Dosimetric evaluation of heterogeneity corrections for RTOG 0236: stereotactic body radiotherapy of inoperable stage I-II non-small-cell lung cancer

Abstract

Purpose: Using a retrospective analysis of treatment plans submitted from multiple institutions accruing patients to the Radiation Therapy Oncology Group (RTOG) 0236 non-small-cell stereotactic body radiotherapy protocol, the present study determined the dose prescription and critical structure constraints for future stereotactic body radiotherapy lung protocols that mandate density-corrected dose calculations.

Method and materials: A subset of 20 patients from four institutions participating in the RTOG 0236 protocol and using superposition/convolution algorithms were compared. The RTOG 0236 protocol required a prescription dose of 60 Gy delivered in three fractions to cover 95% of the planning target volume. Additional requirements were specified for target dose heterogeneity and the dose to normal tissue/structures. The protocol required each site to plan the patient's treatment using unit density, and another plan with the same monitor units and applying density corrections was also submitted. These plans were compared to determine the dose differences. Two-sided, paired Student's t tests were used to evaluate these differences.

Results: With heterogeneity corrections applied, the planning target volume receiving >/=60 Gy decreased, on average, 10.1% (standard error, 2.7%) from 95% (p = .001). The maximal dose to any point >/=2 cm away from the planning target volume increased from 35.2 Gy (standard error, 1.7) to 38.5 Gy (standard error, 2.2).

Conclusion: Statistically significant dose differences were found with the heterogeneity corrections. The information provided in the present study is being used to design future heterogeneity-corrected RTOG stereotactic body radiotherapy lung protocols to match the true dose delivered for RTOG 0236.

Figure 1

The non-coplanar beam arrangement for the 3D conformal planning of the stereotactic radiation treatment (PTV and cord are shown in solid, orientation icon labels: H:head, R:Right, L:Left).

Figure 2

PTV volume distribution and the number of beams (given above each bar) employed for the radiotherapy treatment plan.

Figure 3

Isodose distributions for with unit density (left) and with density corrections (right). The dashed lines are PTV (inside) and 2 cm from PTV (outside). The solid lines are for 60 Gy isodose (inside) and 30 Gy isodose (outside).

Figure 4

Dose volume histograms for PTV coverage with unit density and with density correction, for the case shown in Figure 3, one of the cases with larger than average differences between heterogeneity corrected and unit density plans chosen for illustration.

Figure 5

Doses at isocenter point for all the plans with and without heterogeneity correction.

Figure 6

Percent volume that receives prescription dose 60 Gy or higher, as compared against the protocol required value of 95%, without and with heterogeneity correction.

Figure 7

Percent volume that receives 90% of prescription dose: 54 Gy or higher, as compared against the protocol required value of 99%, without and with heterogeneity correction.