Dosimetric Evaluation of Heterogeneity Corrections for RTOG 0236: Hypofractionated Radiotherapy of Inoperable Stage I/II Non-small Cell Lung Cancer

Article Outline

• Purpose/Objective(s)
• Materials/Methods
• Results
• Conclusions
• Copyright

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Purpose/Objective(s) 

To evaluate treatment plans with and without heterogeneity corrections submitted from multiple institutions accruing patients to RTOG protocol 0236 designed to investigate the benefits of hypofraction irradiation for inoperable stage I/II non-small cell lung cancer.

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Materials/Methods 

Twenty patients from three institutions are evaluated. These institutions were selected because they used convolution/superposition algorithms for treatment planning. All treatment plans used multiple non-opposing, non-coplanar equally-weighted static beams. The protocol required a prescription dose of 60 Gy delivered in three fractions to cover 95% of the volume of the PTV. Additional requirements were specified for target dose heterogeneity and high/low dose spillage into normal tissue. The protocol also has absolute maximum dose limits to any points within the critical structures like spinal cord, esophagus. Dose spillage criteria are that the ratio of the volume of 50% of the prescription dose to the volume of the PTV be no larger than 2.9–3.9 (PTV volume dependent). Additional treatment plans that used the same beam arrangements and monitor units as the plans assuming unit density within the patient, but with heterogeneity corrections applied, were also sent for review. The PTV volumes for the patients studied ranged from 10.7–117 cc, with mean and standard deviation of 45 cc and 28 cc respectively. The number of beams used had a range of 8–12. This study compares the plans that assume unit density throughout the patient to the plans that correct for tissue heterogeneity.

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Results 

With heterogeneity corrections applied, the isocenter dose increased between 0%–22% with mean of 12% and standard deviation of 5%. The volume of PTV receiving 60 Gy or more (V₆₀) decreased from 96% ± 2% to 85% ± 12%. Dose to 95% of the volume (D₉₅) decreased from 60.6 Gy ± 1.4 Gy to 55.9 Gy ± 4.8 Gy. Maximum dose to any point 2 cm or greater away from the PTV in any direction increased from 35.2 Gy ± 7.8 Gy to 38.5 ± 9.6 Gy. Percent lung volume receiving 20 Gy or higher increased from 5.5% ± 2.7% to 5.9% ± 2.9%. Maximum spinal cord dose increased from 9.9 Gy ± 5.7 Gy to 10.9 Gy ± 6.2 Gy. The ratio of prescription isodose volume to the PTV volume meeting the protocol criteria remained similar at over 90%. However, the ratio of the volume of 50% of the prescription dose to the volume of the PTV meeting the protocol criteria decreased from 80% to 35%.

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Conclusions 

Significant differences are found between the calculated doses submitted to meet the requirements of RTOG protocol 0236 and the heterogeneity corrected doses. The volume of PTV receiving prescription dose decreased over 10% on average. Dose to 95% of the PTV volume decreased from an average of 60.6 Gy to 55.9 Gy. Dose spilling to normal tissues significantly increases with heterogeneity corrections applied. The design of the RTOG 0236 protocol was patterned on pilot studies that did not use tissue heterogeneity corrections for the treatment planning. The information provided in the current study will be used for designing future RTOG protocols to exactly match the true dose delivered for 0236. Adjusting the dose for future studies is extremely important given the hypofractionated dose schedule and reduced margins used for RTOG 0236.

The NCI grant U24 CA 81647 is acknowledged for support of the study.