The optimal sequence for clinical utilization of combined radiotherapy and hyperthermia is not known. The clinical trials have resulted in similar responses whether hyperthermia is given before or after radiation. Moreover, studies addressing the best sequence for an entire course of multifractionated hyperthermia and radiation are lacking. In these experiments, the importance of sequencing of heat and irradiation in a multifractionated treatment regimen in RIF‐1 murine tumors was studied. It was observed that a close sequence of heat and irradiation is more beneficial than separate cytotoxic action. When heat and irradiation were given simultaneously, (within 1 hour) 67% to 75% of the tumors were cured. Heat and irradiation given sequentially (the entire course of one following the entire course of the other, each separated by 72 hours) cured 20% of the tumors. No tumors were cured when treated with heat or irradiation alone. The tumor regrowth time (mean tumor doubling time) is much longer in simultaneous treatment than in sequential treatment. It appears that heating first decreases the effectiveness of subsequent irradiation, causing a shorter growth delay than the opposite sequence. Heat alone does not alter the tumor bed permanently, but irradiation seems to do so, resulting in a slower rate of growth upon recurrence.

With the increasing number of high-energy linear accelerators being installed in radiation-therapy facilities, an increasing need exists for carefully measured data concerning the shielding parameters of photon radiation in the energy range above 10 MeV. In this study, the 18 MV X-ray beam from a Varian Clinac 20 linear accelerator was employed. Transmission parameters were measured for the primary X-ray beam in concrete and steel. The tenth value layer for steel was 11.3 +/- 0.6 cm and for concrete 45.0 +/- 2.0 cm. Also measured were the transmission parameters for scattered radiation vs angle in concrete and steel.

The effect of secondary blocking on the output for 7-16 MeV electron beams is investigated for the two most widely used methods of field shaping, i.e.,(a) shield on surface, and (b) shield at end of cone with subsequent air gap. For those two methods differences in output on the order of 10% are observed. These effects are explained in terms of the blocking geometry and its influence on scattered radiation.

The design and construction details of wedge filters for 25-MV x-ray beams are presented. The authors studied how the characteristics of wedged fields affect their clinical use with 25 MV x rays, paying particular attention to the effects of (a) the wedge angle, (b) the variation of the wedge angle with field size, (c) the variation with depth of the isodose angles relative to th central axis, and (d) the wedge factor. The analysis of the data pertinent to the above parameters is presented for 30 degrees, 45 degrees, and 60 degrees wedges. In addition, the authors experimentally confirmed the accuracy of Tatcher's method (7), by which a wide range of wedge angles smaller than the nominal angle can be generated by using a combination of open and wedge beams.

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