PMRT decreases loco-regional recurrence and can increase survival in high-risk breast cancer patients
. The American Society of Clinical Oncology has published treatment guidelines, but has also indicated that the optimal technique for PMRT remains unknown.
Skin in itself may not be part of the clinical target volume (CTV) since recurrence on the skin scar is a rare event
. On the other hand the work from Van Limbergen in breast brachytherapy showed the importance avoiding the terminal branches of the skin microvessel that lay 3 mm below the skin surface
. So we excluded the first 3 mm of the skin surface from our CTV definition.
An underdosage of the most superficial part of the PTV is observed with 6 MV irradiation without bolus due to a larger build-up effect. This can be counterbalanced by an artificial tissue, so-called bolus. Its use enhances superficial dose, but also skin toxicity. On the other hand this might impact also local failure.
In 2004, an e-mail survey was sent to all active physician members of the American Society for Therapeutic Radiology and Oncology, the Canadian Association of Radiation Oncologists and the European Society for Therapeutic Radiology and Oncology. The survey focused on the technical details regarding the use of a bolus in PMRT. The results have been published recently
. In total, 1035 responses were obtained: 642 from the Americas (568 from the USA), 327 from Europe and 66 from Australasia. Respondents from the Americas were significantly more likely to always use a bolus (82%) than the Europeans (31%), as were the Australasians (65%) (P < 0.0001). The results also showed wide variation in the schedule of application (every day [33%] and alternate days [46%]) and thickness used (< 1 cm [35%] and > or = 1 cm [48%]). There is a wide variation in the use of a bolus in PMRT, and this probably translates into a variation in the dose delivered to the skin and may have an effect on local recurrence. No data are available about a possible impact of the use of a bolus and local failure.
The measurements performed in this work are an indicator that PMRT with 4 MV beams is equally good as the optimized
 bolus/non-bolus (40/60) schedule with 6 MV photon beams when target coverage is important. With regard to skin sparing the 4 MV treatment is even advantageous reducing skin dose by about 10% of the prescribed dose. Another feasibility study was performed by Petoukhova et al.
 for head and neck cancers. They found no significant difference between 4 MV and 6 MV radiation beams with regard to dose in and around air cavities.
The measured dose in the build-up region was also compared to the calculations of the Eclipse AAA algorithm. It was found that the precision of the computed dose is around 5%.
Taking the results of this work into account the use of 4 MV photon beams is advantageous to 6 MV with bolus. First, a dose distribution of the same quality can be administered to the PTV, while the use of 4 MV results in better skin sparing. Second, with 4 MV only one treatment plan must be computed instead of two with 6 MV, one without and one with bolus. Finally, no bolus must be placed on the patient which diminishes the potential of for getting the bolus for irradiation.
However, there are also disadvantages of using 4 MV beams for PMRT. In this work a Varian linear accelerator was used with limited dose-rate of 2.5 Gy/min at 4 MV. The dose-rate is 2.5 times lower than of the 6 MV beam resulting in longer irradiation times. This could be an additional problem when the deep-inspiration breath-hold technique, which is standard in our institution for left-sided breast cancer patients, is used. A further disadvantage can be the quality of the dose distribution of two tangential fields with large diameter targets. 4 MV photons may have worse quality of dose distribution in two tangential fields covering large diameter targets.