Evaluation on plan quality and delivery efficiency of dynamic jaws mode in TomoTherapy for left-side breast cancer patients

Abstract

Background: Breast cancer ranks the highest incidence rate of female cancer in Hong Kong regarding the latest statistic from Hong Kong Cancer Registry in 2014(1). Radiotherapy plays an important role in tumour control and reduce subsequent recurrence rate and mortality rate. TomoTherapy®, one of the advanced Intensitymodulate radiotherapy (IMRT), has been used to treat breast cancer, especially for irradiating left breast with lymphatic node involvement. Dynamic Jaw (DJ) Mode in Helical TomoTherapy® (HT), a new feature firstly introduced in ASTRO 2012 by Accuray®, allows the jaws to move continuously during the treatment(2). The dynamic jaws adapt the field width dynamically at the cranial and caudal edges of a target to reduce the cranio-caudal dose penumbra. Several studies claimed that DJ Mode can improve the cranio-caudal dose distribution without prolonging the treatment time in treating different types of cancer including lung, liver, brain, breast and paediatric cancer. Also, studies suggested that DJ with a wider 5.0 cm field width (FW) can replace the conventional fixed jaws (FJ) with 2.5 cm field width, which can maintain the plan quality while reduce the treatment delivery time. However, the study on breast cancer with supraclavicular fossa (SCF) nodal involvement using HT using DJ Mode is limited. This study aims to evaluate the DJ Mode retrospectively by comparing their dosimetric quality with Normal Tissue Complication Probability (NTCP) of organ at risk (OAR) and treatment delivery time with FJ Mode on treating left breast cancer with SCF involvement. The best choice of mode will be suggested to maximize the patient’s benefit. Method: All breast cancer patient with post-mastectomy, who had been irradiated for left-side breast with SCF nodal involvement and planned under HT using DJ Mode with 2.5 cm FW (DJ2.5), from November 2014 to August 2016, at the Department of Radiotherapy in Hong Kong Sanatorium & Hospital, were selected retrospectively for this study. With the same dose constraint and prescription as the treated DJ2.5 plan, two extra plans using DJ mode with 5.0 cm FW (DJ5.0) and FJ mode with 2.5 cm FW (FJ2.5) were computed for plan comparison. Homogeneity index (HI) and several ICRU-recommended dose-volume specifications (e.g. D95) of Planning Target Volume (PTV) for both chest wall (CW) and SCF were used for plan comparison. Several dose-volume specifications with clinical value of OAR and NTCP of heart and lung were used to evaluate the performance in sparing of OAR among three optimization modes. In addition, treatment delivery time and actual modulation factor were also used for comparison. One-way ANOVA and Kruskal-Wallis test were separately used for variables following or not following the normal distribution. All statistical test results were analysed using the GraphPad Prism® (Version 7.00, GraphPad Software, San Diego, CA, US)

Result: The statistical test results showed no significant difference in all the dosimetric quality of PTV and OARs, except for V20 of whole lung. Numerically the plan of DJ2.5 and DJ5.0 showed the best and the worst dosimetric quality correspondingly in term of dose-volume specifications of PTV and OARs among the three optimization modes. Only the dose-volume parameters of larynx and liver, which are located superior and inferior to the targets, illustrated a relatively lower dose in DJ5.0 than FJ2.5. For other OARs, including heart and lung FJ2.5 demonstrated a better OAR sparing than DJ5.0. Radiobiologically, the NTCP of heart and lung for all plans were close to zero, while those NTCPs of DJ2.5 were calculated to be the lowest. The treatment delivery time and actual modulation factor (MF) were found to have significant difference in the statistical test. The average treatment delivery time of DJ5.0 was significantly lower than DJ2.5 and FJ2.5 by almost 40%. The actual MF was significantly higher in FJ2.5 than DJ2.5.

Conclusion: DJ mode demonstrated a better target homogeneity and sparing of organ than FJ mode, given that the same field width was used. Statistical significance is expected to be concluded with larger sample size. The use of wider field width in DJ mode is needed to be justified as the balance is needed to be taken between shorter treatment delivery time and a slightly inferior plan quality with the possible increased chance of secondary malignancy. DJ Mode with justified field width is recommended to be used for treating left-breast cancer patient with SCF involvement.