energies and configurations for a given case. A total of 25 clinical patient cases that each contain volumetric CT studies, primary and secondary delineated targets, and contoured structures were studied: 5 head-and-neck (H&N), 5 prostate, 5 central nervous system (CNS), 5 breast and 5 lung cases. The DICOM plan data were anonymized and exported to the University of Florida optimized radiation therapy (UFORT) treatment planning system. The FMO problem was solved for each case for 5-71 equidistant beams as well as a helical geometry for H&N, prostate, CNS and lung cases, and for 3-7 equidistant beams in the upper hemisphere for breast cases, all with 6 MV, 18 MV and (60)Co dose models. In all cases, 95% of the target volumes received at least the prescribed dose with clinical sparing criteria for critical organs being met for all structures that were not wholly or partially contained within the target volume. Improvements in critical organ sparing were found with an increasing number of equidistant (60)Co beams, yet were marginal above 9 beams for H&N, prostate, CNS and lung. Breast cases produced similar plans for 3-7 beams. A helical (60)Co beam geometry achieved similar plan quality as static plans with 11 equidistant (60)Co beams. Furthermore, 18 MV plans were initially found not to provide the same target coverage as 6 MV and (60)Co plans; however, adjusting the trade-offs in the optimization model allowed equivalent target coverage for 18 MV. For plans with comparable target coverage, critical structure sparing was best achieved with 6 MV beams followed closely by (60)Co beams, with 18 MV beams requiring significantly increased dose to critical structures. In this paper, we report in detail on a representative set of results from these experiments. The results of the investigation demonstrate the potential for IMRT radiotherapy employing commercially available (60)Co sources and a double-focused MLC. Increasing the number of equidistant beams beyond 9 was not observed to significantly improve target coverage or critical organ sparing and static plans were found to produce comparable plans to those obtained using a helical tomotherapy treatment delivery when optimized using the same well-tuned convex FMO model. While previous studies have shown that 18 MV plans are equivalent to 6 MV for prostate IMRT, we found that the 18 MV beams actually required more fluence to provide similar quality target coverage.

Adaptation of telecobalt unit for stereotactic irradiation

We investigated the feasibility of using an isocentric telecobalt unit for advanced treatment techniques, such as stereotactic radiotherapy. To adapt the telecobalt unit (Th780 C) for stereotactic irradiation, collimator inserts of various sizes, collimator mount, and a couch mount suitable for the telecobalt unit were developed, and the characteristics of the narrow beams of Cobalt-60 (60Co) were studied. Comparative study was carried out between the stereotactic radiotherapy plans of 6 MV and 60Co beams using a 3-dimensional (3D) treatment planning system. The beam penumbra of 60Co beams was found to be larger than those of 6 MV beams. The dose-volume histograms (DVH) obtained from the 60Co beam plan were comparable to those obtained from the 6 MV plan. The DVH of nontarget tissue obtained from the plans of the 2 beams were found to be in good agreement to each other. The difference in equivalent fall-off distance (EFOD) for all 3 cases was found insignificant; hence, it can be concluded that the fall-off dose in the dose distribution of the 60Co stereotactic plan is as good as that of the 6 MV stereotactic plan. In all 3 cases for which the treatment plans were compared between 60Co and 6 MV beams, it was observed that the fall-off doses outside the target were similar; therefore, considering 60Co with 5-mm margin is a cost effective alternative for the linac-based stereotactic radiotherapy.

Dosimetric study of the narrow beams of 60Co teletherapy unit for stereotactic radiosurgery

This study explores the possibility of using a telecobalt unit for radiosurgery. A dosimetric study was performed for the narrow beam of Cobalt 60 (60Co) unit with circular radiation fields in diameters of 11, 17, 20, 27, 32, 35, 40, and 44 mm. Percentage depth dose and off-axis ratio were measured with ion chamber and radiographic film. The tissue air ratio values derived from measurements agreed well with the calculated values for all cone sizes and depths, ranging from the depth of maximum ionization of 24 cm in water. A quantitative evaluation of treatment plans with 60Co and 6-MV photon beams was carried out. The penumbra of the narrow beam of 60Co was larger than that of the 6-MV beam by 1.3 mm on average. This difference in penumbra can be attributed to the large source size of 60Co units. The feasibility of using narrow-beam 60Co for stereotactic radiosurgery/radiotherapy is discussed.