A comparison of dosimetry and clinical outcomes in patients receiving photon external beam therapy to the pelvis: A two-year experience with 6MV and 10MV photon energies in cervical cancer patients at Parirenyatwa Radiotherapy Centre in Harare, Zimbabwe.

Abstract

e18768 Background: Radiotherapy efficacy depends on the ability of the treatment machine to deliver optimal radiation dose to target volume while sparing normal tissue using different techniques. The photon energy determines penetrative power. Higher photon energies are generally preferred over low photon energies when treating deep tumours using 3-dimensional conformal radiotherapy (3DCRT). In most low income settings, availability of higher energies (≥10MV) is limited. Elsewhere protocols do not recommend use of lower energies for deep pelvic tumours. There is paucity of data comparing 10MV and 6MV photon energy plans and outcomes for treatment of pelvic tumours with 3DCRT. This study aimed to compare dosimetric and clinical tumour outcomes between the two photon energies which are available in our setting when treating locally advanced cervical cancer (LACC). Methods: We retrospectively analysed medical records for LACC patients who received definitive concurrent chemoradiotherapy (CCRT) at Parirenyatwa Group of Hospitals Radiotherapy Centre for the period between 1st January 2017 and 31st December 2018. Patient’s treatment plans were stratified into two arms according to the photon energies used (10MV and 6MV) and their respective dosimetric and clinical tumour outcomes at three months post-treatment were compared. Results: A total 875 cervical cancer patients were seen during the study period, of these 82 met the inclusion criteria and were evaluated. Out of these, 20(24.4%) and 62(75.6%) patients were planned and/or treated with 10MV and 6MV photon energy respectively. The differences in minimum doses to the planned target volume and dose homogeneity index between the two photon energies were statistically significant (p-values 0.027 and 0.028 respectively), whereas the other dosimetric parameters (maximum & average doses to the planned target volume, conformity index and maximum doses to rectum, bladder, femoral heads and bowel) were not statistically significant (p-values of 0.245, 0.309, 0.130, 0.19, 0.35 and 0.42, and 0.16 respectively). Complete clinical tumour response at 3 months post treatment was 95% in the 10 MV arm compared to 91% in the 6 MV arm. Conclusions: This study showed that the dosimetric and clinical tumour outcomes in LACC patients receiving 3DCRT definitive CCRT using 10MV or 6MV photon energy in our setting are comparable. Follow-up prospective studies to further characterise the application of these photon energies in resource constrained settings is recommended.