Improving cardiac dosimetry: Alternative beam arrangements for intensity modulated radiation therapy planning in patients with carcinoma of the distal esophagus.

PURPOSE: Radiation for carcinoma of the distal esophagus is associated with cardiac perfusion deficits and pericardial effusion. We performed a dosimetric analysis of alternative beam arrangements for use in intensity modulated radiation therapy (IMRT) planning, seeking to lower radiation dose to the heart. METHODS AND MATERIALS: Treatment plans using 4 separate beam arrangements were generated and optimized for 12 patients. Hemispheric and butterfly beam arrangements were compared with plans with posterior and lateral beam entries. Radiotherapy was planned to 50.4 Gy in 28 fractions, using step and shoot IMRT with 6 MV photons. Mean heart dose and volumes of heart and lung receiving up to specified doses (V5-V40) were recorded. Isodose distributions were evaluated for target coverage and normal tissue exposure. RESULTS: IMRT plans utilizing posterior-lateral beam arrangements significantly reduced mean cardiac doses (32.5 ± 3.9 Gy, 33.3 ± 3.2 Gy vs 24.3 ± 3.7 Gy, and 23.4 ± 4.2 Gy, P < .05, paired Student t test with post hoc Bonferroni correction) as well as the total heart volumes receiving at least 20 and 30 Gy. IMRT allowed the maximum cord dose to be limited to less than 40 Gy. While both posterior-lateral beam arrangements lead to improved cardiac dosimetry, mean lung doses as well as V5 and V20 were slightly higher, although within accepted limits. Target coverage, homogeneity, and conformality were similar or improved with the use of alternative beam configurations. CONCLUSIONS: The use of IMRT with posterior-lateral beams can significantly reduce radiation dose to cardiac structures with minimal increased dose to the lung. Future studies will assess the physiologic and clinical impact of cardiac sparing.

Corrections to traditional methods of verifying tangential-breast 3D monitor-unit calculations: use of an equivalent triangle to estimate effective fields.

This paper describes an innovative method for correctly estimating the effective field size of tangential-breast fields. The method uses an "equivalent triangle" to verify intact breast tangential field monitor-unit settings calculated by a 3D planning system to within 2%. The effects on verification calculations of loss of full scatter due to beam oblique incidence, proximity to field boundaries, and reduced scattering volumes are handled properly. The methodology is validated by comparing calculations performed by the 3D planning system with the respective verification estimates. The accuracy of this technique is established for dose calculations both with and without heterogeneity corrections.