Effects of different CT value assignment methods on dose calculations in radiotherapy planning for brain metastases.

BACKGROUND: To study the effects of different CT value assignment methods on the dose calculations in radiotherapy plans for brain metastases, this study will provide a reference for radiotherapy planning design based on MR images. METHODS: All fifty recruited patients underwent CT and MR simulated localization the same day as, but prior to, three dimensional conformal radiation therapy (3D-CRT) or intensity modulated radiation therapy (IMRT) for brain metastases. After rigid registration of both the CT and MR images, the main tissues and organs were delineated on the CT and MR images. The average CT value of each tissue or organ was calculated. Three groups of pseudo-CT were generated by three CT value assignment methods: (I) the whole tissue was assigned 140 HU; (II) cavity, bone and other tissues were assigned -700, 700 and 20 HU, respectively; (III) tissue- and organ-specific CT values were given. The dose distribution was recalculated based on the three groups of pseudo-CT to obtain Plan2, Plan3 and Plan4, accordingly. The resultant radiotherapy plans were considered the original plan (Plan1). Then, the dosimetric differences between these three plans and Plan1 were compared. RESULTS: The average pseudo-CT values of bone and cavity were 731.7±69.3 and -725.5±26.1 HU, respectively. The range of average soft-tissue CT values was from -70 to 70 HU. The dose distribution between Plan1 and Plan2, Plan3 or Plan4 showed some differences, and the differences decreased in turn. The differences in the maximum dose of the lenses can reach 5.0%, 1.5% and 1.2%, respectively, while the differences in other dose parameters (maximum dose, mean dose and D 98% to the PTV, D 5% of the brainstem, and maximum dose of the brainstem, corpus callosum, left eye, right eye) were basically less than 2.0%, 1.2% and 0.8%, respectively. This shows that in the CT value assignment method, the dose calculation error can be greatly reduced by assigning the value to the bone and cavity separately, and if the different soft tissues are distinguished, the error of the dose calculation can be further reduced by more than 30%. In the pixel-by-pixel dosimetric comparison, the areas of more than 1% dose difference between Plan1 and Plan3 as well as Plan4 were mainly distributed near skin while those between Plan1 and Plan2 were mainly distributed at the bone, cavity, bone and soft tissues junction, and the skin near the field. CONCLUSIONS: In summary, a scheme for assigning specific CT values to MRI-based radiotherapy is established. The scheme will provide patients with a dose-free radiotherapy plan. Through the calculation of the differences between the new plans and the old plan, it is found that our scheme can basically control the dose error below 0.8% to meet the clinical requirements.

Study of the dose-volume parameters variation in tumor target volumes and organs at risk during nasopharyngeal carcinoma radiotherapy applying deformation registration.

BACKGROUND: During intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC), the volume of the target volume and the organs at risk (OARs) will change constantly, which may lead to differences between the actual dose received and the initial planned dose. In this study, the cumulative dose of the two plans was obtained by deformable registration. This study provides an approach to evaluate the dose volume of IMRT for the NPC objective. METHODS: From July 2014 to May 2018, eighteen NPC patients who accepted simultaneous integrated boost IMRT were enrolled. All patients underwent simulation CT (CT1) and replanning CT (CT2) scans after 20-25 fractions of radiation therapy. The treatment plans were designed on CT1 and CT2 with the name of Plan1 and Plan2, respectively. The Planreg and Plandef were obtained after registering from CT2 to CT1 using rigidity and deformation technology by Velocity. Then the dose-volume indices of the tumor target volumes and OARs at Plan1, Plan2, Planrig and Plandef were compared. RESULTS: The gross tumor volume (GTV) and the left and right parotid gland volumes decreased by 20.8% (P<0.001), 36.8% (P<0.001) and 37.5% (P<0.001), respectively, from CT1 to CT2. There was no significant difference in the dose-volume index on the GTV and plan gross tumor volume (PGTV) between Plan1 and Plan2. The V30 of the left and right parotid gland and the Dmax of the brainstem, left and right eyeballs, left and right lens, and left and right optic nerves were all lower in Plan2 than in Plan1 (the average decrease was 17.0% to 60.1%). The differences in some dose-volume parameters (including Dmean, D99 of the GTV and PGTV, Dmean of the parotid glands, Dmax of the lens and optic nerves) between Plandef and Plan1 were less than 5%. The differences in some dose-volume parameters (including Dmean, D95 of the GTV and PGTV, Dmean, D50 and V30 of the parotid glands, Dmax of lens and optic nerves) between Planrig and Plan1 were less than 10%. The Dyce Similarity Coefficient of the target volume and OARs after deformation registration were higher than that after rigid registration. CONCLUSIONS: The volume of the GTV and parotid glands were decreased during the IMRT for NPC. The dose-volume indices of the GTV and the OARs in Plandef were similar to those in Plan1. Therefore, the dose-volume indices of Plan1 can be used to evaluate the efficacy of radiotherapy and to predict radioactive damage.

