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Low dose radiation therapy (LDRT) to lungs has shown encouraging results in patients with Covid-19 pneumonia. Though the prescribed dose of LDRT is very low (0.5-1.5 Gy), but there has been some concern regarding the risk of radiation induced carcinogenesis (RIC). Risk is dependent on the amount of radiation exposure and the age at exposure. Most LDRT trials have used conventional AP-PA open fields. Modern technique like IMRT can potentially reduce the organs at risk (OAR) doses thereby minimizing the risk of RIC. We designed a dosimetric study to see if IMRT can reduce the dose to OARs and lessen the risk of RIC in Covid-19 patients.

We retrieved the CT scan data of 10 patients who have been already treated for any malignancy in the region of thorax.  The following selection criteria were used 1) Age >40 years 2) equal number of male and female patient so as to estimate the risk of RIC in breast 3) no previous surgery in the thoracic area and intact thoracic organs and breast 4) complete set of  CT imaging from mandible to L1vertebra. The CT data of each patient was used to delineate the CTV and OAR to generate two parallel plans: one with open fields (Conventional Plan) and one with VMAT. A dose of 1 Gy in single fraction was prescribed to PTV which included both lungs. Mean OAR doses were used to estimate the risk of RIC for both plans and compared. The excess relative risk (ERR) of RIC was estimated using online radiation risk assessment calculator (https://irep.nci.nih.gov/radrat). This tool (RadRT) uses the risk models broadly based on Biological Effects of Ionizing Radiation  (BEIR) VII with some modifications. The ERR values the two plans were compared. For statistical analysis, two tailed Wilcoxon signed rank test was used to compare the dosimetry and ERR between two planning techniques. A p-value of <0.05 was considered significant.

The beam-on time and monitor units (MU) were less with conventional plan but all other DVH parameters  (D₉₅, D_mean, CI and HI) were significantly better with VMAT (p value <0.05 for all). Mean dose to most OAR like esophagus, spinal cord, thyroid and skin was significantly lower with VMAT (p value <0.05 for all). Mean heart dose with conventional plan was not only higher than VMAT plan  (<0.05) but also crossed the prescribed dose. Table 1 shows the comparison of ERR in both the plans. The overall ERR is significantly lower with VMAT as compared to conventional plan (0.357 vs 0.398%, p value <0.05). The ERR for all individual organs except thyroid was significantly lower with VMAT. Even though the mean thyroid dose was significantly less with VMAT than conventional plan, but the ERR for thyroid cancer was comparable with two plans. ERR was significantly less with VMAT. In both plans, the ERR was highest for lung followed by breast and marrow. 

Our dosimetric study shows that IMRT can lessen the risk of RIC in Covid-19 patients undergoing LDRT.