Dosimetric comparison of tomotherapy versus 4- field pelvic box altered fractionation radiotherapy treatment plans for invasive squamous cell carcinoma of the cervix

Summary Altered fractionation radiotherapy features variations in frequency of radiotherapy delivery and/or amount of radiotherapy delivered at each treatment in an attempt to improve radiotherapy efficacy, depending on a given tumour's biologic nature. Several altered fractionation radiotherapy clinical trials for cervix cancer have been completed, but despite promising local control rates, these trials have been unsuccessful due to excessive radiotherapy toxicities. Radiotherapy in these trials was planned using traditional 4-field box techniques. Advanced radiotherapy planning and delivery systems such as helical tomotherapy offer the potential of planning and delivering altered fractionation regimens with improved critical structure avoidance. The purpose of this project is to perform a dosimetric comparison between tomotherapy and 4-field altered fractionation radiotherapy plans for cervix cancer cases. Two planning target volumes (PTVs) were defined for ten cervix cancer cases: PTV 1 contained pelvic lymph nodes, gross tumour, uterus and parametria and PTV 2 contained gross tumour, uterus and parametria. A tomotherapy and 4-field plan was created for each case. A dose of 45 Gy/25 fractions was prescribed to PTV 1 with a concomitant boost of 10 Gy to PTV 2 . Plans were normalized such that 95% of PTV 2 received the prescribed dose. Plans were compared based on dose volume histogram criteria, target volume conformity indices and normal tissue complication and tumour control probability estimates. Tomotherapy plans had significantly better conformity indices for PTV 2 coverage. Four-field plans had significantly higher minimum, median and mean PTV 1 doses. Rectal, bladder and small bowel doses were significantly lower in tomotherapy plans. PTV 2 tumour control probabilities were not significantly different between planning modalities. Bladder, rectal, and small bowel normal tissue complication estimates were significantly lower for tomotherapy plans. This data indicates tomotherapy plans confer superior boost volume coverage and critical structure avoidance compared with traditional 4-field plans for locally advanced cervical cancer. Direct clinical comparisons between tomotherapy and 4-field pelvic box radiotherapy plans for cervix cancer are needed to quantify tomotherapy's clinical benefits in terms of tumour control, radiotherapy toxicity rates and patient quality of life.