Comparative study and significance of measured and calculated doses delivered to the rectum in different plans for treatment of prostate cancer

Objectives: There is a lack of quantitative assessment of the dose delivered to the posterior rectal wall, which is the structure primarily excluded by treatment plans to reduce toxicity. Present study aims at (1) reporting the measured dose of the posterior rectal wall. (2) Comparing direct measurements and calculated doses. (3) Comparing direct measurements and rectal volume doses in 3-D planning. (4) Investigating relationship between dose and rectal morbidity. Materials and Methods: 99 direct measurements were obtained in 74 patients using a minichamber fitted in a specially constructed rectal applicator. Measurements were taken during the actual time of treatment in 25 patients treated by 4 field box technique, 28 by 300 deg. arc, 13 by bilateral 120 deg. arc and 8 by 360 deg. arc rotation. Thirteen patients had 3 measurements using the 3 different arcs to deliver one treatment fraction. The measurements were compared to the dose received by the posterior rectal wall as calculated in a 3-D Dose Volume Histogram (DVH) analysis as well as from the standard plans. Acute rectal symptoms (RTOG) in all patients were correlated to dose measured in the posterior rectal wall. Results: Measured doses to posterior rectal wall were 137, 83, 72, 106 cGy per 180 cGy at isocenter using 4 field box, a 300 deg. rotation, bilateral 120 deg. and 360 deg. arc rotations respectively. Measured doses to the posterior rectal wall showed strong agreement with a mean variance of 1.5% with the standard plan and 3% with the 3-D plan. There is correlation between the doses delivered to the posterior rectal wall and morbidity where patients with higher rectal wall measurements had higher morbidity score. The higher measurements were particularly noted in the 360 deg. arc followed by the 300 deg. arc and the least were noted in bilateral 120 deg. arc rotation. Conclusion: Among the 3 boost techniques investigated, bilateral 120 deg. arc offered the least posterior rectal wall dose with the least morbidity. The technique used for direct rectal dose measurements appears accurate and reproducible Dose as computed for the posterior rectal volume using DVH analysis compares favorably with the direct measurements