Dosimetric comparison of dose accumulation between rigid registration and deformation registration in intensity-modulated radiation therapy for large volume non-small cell lung cancer.

BACKGROUND: To evaluate the cumulative dose to the target volumes and organs at risk (OARs) after replanning during intensity-modulated radiation therapy (IMRT) for large volume non-small cell lung cancer (NSCLC) based on rigid registration and deformation registration technologies. METHODS: Thirty patients with large volume NSCLC who were treated with IMRT were selected, and two four-dimensional computed tomography (4DCT) scans were acquired before radiotherapy and after 20 fractions of radiotherapy. The initial treatment plan (Plan1) based on the average density projection CT (CT1-avg) of the first 4DCT images and the second treatment plan (Plan2) based on CT2-avg of the second 4DCT images were calculated. Then, the dose distributions of Plan2 and Plan1 were accumulated based on rigid and deformation registration technologies to obtain Planrig and Plandef, respectively. Finally, the volume changes of the gross tumor volume (GTV) and OARs between the two CT scans, and the dose-volume parameters among Plan1, Plan2, Planrig and Plandef were compared. RESULTS: Compared with those on the first CT, the mean GTV and heart volume on the second CT decreased by 44.2% and 5.5%, respectively, while the mean volumes of the ipsilateral lung, contralateral lung and total lung increased by 5.2%, 6.2% and 5.8%, respectively. The differences in the above volume parameters between the two CT scans were statistically significant (P<0.05). Compared with those in Plan1, the D95, D98 and V100% values of the IGTV (GTV fusion of 10 CT phases) and planning target volume (PTV) in Plan2 did not change significantly (P>0.05), and those of Planrig and Plandef decreased slightly (P<0.05). The dose-volume parameters of the spinal cord, heart, ipsilateral lung and total lung in Plan2, Planrig and Plandef were significantly lower than those in Plan1 (P<0.05). Among these parameters, V30 and the mean dose to the heart in Plan2, Planrig and Plandef decreased by 27.3%, 16.5%, and 15.3% and 15.2%, 6.6%, and 5.6% compared to those in Plan1, respectively; V20 and the mean dose to the total lung in Plan2, Planrig and Plandef decreased by 15.6%, 4.5%, and 3.7% and 15.7%, 6.2%, and 5.1% compared to those in Plan1, respectively. Some dose-volume parameters (including D95 and D98 to the target volume, V40 of the heart, V20 and the mean dose to the ipsilateral lung and the total lung) of Plandef were slightly higher than those in Planrig (P<0.05). The Dice similarity coefficients (DSCs) of the OARs after deformation registration were significantly higher than those after rigid registration (P<0.05). CONCLUSIONS: The dose-volume parameters of OARs in Plan2 were noticeably different from those in Plan1, so all of these parameters have large deviations in evaluating the actual dose to the OARs. And, the dose-volume parameters obtained by deformation registration can better predict the actual dose than those obtained by rigid registration.