Comparative Analysis of Dose Gradients and Valley Doses in Pelvic Lattice Radiotherapy in RapidArc and IMRT.

PURPOSE

This study evaluated the dose fall off and valley dose percentage in pelvic cancer Lattice Radiotherapy (LRT) using various treatment techniques.

METHODS

Forty five treatment plans were developed for 15 patients undergoing radiotherapy using a linear accelerator. Plans were categorized into three sets: RapidArc (RA), seven-field intensity-modulated radiation therapy (IMRT), and nine-field IMRT, both for high-dose (HD) vertices and the entire planning target volume (PTV). Dose fall-off indices were analyzed using the normalized dose fall-off index (ʌ) to compare the rate of dose decrease beyond HD vertices. Valley dose percentages were determined by analyzing dose profiles between HD vertices to quantify lower dose percentages. Analysis involved averaging normalized dose fall-off index (ʌ) values, valley dose percentages and grouping valley doses to assess variation with respect to center-to-center (CTC) intervals between HD vertices for all the techniques.

RESULTS

RA plans achieved sharper dose fall-off beyond HD vertices compared to seven-field and nine-field IMRT techniques, with decreasing values as distance from the central plane increased. RA plans also exhibited higher valley doses (62.05%) relative to nine-field IMRT (55.02%) and seven-field IMRT (56.56%) for an average CTC distance of 3.75 cm, showing significant variability across CTC intervals.

CONCLUSIONS

RA plans achieve steeper dose fall-off and higher valley doses compared to IMRT, effective for pelvic LRT but less suitable for grid therapy due to challenges with MLCs. Minimal differences in valley doses between nine-field and seven-field IMRT suggest limited impact from beam angles, guiding treatment optimization. The equations derived can be utilized for dosimetric evaluation and clinical planning in grid therapy, emphasizing their practical relevance in treatment strategy development.