High Dose Rate Brachytherapy of Carcinoma of the Cervix: Applicability of Various Dosimetry Systems and Guidelines in the Dose Prescription and Treatment Planning

Purpose: This study is conducted to investigate applicability of various dosimetry systems and guidelines in the dose prescription and treatment planning of high dose rate brachytherapy of carcinoma of the cervix. Material and Methods: Many dosimetry systems were devised to guide the treatment of carcinoma of the cervix. The Manchester system was the first one got wide acceptance among the radiotherapy centers Worldwide due to its reproducibility of dose specification and source distribution. The Manchester system is characterized by doses to points A and B, where dose is prescribed to point A. The definition of point A was modified time to time by different working groups to accommodate technological advancements. Another system recommended by the ICRU relates the dose distribution to the target volume rather than to specific points. The ICRU system for dose specification of brachytherapy for Ca.Cx. recommends an absorbed dose level of 60 Gy as a reference dose level for LDR treatments of the pear shaped 60 Gy isodose reference volume. For the combination of EBRT and ICBT, the reference isodose for ICBT is obtained by subtracting the EBRT dose from a total dose of 60 Gy. The source loading were similar in the ICRU system as that in the Manchester system, which includes entire uterus, cervix and vaginal mucosa. Both systems were effectively adopted for HDR brachytherapy with appropriate dose rate corrections. The ABS had presented guidelines for HDR brachytherapy of Ca.Cx. for CT or MRI based 3D treatment planning and dose delivery. For target contouring, ABS recommends the use of the Groupe European Curietherapie- European Society of Therapeutic Radiation Oncology (GEC-ESTRO) contouring guidelines for both CT and MRI based imaging, and the prescribed dose should cover 90% of the high - risk clinical tumor volume (HR-CTV). Results and Discussion: Comparison of dose distributions of the Manchester / ICRU systems and ABS HR-CTV based dose prescriptions reveals entirely different area coverages. The Manchester / ICRU systems based dose prescriptions cover entire uterus which may have micro-invasive disease and at potential risk of recurrence in the uterus which does not cover in ABS HR-CTV based dose prescription, because the HR-CTV includes the cervix plus tumor extension at the time of brachytherapy, and 1 cm extension above the uterine vessels identified by intravenous contrast or the location where uterus begins to enlarge. On the other hand, there is a significant HR-CTV under coverage, for the patients of large pelvic region and over coverage for small pelvic region, when source loading and dose prescription is done based on the Manchester / ICRU systems. In one of our study, the dose prescription point is defined based on the anatomical variation of the pelvic cavity of the patients treated with HDR brachytherapy, which offers adequate coverage for cervix. Conclusion: Every system or recommendations devised, so far, for HDR (or LDR) brachytherapy of carcinoma of the cervix, has negative and positive points in dose prescription and tumor coverage, or tumor contouring, which sometimes lead to tumor recurrence. Hence it is advice to the practicing radiation oncologist and brachytherapy physicist to consciously individualize the treatment for each patient.