Radiation Therapy Target Objectives for Tonsillar Cancer Treated With Unilateral Radiation Therapy-A Replanning Study...

Unilateral radiation therapy (URT) is an effective treatment strategy in selected patients with lateralized tonsil cancer. However, there is a lack of established planning guidelines for URT treatment, leading to suboptimal optimization of contralateral and midline organs at risk (OARs). This study aimed to reoptimize URT plans to maximize sparing of midline and contralateral OAR's while maintaining target coverage, providing dosimetric guidelines for URT planning.

Treatment plan data for patients treated with URT on TROG 12.01 were reoptimized using Eclipse V16.01. All relevant midline and OAR contours were contoured to align with international OAR consensus guidelines. Treatment plans were reoptimized, aiming for maximal midline and contralateral sparing while maintaining TROG 12.01 protocol-mandated target coverage (gross tumor volume: D100% > 95%; planning target volume: D95% > 95%). A second reoptimization was performed to align with NRG acceptance criteria (planning target volume: D95% > 100%).

All reoptimized plans achieved the TROG 12.01 and NRG protocol target volume coverage. Clinically meaningful reductions in dose to the pharyngeal constrictors (trial-delivered plan vs TROG reoptimized plan, median of mean dose, 48.5 Gy vs 37.4 Gy, estimated mean difference [ED] -9.5Gy, P < .001), contralateral submandibular gland (14.9 vs 6.7 Gy, ED -11.0, P < .001) and larynx (22.1 vs 12.1, ED -12.2, P < .001). Improvements were also seen in contralateral parotid gland (9.2 vs 4.9, ED -5.0, P < .001) and oral cavity (39.5 vs 35.9, ED -3.0, P < .001), which were considered less likely to be clinically relevant. There was no increase in dose to the ipsilateral or contralateral mandible.

The results of this reoptimization study provide planning guidelines to guide dosimetrists and radiation oncologists on achievable OAR sparing when delivering URT.