THE EFFECT OF CUSTOMIZED BEAM SHAPING ON NORMAL TISSUE COMPLICATIONS IN RADIATION-THERAPY OF PAROTID-GLAND TUMORS

The impact of customized beam shaping was studied for five patients with parotid tumors treated with a paired wedged field technique. For each patient two plans were generated. The standard plan had unblocked portals with field sizes defined by the largest target contour found in any CT slice. In the second plan customized beam's eye view (BEV) designed blocks were added to both beams. The differences in the dose distributions between the two types of plans were evaluated using dose-volume histograms (DVH). As expected, the dose distribution within the target volume showed no difference. However, a considerable sparing of normal tissue was observed for the plans with customized blocks. The volume of unnecessary exposed normal tissue that received more than 90% of the prescribed dose, was reduced by a factor of about 4: from 165 to 44% on average, if the volume is expressed as a percentage of the target volume in each patient. In particular, the homolateral mandible showed a mean decrease of 21% of integral dose when blocks were used. Normal tissue complication probabilities (NTCP) were calculated. For a tumor dose of 70 Gy, the average bone necrosis probability was reduced from 8.4% (no blocks) to 4.1% (blocks). For other normal tissues such as nervous tissue, other soft tissues and bones a substantial reduction of integral dose was found for all patients when individual blocks were used.