Implementation of multiple isocentre treatment for dynamic radiosurgery

Dynamic radiosurgery at the Toronto - Bayview Regional Cancer Centre, 1988-2007

Dynamic radiosurgery was first developed in Montreal and was subsequently adopted at the Toronto-Bayview Regional Cancer Centre in 1988. At that time radiosurgery was in its infancy in Canada. The opportunity of offering highly conformal radiation treatments for intracranial targets presented numerous technical challenges notably in the area of quality assurance. This review chronicles the development of radiosurgery at the Toronto-Bayview Regional Cancer Centre and summarises the successes and failures of the program over the following two decades.

Fractionated (split dose) radiosurgery in patients with recurrent brain metastases: implications for survival

Radiosurgery is conventionally prescribed for brain metastases with a single dose of radiation. Fractionation has been advocated to improve tumour control. A multivariate analysis of prognostic factors including fractionation has been performed in two consecutive prospective radiosurgery protocols with and without fractionation in order to identify an association, if any, between fractionation and survival. A surgically applied stereotactic head frame was used. Radiosurgery planning was based on a contrast-enhanced CT. Sixty-nine patients underwent the two-fraction regimen and 35 patients had a single treatment. Multivariate analysis showed that the presence of extracranial malignancy, performance status, multiple brain metastases, patient gender and the time from the initial treatment to radiosurgery were independent determinants for survival. Fractionation was also an independent determinant with two-fraction patients surviving a median of 30 weeks versus single fraction patients who survived a median of 16 weeks. Fractionated radiosurgery was associated with improved survival and deserves further investigation.

Advantages of mini-multileaf in stereotactic radiotherapy

Over the past few decades, cones of different diameter (12.5 mm to 40 mm) were used for treatment of intracranial lesions. These give very focused dose delivery to the target with minimum dose to outside normal brain tissues. This study is intended to compare the older method of arc-based stereotactic treatments using cones with the new mini-multileaf collimator (mMLC). Treatment plans are made for various sites of intracranial lesions with the cones and mMLC. In case of nonspherical lesions, more than one isocenter is used to get an optimum dose distribution with cones, while a single isocenter is sufficient with mMLC. Treatment plans are compared for irregular lesions using cones with multiple isocenters and mMLC. It is observed that conformity index and dose heterogeneity are better for mMLC based treatments.

Fractionated stereotactic radiotherapy for the treatment of vestibular schwannomas: combined experience of the Toronto-Sunnybrook Regional Cancer Centre and the Princess Margaret Hospital

PURPOSE

To evaluate the efficacy and toxicity of fractionated stereotactic radiotherapy (FSRT) for vestibular schwannomas in patients treated at two university-affiliated hospitals.

METHODS AND MATERIALS

Thirty-nine patients were treated between April 1996 and September 2000. The median age was 56 years (range: 29-80), and median maximal tumor diameter was 20 mm (range: 9-40). A total of 11 patients had fifth and/or seventh cranial nerve dysfunction before irradiation; 2 patients had only facial weakness, 5 patients had only facial numbness, and 4 patients had both facial weakness and numbness. Thirty-three patients were treated with primary FSRT, and 6 patients were treated for recurrent or persistent disease after previous surgery. All patients were treated with 6-MV photons using a stereotactic system with a relocatable frame. The 39 patients received 50 Gy in 25 fractions over 5 weeks. Median follow-up was 21.8 months (range: 4.4-49.6).

RESULTS

Local control was achieved in 37 patients (95%). Two patients experienced deterioration of their symptoms at 3 and 20 months as a result of clinical progression in one case and tumor progression in the other and underwent surgery post FSRT. A total of 19/28 (67.9%) patients preserved serviceable hearing after FSRT. Deterioration of the facial and trigeminal nerves was observed in only 2 patients who were treated with surgery post FSRT.

CONCLUSION

FSRT provided excellent tumor control with minimal morbidity and good hearing preservation in this cohort of patients. Longer follow-up is required to confirm long-term control rates.

CARABEAMER: a treatment planner for a robotic radiosurgical system with general kinematics

Stereotactic radiosurgery is a minimally invasive procedure that uses a focused beam of radiation as an ablative instrument to destroy brain tumors. To deposit a high dose of radiation in a tumor, while reducing the dose to healthy tissue, a large number of beams are crossfired at the tumor from multiple directions. The treatment planning problem (also called the inverse dosimetry problem) is to compute a set of beams that produces the desired dose distribution. So far its investigation has focused on the generation of isocenter-based treatments in which the beam axes intersect at a common point, the isocenter. However this restriction limits the applicability of the treatments to tumors which have simple shapes. This paper describes CARABEAMER, a new treatment planner for a radiosurgical system in which the radiation source can be arbitrarily positioned and oriented by a six-degree-of-freedom manipulator. This planner uses randomized techniques to guess a promising initial set of beams. It then applies space partitioning and linear programming techniques to compute the energy to be delivered along each beam. Finally, it exploits the results of the linear program to iteratively adapt and improve the beam set. Experimental results obtained with CARABEAMER on both patient and synthetic cases are presented and discussed. These results demonstrate that a radiosurgical system with general kinematics can deliver treatments in which the region receiving a high dose closely matches the shape of the tumor, even in complicated cases. They also suggest new research directions which are discussed at the end of the paper.