Surface Dose Measurements in Chest Wall Postmastectomy Radiotherapy to Achieve Optimal Dose Delivery with 6 MV Photon Beam.

Aim: A tissue-equivalent bolus of sufficient thickness is used to overcome build up effect to the chest wall region of postmastectomy radiotherapy (PMRT) patients with tangential technique till Radiation Therapy Oncology Group (RTOG) Grade 2 (dry desquamation) skin reaction is observed. The aim of this study is to optimize surface dose delivered to chest wall in three-dimensional radiotherapy using EBT3 film.

Materials And Methods: Measurements were conducted with calibrated EBT3 films with thorax phantom under "open beam, Superflab gel (0.5 cm) and brass bolus conditions to check correlation against TPS planned doses. Eighty-two patients who received 50 Gy in 25# were randomly assigned to Group A (Superflab 0.5 cm gel bolus for first 15 fractions followed by no bolus in remaining 10 fractions), Group B or Group C (Superflab 0.5 cm gel or single layer brass bolus, respectively, till reaching RTOG Grade 2 skin toxicity).

Results: Phantom measured and TPS calculated surface doses were within - 5.5%, 4.7%, and 8.6% under open beam, 0.5 cm gel, and single layer of brass bolus applications, respectively. The overall surface doses (OSD) were 80.1% ±2.9% ( = 28), 92.6% ±4.6% ( = 28), and 87.4% ±4.7% ( = 26) in Group A, B, and C, respectively. At the end of treatment, 7 out of 28; 13 out of 28; and 9 out of 26 patients developed Grade 2 skin toxicity having the OSD value of 83.0% ±1.6% ( = 7); 93.7% ±3.2% ( = 13); and 89.9% ±5.6% ( = 9) in Groups A, B, and C, respectively. At the 20-23 fraction, 2 out of 7; 6 out of 13; and 4 out of 9 patients in Groups A, B, and C developed a Grade 2 skin toxicity, while the remaining patients in each group developed at the end of treatment.

Conclusions: Our objective to estimate the occurrence of optimal dose limit for bolus applications in PMRT could be achieved using clinical EBT3 film dosimetry. This study ensured correct dose to scar area to protect cosmetic effects. This may also serve as quality assurance on optimal dose delivery for expected local control in these patients.