(P009) Assessment of Lumpectomy Cavity Radiotherapy Boost Volume With Repeat CT Simulation

Abstract

Lumpectomy cavity (LC) radiotherapy boost after breast conserving surgery and whole breast irradiation (WBRT) reduces local cancer recurrence. Radiotherapy boost correlates with increased rates of fibrosis and may have worse cosmetic outcomes. LC volumes and subsequent boost volumes may decrease with increasing time from surgery. This study evaluates the timing of CT simulation and radiotherapy technique for cavity directed radiotherapy boost. We seek to determine if repeat CT simulation results in smaller boost volumes which may minimize treatment related toxicity and maximize local tumor control. A retrospective chart review examined 47 consecutive patients receiving WBRT with LC boost after breast conserving surgery. All patients underwent CT simulation prior to WBRT and repeat CT simulation prior to LC boost. The WBRT prescription dose ranged from 4240-5040cGy and boost from 1000-1600cGy. The LC was contoured on the pre and post CT images and delineated by surgical clips and seroma cavity. 40 of 47 patients reviewed were found to have decreased LC volumes. The mean initial and repeat LC volumes were 71.1cc and 42.6cc respectively. Initial volumes greater than 60cc had a greater percentage decrease in LC volume at time of repeat simulation, 42% vs 30% (p=0.01). Patients who underwent initial LC evaluation less than 4 weeks after surgery had a greater percent decrease in mean repeat cavity volumes compared to those greater than 4 weeks out from surgery, 53% vs 24%. Patients undergoing WBRT with LC radiotherapy boost were found to have decreased cavity volume with repeat CT simulation. Factors that correlate to reduced volume are larger initial cavity volume and shorter time interval from surgery. These factors help to identify patients who will benefit from repeat CT simulation prior to boost planning and will subsequently have smaller volumes of radiation to normal breast tissue.