Date Published: February 05, 2018
Author(s): Kaley Woods, Percy Lee, Tania Kaprealian, Isaac Yang, Ke Sheng.
This study investigates whether 4π noncoplanar radiation therapy can spare the cochleae and consequently potentially improve hearing preservation in patients with acoustic neuroma who are treated with radiation therapy.
Clinical radiation therapy plans for 30 patients with acoustic neuroma were included (14 stereotactic radiation surgery [SRS], 6 stereotactic radiation therapy [SRT], and 10 intensity modulated radiation therapy [IMRT]). The 4π plans were created for each patient with 20 optimal beams selected using a greedy column generation method and subsequently recalculated in Eclipse for comparison. Organ-at-risk (OAR) doses, homogeneity index, conformity, and tumor control probability (TCP) were compared. Normal tissue complication probability (NTCP) was calculated for sensorineural hearing loss (SNHL) at 3 and 5 years posttreatment. The dose for each plan was then escalated to achieve 99.5% TCP.
4π significantly reduced the mean dose to both cochleae by 2.0 Gy (32%) for SRS, 3.2 Gy (29%) for SRT, and 10.0 Gy (32%) for IMRT. The maximum dose to both cochleae was also reduced with 4π by 1.6 Gy (20%), 2.2 Gy (15%), and 7.1 Gy (18%) for SRS, SRT, and IMRT plans, respectively. The reductions in mean/maximum brainstem dose with 4π were also statistically significant. Mean doses to other OARs were reduced by 19% to 56% on average. 4π plans had a similar CN and TCP, with a significantly higher average homogeneity index (0.93 vs 0.92) and significantly lower average NTCP for SNHL at both 3 years (30.8% vs 40.8%) and 5 years (43.3% vs 61.7%). An average dose escalation of approximately 116% of the prescription dose achieved 99.5% TCP, which resulted in 32.6% and 43.4% NTCP for SNHL at 3 years and 46.4% and 64.7% at 5 years for 4π and clinical plans, respectively.
Compared with clinical planning methods, optimized 4π radiation therapy enables statistically significant sparing of the cochleae in acoustic neuroma treatment as well as lowering of other OAR doses, potentially reducing the risk of hearing loss.
Acoustic neuroma, also known as vestibular schwannoma, is a benign brain tumor arising from the eighth cervical nerve. There are 2000 to 3000 new cases of benign acoustic neuroma diagnosed in the United States each year, approximately 25% of which are treated with radiation therapy.1 Due to its benign nature, the prognosis for patients with acoustic neuroma is typically very good, and with proper surveillance and treatment, no decrease in lifespan is expected. Therefore, the long-term posttreatment toxicity must be heavily weighted for these patients. Although the complication rates are much lower than with surgery,2, 3, 4 some patients experience radiation-induced side effects after treatment. Up to 40% of patients may experience middle ear side effects such as otitis media during treatment,5 which can cause tinnitus, dizziness, and pain. Almost half of patients may also experience some degree of sensorineural hearing loss (SNHL), which continues to worsen for years after treatment.6, 7, 8, 9, 10, 11, 12
4π radiotherapy achieves significantly greater normal tissue sparing compared with radiation therapy techniques that are typically used in acoustic neuroma treatment. These major reductions in cochlear dose may reduce the risk of normal tissue complications such as hearing loss and enable the safe escalation of prescription doses to potentially improve tumor control rates.