SUMMARY To evaluate the use of single large doses spatially fractionated radiation (GRID) therapy either alone or in combination with conventional radiation in the palliative treatment of advanced cancer. Sixty-one patients with advanced cancer who had exhausted conventional approaches to palliative treatment with surgery, chemotherapy and/or radiotherapy were treated with high dose GRID therapy using high energy photons. Seventytwo symptomatic areas of disease were irradiated. A 50:50 GRID (open to closed areas) was utilized and a single fraction of 1,000-2,000 cGy to d, was delivered to the open areas using a single field with either 6 M V or 25 MV photons. Follow-up ranged from 2 weeks to 28 months. Short follow-up times in some patients was due to their end stage disease. Patients were analyzed for palliation of symptoms and normal tissue morbidity. Sixty-four of 72 treatments were evaluable for palliative response. The results of treatment indicated a 27% (17/64) complete palliative response. A partial response was obtained in 64% (41164). Overall response rate was 91%. Pain was the primary reason for treatment. Twenty-eight percent complete pain relief and 62% partial pain relief was achieved with GRID therapy. Symptoms related to large tumor masses were completely relieved in 19% and partially relieved in 71 Oh. No acute morbidity was observed in spite of the large single doses delivered. Eight patients who were followed 12 to 28 months have shown no late morbidity. The use of spatially fractionated GRID therapy to deliver large single doses of radiation for palliative treatment has proven effective, especially for patients with short life expectancies. Further optimization of the GRID radiation distribution and maximum tolerable doses need to be established.

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The dosimetric characteristics of a multileaf collimator (MLC) retrofitted to a SL25 linear accelerator have been investigated. Central-axis depth dose, surface dose, penumbra, beam flatness and symmetry, field size factors, beam transmission through leaves and/or diaphragms, and leakage between the leaves were measured. Quantitative measurements of all beam parameters show good agreement with the design specifications of the manufacturer. No changes were observed in flatness, symmetry, penumbra, and penetration for both 6- and 25-MV photon beams when compared to the values for the standard collimator. No significant differences were observed in the penumbra as a function of leaf position. Transmission measurements in areas shielded by either X diaphragms or leaves plus diaphragms are less than 1% of dose within open field. The average leakage between leaves is about 2.5% for 6-MV and 3.5% for 25-MV photon beams. The peak value of the leakage at any point between leaves is less than 5%. The dosimetric features of shaped fields using the MLC are comparable to those of alloy shaped fields with the standard SL25 collimator.

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